AN INTRODUCTION TO ENTOMOLOGY
AN
INTRODUCTION
TO
ENTOMOLOGY
BY
JOHN HENRY COMSTOCK
PROFESSOR OF ENTOMOLOGY AND GENERAL INVERTEBRATE
ZOOLOGY, EMERITUS, IN CORNELL UNIVERSITY
SECOND EDITION, ENTIRELY REWRITTEN
ITHACA, N. Y.
T E COMSTOCK PUBLISHING CO.
1920
COPYRIGHT I92O
BY
THE COMSTOCK PUBLISHING COMPANY
SPRINTED IN UNITED STATES OF AMERICA
PRESS OF W. F. HUMPHREY
GENEVA, N. V.
TO
MY OLD STUDENTS
WHOSE YOUTHFUL ENTHUSIASM WAS A CONSTANT INSPIRATION DURING
THE LONG PERIOD OF MY SERVICE AS A TEACHER THIS EFFORT
TO CONTINUE TO AID THEM IS AFFECTIONATELY
INSCRIBED
5t83f>2
PREFACE TO PART I
THE following pages constitute the first part of a text-book of
entomology that the writer has in preparation. This first part
is published in advance of the completion of the entire work in
response to the request of some teachers who desire that it be avail-
able for the use of their classes.
The early publication cf this part of the book will not only render
it immediately available but will also afford an opportunity for the
suggestion of desirable changes to be made before it is incorporated
in the complete work. Such suggestions are earnestly invited by
the writer.
In writing this text-book much use has been made of material
published in my earlier works, notably in "An Introduction to
Entomology" published in 1888 and long out of print, "A Manual
for the Study of Insects," in the preparation of which I was aided by
Mrs. Comstock, and in the "Wings of Insects," more recently pub-
lished. The more important of the other sources from which material
has been drawn are indicated in the text and in the bibliography at
the end of the volume. References to the bibliography are made in
the text by citing the name of the author and the year in which the
paper quoted was published.
A serious obstacle to the popularization of Natural History is the
technical names that it is necessary to use. In Border to reduce this
difficulty to a minimum the pronunciation of these names is indicated
by indicating the length of the vowel that receives the primary
accent.
The original figures and the copies of published figures in the first
chapter were drawn by Miss Anna C. Stryke; those in the three
following chapters, by Miss Ellen Edmonson. I am deeply indebted
to each of these artists for the painstaking care shown in her work.
Two objects are kept constantly in mind in the preparation of the
text-book of which this volume is a part: first, to aid the student in
laying a firm foundation for his entomological studies; and second,
to make available, so far as possible in the limited space of a hand-
book, a knowledge of the varied phenomena of the insect world.
It is hoped that those who use this book will find delight in acquiring
a more intimate acquaintance with these phenomena.
JOHN HENRY COMSTOCK.
ENTOMOLOGICAL LABORATORY
CORNELL UNIVERSITY
JUNE
TABLE OF CONTENTS
PART I. THE STRUCTURE AND METAMORPHOSIS
OF INSECTS
CHAPTER I
PAGES
THE CHARACTERISTICS OF INSECTS AND THEIR NEAR RELATIVES i
Phylum Arthropoda I
List of the classes of the Arthropoda 2
Table of the classes of the Arthropoda
Class Onychophora
Class Crustacea \ .......... ./. 6
Class Palaeostracha .-. J 8
Class Arachnida .X /. 9
Class Pycnogonida /. 10
Class Tardigrada : 12
Class Pentastomida 14
Class Diplopoda 15
Class Pauropoda 14
Class Chilopoda 28
Class Symphyla 23
Class Myrientomata 26
Class Hexapoda 20
CHAPTER II
THE EXTERNAL ANATOMY OF INSECTS 29
I. THE STRUCTURE OF THE BODY- WALL
a. The three layers of the body- wall 29
The hypodermis 29
The trichogens 30
The cuticula 30
Chitin 30
Chitinized and non-chitinized cuticula 3°
The epidermis and the dermis 3*
The basement membrane 31
b. The external apophyses of the cuticula 31
The cuticular nodules 31
The fixed hairs. 3*
The spines 32
c. The appendages of the cuticula 32
The spurs ... 32
TABLE OF CONTENTS
The setae 32
The taxonomic value of setae 33
A classification of setae 33
(1) The clothing hairs 33
(2) The glandular hairs 33
(3) The sense-hairs 33
d. The segmentation of the body 34
The body-segments, somites or metameres 34
The transverse conjunctivae 34
e . The segmentation of the appendages 34
/. The divisions of a body-segment 34
The tergum, the pleura, and the sternum . 34
The lateral conjunctivae 35
The sclerites 35
The sutures 35
The median sutures 35
The pilif erous tubercles of larvae 35
The homologizing of sclerites 35
g. The regions of the body -. . . . 36
2. THE HEAD
a. The corneas of the eyes 36
The corneas of the compound eyes 36
The corneas of the ocelli 3?
b The areas of the surface of the head 3?
The front 3?
The clypeus 38
The labrum 38
The epicranium 38
Thevertex 39
The occiput 39
Thegenae. 39
The postgenae 39
The gula 39
The ocular sclerites , 39
The antennal sclerites 39
The trochantin of the mandible 4°
The maxillary pleurites 4°
The cervical sclerites 4°
c. The appendages of the head 4°
The antennae , 40
The mouth-parts . 42
The labrum 42
The mandibles 42
The maxillalac 42
The maxillae 42
The labium or second maxillae 45
TABLE OF CONTENTS xi
The epipharynx 46
The hypopharynx 47
d . The segments of the head 47
3. THE THORAX
a. The segments of the thorax 48
The prothorax, mesothorax, and metathorax 48
The alitrunk 49
The propodeum or the median segment 49
b. The sclerites of a thoracic segment 49
The sclerites of a tergum 49
The notum 49
The postnotum or the postscutellum 50
The divisions of the notum 50
The patagia 50
The parapsides 51
The sclerites of the pleura 51
The episternum • • • • 51
The epimerum 51
The preepisternum 51
The paraptera 51
The spiracles 52
The peritremes • • 52
The acetabula 52
The sclerites of a sternum 52
c. The articular sclerites of the appendages 53
The articular sclerites of the legs ^ 53
The trochantin • 53
The antecoxal piece 54
The second antecoxal piece 54
The articular sclerites of the wings 54
The tegula 54
The axillaries 54
d. The appendages of the thorax ; 55
The legs. 56
The coxa 56
The styli • 56
The trochanter 57
The femur 57
The tibia 57
The tarsus 57
The wings 58
The different types of wings 59
The margins of wings 60
The angles of wings 60
The axillary cord 60
The axillary membrane 60
The alula ^
The axillary excision 61
TABLE OF CONTENTS
The posterior lobe 61
The methods of uniting the two wings of each side 61
The hamuli 61
The frenulum and the frenulum hook 61
The jugum 61
Thefibula 62
The hypothetical type of the primitive wing- venation 62
Longitudinal veins and cross- veins 64
The principal wing- veins 64
The chief branches of the wing- veins 64
The veins of the anal area 65
The reduction of the number of the wing- veins 65
Serial veins 67
The increase of the number of the wing- veins 68
The accessory veins 68
The intercalary veins , 69
The adventitious veins 70
The anastomosis of veins 70
The named cross-veins 71
The arculus .72
The terminology of the cells of the wing 72
The corrugations of the wings 73
Convex and concave veins 73
The furrows of the wing 73
The bullae 74
The ambient vein 74
The humeral veins 74
The pterostigma or stigma 74
The epiplurae '.' 74
The discal cell and the discal vein 74
The anal area and the preanal area of the wing 75
4. THE ABDOMEN 75
a. The segments of the abdomen 75
b. The appendages of the abdomen 76
The styli or vestigial legs of certain Thysanura 76
The collophore of the Collembola 76
The spring of the Collembola 76
Thegenitalia 76
The cerci 77
The median caudal filament 78
The prolegs of larvae 78
5. THE MUSIC AND THE MUSICAL ORGANS OF INSECTS 78
a. Sounds produced by striking objects outside of the body 79
b. The music of flight 8p
TABLE OF CONTENTS xiii
C. Stridulating organs of the rasping type 81
The stridulating organs of the Acridiidae 82
The stridulating organs of the Gryllidae and the Locustidse . 83
Rasping organs of other than orthopterous insects 87
d. The musical organs of a cicada v. . . . 89
e. The spiracular musical organs 91
/. The acute buzzing of flies and bees 91
g. Musical notation of the songs of insects 92
h. Insect choruses 93
CHAPTER III
THE INTERNAL ANATOMY OF INSECTS 94
I. THE HYPODERMAL STRUCTURES 95
a. The internal skeleton 95
Sources of the internal skeleton 95
Chitinized tendons 95
Inyaginations of the body- wall or apodemes 95
The tentorium 96
The posterior arms of the tentorium 96
The anterior arms of the tentorium 97
The dorsal arms of the tentorium • 97
The frontal plate of the tentorium 97
The endothorax 97
The pragmas 97
The lateral apodemes 98
The furcae ^ 98
b. The hypodermal glands 98
The molting-fluid glands 99
Glands connected with setae 99
Venomous setae and spines . 100
Androcoriia 100
The specific scent-glands of females 100
Tenent hairs 100
The osmeteria 101
- Glands opening on the surf ace of the body 102
Wax-glands 102
Froth-glands of spittle insects 102
Stink-glands 102
The cephalic silk-glands 103
The salivary glands 104
2. THE MUSCLES I<>4
3. THE ALIMENTARY CANAL AND ITS APPENDAGES 107
a. The more general features IO7
The principal divisions 108
Imperf orate intestines in the larvae of certain insects 108
TABLE OF CONTENTS
b. The fore-intestine , 109
The layers of the fore-intestine 109
The intima 109
The epithelium 109
The basement membrane 109
The longitudinal muscles 109
The circular muscles 109
The peritoneal membrane 109
The regions of the fore-intestine 109
The pharynx v 109
The oesophagus no
The crop no
The proventriculus no
The cesophageal valve in
c. The mid-intestine in
The layers of the mid-intestine in
The epithelium 112
Theperitrophic membrane 112
d. The hind-intestine 112
The layers of the hind-intestine ..."... 112
The regions of the hind-intestine 113
The Malpighian vessels 113
The Malpighian vessels as silk-glands 113
The caecum 113
The anus 113
4. THE RESPIRATORY SYSTEM 113
a. The open or holopneustic type of respiratory organs 114
1. The spiracles 114
The position of the spiracles 114
The number of spiracles 114
Terms indicating the distribution of the spiracles 115
The structure of spiracles 116
The closing apparatus of the tracheae 116
2. The trochees 1 16
The structure of the tracheae , 117
j. The tracheoles 1 18
4. The air-sacs 1 1 8
5. Modifications of the open type of respiratory organs 1 19
b. The closed or apneustic type of respiratory organs 119
J. The Tracheal gills 119
2. Respiration of parasites 120
j. The blood-gills 120
TABLE OF CONTENTS xv
5. THE CIRCULATORY SYSTEM 121
The general features of the circulatory system 121
The heart I2I
The pulsations of the heart <•.... 122
The aorta 122
The circulation of the blood 122
Accessory circulatory organs 122
6. THE BLOOD 122
7. THE ADIPOSE TISSUE 123
8. THE NERVOUS SYSTEM 123
a. The central nervous system 123
b. The oesophageal sympathetic nervous system 125
c. The ventral sympathetic nervous system 127
d. The peripheral sensory nervous system 128
9. GENERALIZATIONS REGARDING THE SENSE-ORGANS OF INSECTS .. I2Q
A classification of the sense-organs 129
The cuticular part of the sense-organs 130
IO. THE ORGANS OF TOUCH 131
II. THE ORGANS OF TASTE AND SMELL 132
12. THE ORGANS OF SIGHT 134
a. The general features 134
The two types of eyes 134
The distinction between ocelli and compound eyes 134
The absence of compound eyes in most of the Apterygota 135
The absence of compound eyes in larvae 135
b. TheocelH 135
The primary ocelli 135
The adaptive ocelli 136
The structure of a visual cell 137
The structure of a primary ocellus 137
Ocelli of Ephemerida 139
c. The compound eyes 139
The physiology of compound eyes H1
The theory of mosaic vision I41
Day-eyes I42
Night-eyes X43
Eyes with double function I43
Divided eyes X44
The tapetum J44
xvi TABLE OF CONTENTS
13. THE ORGANS OF HEARING 145
a. The general features 145
• The tympana 145
The chordotonal organs 145
The scolopale and the scolopophore ^5
The integumental and the subintegumental scolopophores 146
The structure of a scolopophore 146
The structure of a scolopale 147
The simpler forms of chordotonal organs 147
The chordotonal ligament 147
b. The chordotonal organs of larvae 148
c. The chordotonal organs of the Acridiidae 148
d. The chordotonal organs of the Locustidae and of the Gryllidse 149
The trachea of the leg 1 50
The spaces of the leg . . . .. 151
The supra-tympanal or subgenual organ 151
The intermediate organ 1 52
Siebold's organ or the crista acustica 152
e. Johnston's organ 152
14. SENSE-ORGANS OF UNKNOWN FUNCTIONS
The sense-domes or the olfactory pores 154
I 15. THE REPRODUCTIVE ORGANS
a. The general features 156
Secondary sexual characters 157
b. The reproductive organs of the female 157
The general features of the ovary 1 57
The wall of an ovarian tube 158
The zones of an ovarian tube 158
The contents of an ovarian tube 1 58
The egg-follicles 158
The functions of the follicular epithelium 159
The ligament of the ovary '. 159
Thfr oviduct 159
The egg-calyx 159
The vagina 159
The spermatheca 159
The bursa copulatrix . .- 159
The colleterial glands 160
c. The reproductive organs of the male 160
The general features of the testes 160
The structure of a testicular follicle 161
The spermatophores 162
Other structures. . ..-.., 162
TABLE OF CONTENTS xvii
1 6. THE SUSPENSORIA OF THE VISCERA
The dorsal diaphragm 162
The ventral diaphragm 163
The thread-like suspensoria of the viscera t. . . . 163
17. SUPPLEMENTARY DEFINITIONS
The cenocytes 163
The pericardial cells 164
The phagocytic organs 164
The light-organs 165
CHAPTER IV
THE METAMORPHOSIS OF INSECTS 166
I. THE EXTERNAL CHARACTERISTICS OF THE METAMORPHOSIS OF INSECTS
a. The egg ! 166
The shape of the egg 167
The sculpture of the shell 167
The micrpphyle 167
The number of eggs produced by insects 168
Modes of laying eggs 168
Duration of the egg-state 170
b. The hatching of young insects 171
The hatching spines 171
c. The molting of insects 171
General features of the molting of insects .. . 171
The molting fluid 172
The number of postembryonic molts 172
Stadia r 172
Instars 172
Head^measurements of larvae 173
The reproduction of lost limbs 173
d. Development without metamorphosis 174
The Ametabola 174
e. Gradual metamorphosis I7if
The Paurometabola 176
The term nymph I76
Deviations ft om the usual type 176
The Saltitorial Orthoptera 177
The Cicadas 177
The Coccidae 177
The Aleyrodidae 177
The Aphididse 177
The Thysanoptera 177
f. Incomplete metamorphosis 17%
The Hemimetabola 179
Thetermnaiad 179
Deviations from the usual type I^°
The Odonata I8°
The Ephemerida I8°
xviii TABLE OF CONTENTS
g. Complete metamorphosis 180
The Holometabola 180
The term larva 180
Theadaptive characteristics of larvae 181
The different types of larvae 183
The prepupa 185
Thepupa 186
The chrysalis 186
Active pupae 187
The cremaster 187
The cocoon 188
Modes of escape from the cocoon 188
The puparium 190
Modes of escape from the puparium 190
The different types of pupae 190
The imago 191
h. Hypermetamorphosis 191
*. Viviparous insects 191
Viviparity with parthenogenetic reproduction 192
Viviparity with sexual reproduction *. . . 193
j. Neoteinia 194
2. THE DEVELOPMENT OF APPENDAGES 194
a. The development of wings 195
The development of the wings of nymphs and naiads 195
The development of the wings in insects with a complete meta-
morphosis 195
b. The development of legs 197
The development of the legs of nymphs and naiads 198
The development of the legs in insects with a complete meta-
morphosis 198
c. The development of antennae 199
d. The development of mouth-parts 200
e. The development of the gential appendages 201
3. THE DEVELOPMENT OF THE HEAD IN THE MUSCHXE 2O2
4. THE TRANSFORMATION OF THE INTERNAL ORGANS 204
BIBLIOGRAPHY 206
INDEX 213
THE STRUCTURE AND METAMORPHOSIS
OF INSECTS
CHAPTER I
THE CHARACTERISTICS OF INSECTS AND OF THEIR
NEAR RELATIVES
PHYLUM ARTHROPODA
The Arthropods
IF an insect, a scorpion, a centipede, or a lobster be examined,
the body will be found to be composed of a series of more or less
similar rings or segments joined together; and some of these seg-
ments will be found to bear jointed legs (Fig. i) . All animals possess-
ing these characteristics are classed together
as the Arthropoda, one of the chief divisions or
phyla of the animal kingdom.
A similar segmented form of body is found
among worms; but these are distinguished
from the Arthropoda by the absence of legs.
It should be remembered that many animals
commonly called worms, as the tomato-worm,
the cabbage-worm, and others, are not true
worms, but are the larvae of insects (Fig. 2).
The angle-worm is the most familiar example
of a true worm.
In the case of certain arthropods the dis-
tinctive characteristics of the phylum are
not evident from a cursory examination.
This may be due to a very generalized condi-
tion, as perhaps is true of Peripatus; but in
Fig. i. — An arthropod, most instances it is due to "a secondary modifi-
cation of form, the result of adaptation to
special modes of life. Thus the segmentation of the body may be
Fig. 2. — A larva of an insect.
(1)
AN INTRODUCTION TO ENTOMOLOGY
obscured, as in spiders and in mites (Fig. 3) ; or the jointed append-
ages may be absent, as in the larvae of flies (Fig. 4), of bees, and of
many other insects. In all of these cases, however, a careful study
of the structure of the animal, or
of its complete life-history, or of
other animals that are evidently
closely allied to it removes any
doubt regarding its being an
arthropod.
The phylum Aithropoda is
the largest of the phyla of the
animal kingdom, including many
more known species than all the
other phyla taken together. This
vast assemblage of animals in-
Fig. 3. — A mite, an . ••**•'•
arthropod in which the eludes forms differing widely in
segmentation of the structure, all agreeing, however,
body is obscured. The . ,,
• southern cattle-tick in ^ne possession oi tne essential
Boophilus annulatus. ' characteristics of the Aithropoda.
Several distinct types of arthropods are recognized ;
and those of each type are grouped together as a class.
The number of distinct classes that should be recog-
nized, and the relation of these classes to each other are
matters regarding which there are still differences of
opinion ; we must have much more knowledge than we
now possess before we can speak with any degree of
certainty regarding them.
Fig.4.-Larva
of a fly, Tip-
id a abdomi-
n ali s; an
arthropod in
which
Each of the classes enumerated below is regarded by development
all as a distinct group of animals ; but in some cases there
may be a question whether the group should be given
the rank of a distinct class or not. The order in which the classes
are discussed in this chapte'r is indicated in the following list.
of the legs is
retarded.
LIST OF THE CLASSES OF
I. THE MOST PRIMITIVE ARTHROPODS
Class Onychophora, page 4
II. THE AQUATIC SERIES
Class Cruiticea, page 6
Class Palaeostracha, page 8
III. AN OFFSHOOT OF THE AQUATIC
Class Arachnida, page 9
THE ARTHROPODA
SERIES, SECONDARILY AERIAL
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 3
IV. DEGENERATE ARTHROPODS OF DOUBTFUL POSITION
Class Pychnogonida, page 10
Class Tardigrada, page 1 2
Class Pentastomida, page 14
V. THE PRIMARILY AERIAL SERIES
Class Onychophora (See above)
Class Diplopoda, page 1 5
Class Pauropoda, page 18
Class Chilopoda, page 20
Class Symphyla, page 23
Class Myrientomata, page 24
Class Hexapoda, page 26
TABLE OF CLASSES OF THE ARTHROPODA
A . Worm-like animals, with an unsegmented body, but with many,
un jointed legs. '. ONYCHOPHORA
A A. Body more or less distinctly segmented except in a few degen-
erate forms.
B. With two pairs of antennae and at least five pairs of legs;
respiration aquatic/ CRUSTACEA
BB. Without or apparently without antennae.
C. With well-developed aquatic respiratory organs.
PAL^OSTRACHA
CC. With well-developed aerial respiratory organs or with-
out distinct respiratory organs.
D. With well-developed aerial respiratory organs.
E. Body not resembling that of the Thysanura in form.
ARACHNIDA
EE. Body resembling that of the Thysanura in form
(Family Eosentomidae) MYRIENTOMATA
DD. Without distinct respiratory organs.
E. With distinctly segmented legs.
F. Body resembling that of the Thysanura in form, but
without antennae, and with three pairs of thoracic
legs and three pairs of vestigial abdominal legs
(Family Acerentomidae) MYRIENTOMATA
FF. With four or five pairs of ambulatory legs;
abdomen vestigial PYCHNOGONIDA
EE. Legs not distinctly segmented.
F. With four pars of le^s in. ths adult instar.
TARDIGRADA
4 AN INTRODUCTION TO ENTOMOLOGY
FF. Larva with two pairs of legs, adult without
legs PENTASTOMIDA
BBB. With one pair, and only one, of feeler-like antennae.
Respiration aerial.
C. With more than three pairs of legs, and without wings.
D, With two pairs of legs on some of the body-segments.
DIPLOPODA
D D . With only one pair of legs on each segment of the body .
E. Antennae branched PAUROPODA
EE. Antennae not branched.
F. Head without a Y-shaped epicranial suture.
Tarsi of legs with a single claw each. Opening of
the reproductive organs near the caudal end of
the body CHILOPODA
FF. Head with a Y-shaped epicranial suture, as in
insects. Tarsi of legs with two claws each.
Opening of the reproductive organs near the head.
SYMPHYLA
CC. With only three .pairs of legs, and usually with wings in
the adult state HEXAPODA
CLASS ONYCHOPHORA
The genus Peripatus of authors
The members of this class are air-breathing animals, with a nearly
cylindrical, unsegmented body, which is furnished with many pairs of
unjointed legs. The reproductive organs open near the hind end of the body.
The class Onychophora occupies the position of a connecting link
between the Arthropoda and the phylum Annulata or worms; and is
therefore of the highest interest to students of systematic zoology.
All known members of this class have been included until recently in a
single genus Peripatus; but now the fifty or more known species are
distributed among nearly a dozen genera.
The body
(Fig. 5) is nearly
cylindrical, cat-
erpillar - like in
form, but is un-
segmented ex-
ternally. It is
Fig. 5.—Peripaloides nova-zealandica. furnished with
many pairs cf legs, the number of which varies in different species.
The legs have a ringed appearance, but are not distinctly jointed:
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 5
The head bears a pair of ringed antennae (Fig. 6) ; behind these on
the sides of the head, there is a pair of short appendages termed oral
papillae. The mouth opening is surrounded by a row of lobes which
constitute the lips, and between these in the anterior part of the
mouth-cavity there is an obtuse pro-
jection, which bears a row of qhitinous
points. Within the mouth cavity there
are two pairs of hooked plates, which
have been termed the mandibles, the
two plates of each side being regarded
as a single mandible.
Although the body is unsegmented
externally, internally there are evi-
dences of a metameric arrangement of
parts. The ventral nerve cords, which
at first sight appear to be without
ganglia, are enlarged opposite each
pair of legs, and these enlargments
are regarded as rudimentary ganglia. Fig, 6.-Ventral view of the head
We can, therefore speak of each sec-
tion of a body corresponding to a
pair of appendages as a segment. The
metameric condition is farther indicated by the fact that most of
these segments contain each a pair of nephridia;~^each nephridium
opening at the base of a leg.
The respiratory organs are short tracheae, which are rareiy
branched, and in which the tsenidia appear to be rudimentary.* In
some species, the spiracles are distributed irregularly; in others, they
are in longitudinal rows.
The sexes are distinct. The, reproductive organs open near the
hind end of the body, either between the last or the next to the last
pair of r legs.
The various species are found in damp situations, under the bark
of rotten stumps, under stones or other objects on the ground. They
have been found in Africa, in Australia, in South America, and in the
West Indies.
Their relationship to the Arthropoda is shown by the presence of
paired appendages, one, or perhaps two, pairs of which are modified as
jaws; the presence of tracheae which are found nowhere else except
*It is quite possible that the "short tracheae" described by writers on the
structure of these animals are tracheoles. See the account of the distinguishing
features of tracheae and tracheoles in Chapter III.
and first pair of legs of Peri-
paloides; a, antenna; o, oral
papilla.
6
AN INTRODUCTION TO ENTOMOLOGY
in the Arthropoda; the presence of paired ostia in the wall of the
heart ; and the presence of a vascular body cavity and pericardium.
They resemble the Annulata in having a pair of nephridia in most
of the segments of the body corresponding to the pairs of legs, and in
having cilia in the generative tracts.
An extended monograph of the Onychophora was published by
Bouvier ('o5-'o7).
CLASS CRUSTACEA
Crustaceans
The members of this class are
aquatic arthropods, which breathe
by true gills. They have two
pairs of antenna and at least five
pairs of legs. The position of the
openings of the reproductive organs
varies greatly; but as 'a rule they
are situated far forward.
The most familiar examples
of the Crustacea are the cray-
fishes, the lobsters, the shrimps,
and the crabs. Cray-fishes (Fig.
7) abound in our brooks, and are
often improperly called crabs.
The lobsters, the shrimps, and
Fig< 7>_A cray-fish. the true crabs live in salt
water.
Excepting Limulus, the sole living representative of the class
described next, the Crus-
tacea are distinguished
from all other arthro-
pods by their mode of
respiration, being the
only ones that breathe
by true gills. Many in-
sects live in water and
are furnished with gill-
like organs; but these
are either tracheal gills or
blood-gills, organs which
differ essentially in struc- ^ 8._Mtaute crulltaoellnll! a, Daphnia;
ture from true gills, as Cypridopsis; c, Cyclops.
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES V
described later. The Crustacea also differ from other Arthropoda
in having two pairs of antennae. Rudiments of two pairs of antennae
have been observed in the embryos of many other arthropods ; but
in these cases one or the other of the two pairs of antennae fail
to develop.
The examples of crustaceans named above are the more con-
spicuous members of the class; but many other smaller forms abound
both in the sea and in fresh water. Some of the more minute fresh-
water forms are almost sure to occur in any fresh- water aquarium.
In Figure 8 are repre-
sented three of these
greatly enlarged. The
minute crustaceans form
an important element in
the food of fishes.
Some crustaceans live
in damp places on land,
and are often found by
collectors of insects;
those most often ob-
served are the sow-bugs
(Oniscoida), which fre-
quently occur about
Fig. 9.— A sow-bug, Cylisticus convexus (From water-Soaked wood.
Richardson after Sars). Figure 9 represents one
of these.
As there are several, most excellent text books devoted to the
Crustacea, it is unnecessary to discuss farther this class in this place.
AN INTRODUCTION TO ENTOMOLOGY
CLASS PAL^OSTRACHA
The King-crabs or Horseshoe-crabs
The members of this class
are aquatic arthropods, which
resemble the Crustacea in that
they breathe by true gills, but
in other respects are closely
allied to the Arachnida. They
are apparently without
antenna, the appendages hom-
ologous to antenna being not
feeler-like. The reproductive
organs open near the base of
the abdomen.
The class Palseostracha
is composed almost entirely
of extinct forms, there being
living representatives of only
a single order, the Xiphosura,
and this order is nearly
extinct; for of it there re-
mains only the genus
ULmulus, represented by
only five known species.
The members of this
genus are known as king-
crabs or horseshoe-crabs ;
the former name is sug-
gested by the great size of some of the species; the latter, by
shape of the cephalothorax (Fig. 10).
The king-crabs are marine; they are found on our Atlantic Coast
from Maine to Florida, in the West Indies, and on the eastern shores
of Asia. They are found in from two to six fathoms of water on
sandy and muddy shores; they burrow a short distance in the sand
or mud and feed chiefly on worms. The single species of our coast is
Llmulus polyphemus.
Fig. 10. — A horseshoe crab, Limidus (After
Packard).
the
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 9
CLASS ARACHNIDA
.Scorpions, Harvestmen, Spiders, Mites, and others
The members of this class are air-breathing arthropods, in which the
head and thorax are usually grown together, forming a cephalothorax,
which have four pairs of legs, and which apparently have no antenna.
The reproductive organs open near the base of the abdomen.
Fig. i i b
Fig. ii. — Arachnids: a, a scorpion; b, a harvestman.
c, a spider; d, an itch-mite, from below and from
above.
The Arachnida abound wherever insects occur, and are often
mistaken for insects. But they can be easily distinguished by the
characters given above, even in those cases where an exception occurs
to some one of them. The more important of the exceptions are the
following : in one order, the Solpugida, the head is distinct from the
10 AN INTRODUCTION TO ENTOMOLOGY
thorax; as a rule the young of mites have only six legs, but a fourth
pair is added during growth ; and in the gall-mites there are only four
legs.
The Arachnida are air-breathing; but it is believed that they
have been evolved from aquatic progenitors. Two forms of respira-
tory organs exist in this class : first, book-lungs ; and second, tubular
tracheae. Some members of it possess only one of these types ; but
the greater number of spiders possess both.
A striking characteristic of the Arachnida, which, however, is also
possessed by the Palseostracha, is the absence of true jaws. In other
arthropods one or more pairs of appendages are jaw-like in form and
are used exclusively as jaws ; but in the Arachnida the prey is crushed
either by the modified antennae alone or by these organs and other
more or less leg-like appendages. The arachnids suck the blood of
their victims by means of a sucking stomach; they crush their prey,
but do not masticate it so as to swallow the solid parts.
In the Arachnida there exist only simple eyes.
The reproductive organs open near the base of the abdomen on the
ventral side. In this respect the Arachnida resemble Limulus, the
millipedes, and the Crustacea, and differ from the centipedes and
insects.
Among the more familiar representatives of this class are the
scorpions (Fig. u, a), the harvestmen (Fig. n, &), the spiders (Fig.
1 1 , c] , and the mites (Fig. 1 1 , d) .
As the writer has devoted a separate volume (Comstock, '12) to
the Arachnida, it will not be discussed farther in this place.
CLASS PYCNOGONIDA
The Pycnogonids
The members of this class are marine arachnid-like arthropods, in
which the cephalothcrax bears typically seven pairs of jointed appen-
dages, but in a few forms there are eight pairs, and in some the anterior
two or three pairs are absent; and in which the abdomen is reduced to a
legless, unsegmented condition. They possess a circulatory system, but
no evident respiratory organs. The reproductive organs open through
the second segment of the legs; the number of legs bearing these opening
varies from one to five pairs.
The Pycnogonida or pycnogonids are marine animals, which bear
a superficial resemblance to spiders (Fig. 12). Some of them are
found under stones, near the low water line, on sea shores; but they
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 11
are more abundant in deep water. Some are found attached to sea-
anemones, upon which they probably prey; others are found climbing
Fig. 12. — A pycnogonid, Nymphon hispidum: r, chelophore; p,
palpus; o, ovigerous legs; /, /, /, /, ambulator/ legs; ab, abdo-
men (After Hoeck).
over sea-weeds and Hydroids; and sometimes they are dredged in
great numbers from deep water.
They possess a suctorial proboscis. In none of the appendages are
the basal segments modified into organs for crushing the prey.
The cephalothorax comprises almost the entire body ; the abdomen
being reduced to a mere vestige, without appendages, and with no
external indication of segmentation. But the presence of two pairs
of abdominal ganglia indicates that originally the abdomen consisted
of more than one segment.
There are typically seven pairs of appendages; but a few forms
possess eight pairs ; and in some the first two or three pairs are absent.
The appendages, when all are present, consist of a pair of chelophores,
each of which when well-developed consists of one or two basal seg-
ments and a chelate "hand;" the palpi, which are supposed to be
tactile, and which have from five to ten joints when well-developed;
the ovigerous legs, which are so-called because in the males they are
used for holding the mass of eggs beneath the body; and the ambula-
tory legs, of which there are usually four pairs, but a few forms possess
a fifth pair. The ambulatory legs consist each of eight segments and
a terminal claw.
The only organs of special sense that have been found in these
animals are the eyes. These are absent or at least very poorly
12 AN INTRODUCTION TO ENTOMOLOGY
developed in some forms, especially those that are found in very deep
water, i. e. below four or five hundred fathoms. When well -developed
they are simple, and consist of two pairs, situated on a tubercle, on
the head or the first compound segment of the body, the segment that
bears the first four pairs of appendages.
The reproductive organs open in the second segment of the legs.
In some these openings occur only in the last pair of legs ; in others, in
all of the ambulatory legs.
Very little is known regarding the habits of these animals. The
most interesting features that have been observed is perhaps the fact
that the males carry the eggs in a mass, held beneath the body by the.
third pair of appendages, the ovigerous legs, and also carry the young
for a time.
As to the- systematic position of the class Pycnogonida, very little
can be said. These animals are doubtless arthropods, and they are
commonly placed near the Arachnida.
CLASS TARDIGRADA
The Tardigrades cr Bear Animalcules
The members of this class are very minute segmented animals, with
four pairs of legs, but without antennas or mouth-appendages, and without
special circulatory or respiratory organs; the reproductive organs open
into the intestine.
The Tardigrada or tardigrades are microscopic animals, measuring
from one seventy-fifth to one twenty-fifth of an inch in length. They
are somewhat mite-like in appearance; but are very different from
mites in structure (Fig. 13 and 14).
The head bears neither antennae nor mouth-appendages. The
four pairs of legs are short, un jointed, and are distributed along the
entire length of the body, the
fourth pair being at the cau-
dal end. Each leg is termin-
ated by claws, which differ in
number and form in different
genera.
The more striking features
of the internal structure of
Fig. ,3.-A tardigrade (After Doy^re).
special circulatory and respiratory organs; the presence of a pair of
chitinous teeth, either in the oral cavity or a short distance back of
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 13
it; the presence of Malpighian tubules; the unpaired condition of
the reproductive organs of both sexes; and the fact that these organs
open into the intestine. The central nervous system consists of a
brain, a subcesophageal ganglion, and a ventral chain of four ganglia,
connected by widely separated connectives.
The tardigrades are very abundant, and are very widely dis-
tributed. Some live in fresh water, a few are marine, but most of
them live in damp places, and especially on the roots of moss, growing
in gutters, on roofs or trees, or in ditches.
But although they are common, their
minute size and retiring habits result in1
their being rarely seen except by those
who are seeking them.
Many of them have the power of
withstanding desiccation for a long period.
This has been demonstrated artificially by
placing them on a microscopic slide and
allowing the mositure to evaporate
slowly. The body shrinks, its skin
becomes wrinkled, and finally it assumes
the appearance of a grain of sand in
which no parts can be distinguished. In
this state they can remain, it is said, for
years; after which, if water be added,
the body swells, assumes its normal form,
and after a time, the creatures resume
their activities.
*> -. .... Fig. 14. — A tardigrade (After
Regarding the systematic position of Doyere).
this class of animals nothing definite can
be stated beyond the fact that they are doubtless arthropods. Their
relationship to the other classes of arthropods has been masked by
degenerative modifications. They are placed here near the end of
the series of classes of arthropods, merely as a matter of convenience,
in what may be termed, an appendix to the arthropod series, which
includes animals of doubtful relationships.
14
AN INTRODUCTION TO ENTOMOLOGY
CLASS PENTASTOMIDA
The Pentastomids or Linguatulids
The members of this class are degenerate, worm-like, parasitic
arthropods, which in the adult state have no appendages, except two pairs
of hooks near the mouth; the larvcz have two pairs of short legs. These
animals possess neither circulatory nor respiratory organs. The
reproductive organs of the male open a short distance behind the mouth;
those of the female near the caudal end of the body.
The Pentastomida or pentastomids are worm-like creatures, whose
form has been greatly modified by their parasitic life. The adults
bear little resemblance to any other arthropods. Representatives of
three genera are known. These are Lingudtula in which the body is
fluke-like in form (Fig. 15) and superficially annula ted; Porocephalus,
in which the body is cylindrical (Fig. 16) and ringed; and Reighardia,
which is devoid of annulations, and with poorly developed hooks and
a mouth-armature.
The arthropodan nature of these animals is
indicated by the form of the larvae, which although
greatly degenerate, are less so than the adults,
having two pairs of legs (Fig. 17).
-/ ,2
-oe
Fig. ^ 1 5. — A pentasto-
mict, I.inguatula
tcenioides, f :male at
the time of copula-
tion: h, hooks; oe,
oesophagus, rs, re-
ceplncula . seminis,
one of which is still
empty; i, intestine;
OT, ovary; va, vagina
(From Lang after
Leuckart).
Fig. 1 6. — A pentastomid,
Porocephalus annulalus;
a, ventral view of head,
greatly enlarged; b,
ventral view of animal,
slightly enlarged (After
Shipley).
/
Fig. 17 — A pentastomid, larva of
Porocephalus proboscideus, seen
from below, highly magnified: I ,
boring anterior end; 2, first pair
of chitinous processes seen be-
tween the forks of the second pair ;
3, ventral nerve ganglion; 4, 'ali-
mentary canal; 5, mouth; 6 and
7, gland cells (From Shipley after
Stiles).
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 15
Like many of the parasitic worms, these animals, in some cases at
least, pass their larval life in one host, and complete their development
in another of a different species ; some larvae being found in the bodies
of herbivorous animals and the adults in predacious animals that feed
on these herbivorous hosts. •
The systematic position of the pentastomids is very uncertain.
They have been considered by some writers to be allied to the mites.
But it seems better to merely place them in this appendix to the
arthropod series until more is known of their relationships.
CLASS DIPLOPODA
The Millipedes or -Diplopods
The members of this class are air-breathing arthropods in which the
head is distinct, and the remaining segments of the body form a continuous
region. The greater number of the body-segments are so grouped that
each apparent segment bears two pairs of legs. The antenna are short
and very similar to the legs. The openings of the reproductive organs are
paired, and situated behind the second pair of legs.
Fig. 1 8.— A millipede, Spirobolus marginatus.
The Diplopoda and the three following classes were formerly
grouped together as a single class, the Myriapoda. But this grouping
has been abandoned, because it has been found that the Chilopoda are
more closely allied to the insects than they are to the Diplopoda; and
the Pauropoda and Symphyla are both very distinct from the Diplo-
poda on the one hand and the Chilopoda on the other. Owing to the
very general and long continued use of the term Myriapoda, the
student who. wishes to look up the literature on these four classes
should consult the references under this older name.
The most distinctive feature of the millipedes is that which sug-
gested the name Diplopoda for the class, the fact that throughout the
greater part of the length of the body there appears to be two pairs of
legs borne by each segment (Fig 18).
This apparent doubling of the appendages is due to a grouping o.
the segments in pairs and either a consolidation of the two terga of
16
AN INTRODUCTION TO ENTOMOLOGY
each pair or the non-development of one of them; which of these
alternatives is the case has not been definitely determined.
It is clear, however, that there has been a grouping of the seg-
ments in pairs in the region where the appendages are doubled, for
corresponding with each tergum there are two sterna and two pairs of
spiracles. .
A few of the anterior body segments, usually three or four in
number, and sometimes one or two of the caudal segments remain
single. Frequently one of the anterior single segments is legless, but
the particular segment that lacks legs differs in the different families.
The head, which is as distinct as is the head of insects, bears the
antennae, the eyes, and the mouth-parts. The antennae are short,
and usually consist each of seven segments. The eyes are usually
represented by a group of ocelli on each side of the
head; but the ocelli vary greatly in number, and are
sometimes absent. The mouth-parts consist of an P
upper lip or labrum; a pair of mandibles; and a pair
of jaws, which are united at the base, forming a large
plate, which is known as the gnathochilarium. In
the genus Polyocenus there is a pair of jaws between
the mandibles and the gnathochilarium, which have
been named the maxillula.
The labrum is merely the anterior part of the FJ£- I9-7~A mandi-
11 f 4-t, t. j j • • • ble of Julus; c,
upper wall of the head and, as in insects, is not an cardo; d,d, teeth;
appendage. The mandibles, in the forms in which
they are best developed, are fitted for biting, and
consist of several parts (Fig. 19) ; but in some forms
they are vestigial. The gnathochilarium (Fig. 20) is
complicated in structure, the details of which vary greatly in different
genera.
m, muscle; ma,
mala; p, pecti-
nate plate; s,
stipes (After
Latzel).
- st
Pm-
Fig. 20. — The gnathochilarium or second jaws of three diplopods; A, Spirostrep-
ins; B, Julus; C, Glomeris: r, cardo; h, hypostoma; Ig, linguae;
pm, promentum; st, stipes (After Silvestri).
m, men turn ;
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 17
mxl
In one subdivision of the class Diplopoda, which is represented
by the genus Polyxenus and a few others, the mandibles are one-
jointed; and be-
**xl tween the mandi-
bles and the
gnathochilarium
there is a pair of
one -jointed jaws,
which have not
been found in
other diplopods ;
these are the maxil-
lulse(Fig. 21). The
correspondence of
the parts of the
gnathochilarium of
Polyxenus and its
allies with the parts
of the gnathocil-
lariurrrof other di-
plopods has not
been satisfactorily
determined.
Most of our more common millipedes possess stink- glands, which
open by pores on a greater or less number of the body segments.
These glands are the only means of defence possessed by millipedes,
except the hard cuticula protecting the body.
The millipedes as a rule are harmless, living in damp places and
feeding on decaying vegetable matter; but there are a few species
that occasionally feed upon growing plants.
For a more detailed account of the Diplopoda see Pocock ('n).
la
Fig. 21. — The second pair of jaws, maxillulae, and the
third pair of jaws, maxillae or gnathochilarium, of
Polyxenus; the parts of the maxillae or gnathochila-
rium are stippled and some are omitted on the right
side of the figure: mb, basal membrane of the labium ;
la, "labium" of Carpenter, perhaps the mentum and
promentum of the gnathochilarium; mx, basal seg-
ment of the maxilla, perhaps the stipes of the
gnathochilarium; mx. lo, lobe of the maxilla; mx. p,
maxillary palpus; h, tongue or hypopharynx; mxl,
maxillula; fl. flagellate process (After Carpenter).
18
AN INTRODUCTION TO ENTOMOLOGY
CLASS PAUROPODA
The Pauropods
The members of this class are small arthropods in which the head is
distinct, and the segments of the body form a single continuous region.
Most of the body-segments bear each a single pair of legs. Although
most of the terga of the body-segments are usually fused in couples, the
legs are not grouped in double pairs as in the Diplopoda, The antenna
are branched. The reproductive organs open in the third segment back
of the head.
The Pauropoda or pauropods are minute creatures, the described
species measuring only about one twenty-fifth inch in length, more
or less. They resemble centipedes in the elongated form of the body
and in the fact that the legs are not grouped in double pairs as in the
Diplopoda, although the terga of the body-region are usually fused in
couples. These characteris-
tics are well-shown by the
dorsal and ventral views of
Pauropus (Fig. 22 and 23).
Although the pauropods
resemble the chilopods in
the distribution of their legs,
they differ widely in the
position of the openings of
the reproductive organs.
These open in the third seg-
ment back of the head ; that
of the female is single, those
of the male are double.
The head is distinct from
the body-region. It bears
one pair of antennae and two
pairs of jaws; the eyes are
absent but there is an eye-
Fig. 22.— A pauropod, like spot on each side of the Fig. 23.— Pauropus
Pauropus huxleyi, dor- head (pie 24.} The first huxleyi, ventral as-
sal aspect (After Ken- pect (After Lub-
yon). pair of jaws are large, one- bock).
jointed mandibles; the
second pair are short pear-shaped organs. Between these two pairs
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 19
Fig. 24. — Eurypauropus spino*
sus; face showing the base of
the antennas, the mandibles,
and the eye-like spots (After
Kenyon).
of jaws, there is a horny framework forming a kind of lower lip to the
mouth (Fig. 25). The homologies of the mouth-parts with those of
the allied classes of arthropods have not
been determined.
The body-region consists of twelve
segments. This is most clearly seen by
an examination of the ventral aspect of
the body. When the body is viewed from
above the number of segments appears to
be less, owing to the fact that the terga of
the first ten segments are fused in
couples. Nine of the body-segments bear
well-developed legs. The appendages of
the first segment are vestigial, and the
last two segments bear no appendages.
The most distinctive feature of mem-
bers of this class is the form of the
antennas, which differ from those of all
other arthropods in structure. Each
antenna (Fig. 26) consists of four short
oasal segments and a pair of one- jointed
branches borne by the fourth segment.
One of these branches bears a long, many-
ringed filament with a rounded apical
knob; and the other branch bears two
such filaments with a globular or pear-
shaped body between them. This is prob-
ably an organ of special sense.
The pauropods live under leaves and
stones and in other damp situations.
Representatives of two quite distinct families are found in this
country and in various other parts of the world. In addition to these
a third family, the
Brachypauropodida,
is found in Europe.
In this family the
pairs of terga consist
each of two distinct
plates. Our two
families are the fol-
Fig. 26. — Antenna of Eurypauropus sp^nosus
(After Kenyon). lowing:
Fig. 25. — Mouth-parts of Eury-
pauropus ornatus; md, man-
dible; mx, second jaws; /,
lower lip (After Latzel).
20
AN INTRODUCTION TO ENTOMOLOGY
Family Pauropodidce. — In members of this family the head is
not covered by the first tergal plate and the anal segment is not
covered by the sixth tergal plate.
The best known representatives of this
family belong to the genus Pauropus (Fig.
22). This genus is widely distributed, represen-
tatives having been found in Europe and in both-
North and South America. They are active,
measure about one twenty-fifth inch in length,
and are white.
Family Eurypauropida. — The members of
this family are characterized by the wide form
of the body, which bears some resemblance to
that of a sow-bug. The head is concealed by the
first tergum of the body-region; and the anal
segment, by the penultimate tergum. Our most
familiar representative is Eurypauropus spinosus
(Fig. 27). This, unlike Pauropus, is slow in its
movements.
Fig. 27. — Eurypauro-
sspinos
myon).
PUS spinosus (After
Kei
CLASS CHILOPODA
The Centipedes or Chilopods
The members of this class are air-breathing arthropods in which the
head is distinct, and the remaining segments of the body form a continuous
region. The numerous pairs of legs are not grouped in double pairs, as
in the Diplopoda. The antenna are long and many-jointed. The
appendages of the first body-segment are jaw-like and function as organs
of offense, ike poison-jaws. The opening of the reproductive organs is
in the next to the last segment of the body.
The animals constituting the class Chilopoda or chilopods are
commonly known as centipedes. They vary to a considerable degree
in the form of the body, but in all except perhaps the sub-class
Notostigma the body-segments are distinct, not grouped in couples
as in the diplopods (Fig. 28). They are sharply distinguished from
the three preceding classes in the possession of poison- jaws and in
having the opening of the reproductive organs at the caudal end of
the body
The antennae are large, flexible, and consist of fourteen or more
segments. There are four pairs of jaws including the jaw-like
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 21
appendages of the first body-segment. These are the mandibles
(Fig. 29, A), which are stout and consist each of two segments; the
maxillce (Fig. 29, B, a), which are foliaceous,
and usually regarded as biramous; the ^ second
maxilla or palpognaths, which are leg-like in
form, consisting of five or six segments, and
usually have the coxae united on the middle
line of the body (Fig. 29, B, b), and the poison-
claws or toxicognaths, which are the appendages
of the first body-segment (Fig. 29, C).
The poison-claws consist each of six seg-
ments, of which the basal one, or coxa is usually
fused with its fellow, the two forming a large
coxal plate, and the distal one is a strong pierc-
ing fang in which there is the opening of the
duct leading from a poison gland, which is in
the appendage.
The legs consist typically of six segments,
of which the last, the tarsus, is armed with a
single terminal claw. The last pair of legs are
directed backwards, and are often greatly
modified in form.
The class Chilopoda includes two quite distinct groups of animals
which are regarded by Pocock ('n) as sub-classes, the Pleuro-
stigma and theNoto-
stigma. The names t*&M ffi&.JF\ C
of the sub-classes
refer to the position , A
of the spiracles.
SUB-CLASS
PLEUROSTIGMA
The typical Centipedes
In the typical cen-
tipedes, the sub-class
Pleurostigma, the
spiracles are paired
and are situated in the sides of the segments that bear them. Each
leg-bearing segment contains a distinct tergum and sternum, the
number of sterna never exceeding that of the terga. The eyes
Fig. 28. — A centipede
Bothropolys multi-
dentatus.
Fig. 29. — Mouth-parts of a centipede, Geophilus flam-
dus. A, right mandible, greatly enlarged. B, the
two pairs of maxillae, less enlarged; a, the united
coxse of the maxillae; 6, the united coxae of the
second maxillae or palpognaths. C, the poison claws
or toxicognaths (After Latzel)
22
AN INTRODUCTION TO ENTOMOLOGY
when present are simple ocelli; but there may be a group of ocelli
on each side of the head. Figure 28 represents a typical centipede.
SUB-CLASS NOTOSTIGMA
Scutigera and its Allies
In the genus Scutigera and its allies,
which constitute the sub-class Notostigma,
there is a very distinctive type of respiratory
organs. There is a single spiracle in each
of the spiracle-bearing segments, which are
seven in number. These spiracles open in
the middle line of the back, each in the hind
margin of one of the seven prominent terga
of the body-region. Each spiracle leads into
a short sac from which the tracheal tubes
extend into the pericardial blood-sinus.
There are fifteen leg-bearing segments in
the body region; but the terga of these
segments are reduced to seven by fusion and
suppression.
The eyes differ from those of all other
members of the old group Myriapoda in
being compound, the ommatidia resembling
in structure the ommatidia of the compound
eyes of insects.
The following species is the most familiar
representative of the Notostigma.
The house centipede, Scutigera forceps. —
This centipede attracts attention on account
of the great length of its appendages
(Fig. 30), and the fact that it is often seen,
in the regions where it is common, running on the walls of rooms in
dwelling houses, where it hunts for flies and other insects. It prefers
damp situations; in houses it is most frequently found in cellars,
bathrooms, and closets. Sometimes it becomes very abundant in
conservatories,- living among the stored pots and about the heating
pipes. It is much more common in the South than in the North.
\
Fig. 30. — Scutigera forceps.
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 23
The body of the adult measures an inch or a little more in
length. It is difficult to obtain perfect specimens, as they shed
their legs when seized.
CLASS SYMPHYLA
The Symphylids
The members of this class are small
arthropods in which the head is distinct, and
the segments of the body form a single con-
tinuous region. Most of the body-segments
bear a single pair of legs. The antenna are
very long and many-jointed. The head bears
a Y-shaped epicranial suture, as in insects.
The opening of the reproductive organs is in
the third segment behind the head.
The class S^mphyla includes a small
number of many-legged arthropods which
exhibit striking affinities with insects, and
especially with the Thysanura. The body
is centipede-like in form (Fig 31). The
head is distinct, and is not bent down
as it is in the diplopods and pauro-
pods ; it is shaped as in Thysanura and
bears a Y-shaped epicranial suture. The
body-region bears fifteen terga, which are
distinct, Dot grouped in couples as in the
two preceding classes; and there are
eleven or twelve pairs of legs.
The antennae are long and vary greatly
in the number of the segments. There are
no eyes. Four pairs of jaws are present ;
these are the mandibles, the maxillulae,
the maxillae, and the second maxillae or
labium.
The mandibles (Fig. 3 2 , md) are two-
jointed; the maxillula (Fig. 33, m) are
small, not segmented, and are attached to a median lobe or
hypopharynx (Fig. 33, k); they are hidden when the mouth-parts
are viewed from below as represented in Figure 3 2 ; the maxilla (Fig.
F g. 31 . — S'.olopendrella
i( After Latzel).
''ig. 32. — Mouth-parts of
Scolopendrella seen from
below: md, mandible; mx,
maxillae; s, stipes; p, pal-
Eus; /, second maxillae or
ibium. The mandible on
the right side of the figure
is omitted (After Hansen).
24
AN INTRODUCTION TO ENTOMOLOGY
and maxillulae (m)
of Scolopendrella
(After Hansen).
32, mx) resemble in a striking degree the maxillae of insects, consisting
of a long stipes, (5), which bears a minute palpus, (p), and an outer
and inner lobe; the second maxilla or labium (Fig.
3 2 , /) also resembles the corresponding part of the
more generalized insects, being composed of a pair
of united gnathites.
The legs of the first pair are reduced in size and
in the number of their segments. The other legs
consist each of five segments; the last segment
bears a pair of claws. Excepting the first two
pairs of legs, each leg bears on its proximal seg-
ment a slender cylindrical process, the parapodium (Fig. 34, p).
These parapodia appear to correspond with the styli of the
Thysanura.
At the caudal end of the body there is a pair of
appendages, which are believed to be homologous
P '~* _**?$>$ with the cerci of insects (Fig. 3 5 , c) .
A striking peculiarity of the symphylids is that
Fig. 34- — A leg of ^ey pOSSess only a single pair of tracheal tubes,
Scolopendrella; , . , . r • i •*. u ^ • *t
£, parapodium. which open by a pair of spiracles, situated in the
head beneath the insertion of the antennae.
The members of this class are of small size, the
larger ones measuring about one-fourth inch in
length. They live in earth under stones and decay-
ing wood, and in other damp situations. Imma-
ture individuals possess fewer body-segments
and legs than do adults.
Less than thirty species have been described ;
but doubtless many more remain to be discovered.
The known species are classed in two genera : pig. 35.__The caudal
Scolopendre.la and Scutigerella. In the former the e0nd °* the bcdy °,f
, P jn 111 Scolopendrella; I,
posterior angles of the terga are produced and ieg; c, cercus (After
angular; while in the latter they are rounded. Latzel).
A monograph of the Symphyla has been published by Hansen ('03) .
CLASS MYRIENTOMATA
The Myrientoniatids
The members of this class are small arthropods in which the body is
elongate, as in the Thysanura, fusiform, pointed behind, and depressed;
it may be greatly extended and retracted. The antenna and cerci are
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 25
absent. The oral appratus is suctoral, and consists of three pairs of
gnathites. There are three pairs of thoracic legs, and three pairs of
vestigial abdominal legs. The abdomen is composed of eleven segments
and a telson. The opening of the reproductive organs is unpaired, and
near the hind end of the body. The head bears a pair of organs, termed
pseudoculi, the nature of which has not been definitely determined.
The known members of this
class are very small arthropods,
the body measuring from one-
fiftieth to three-fiftieths of an
inch in length. The form of the
body is shown by Figure 36.
These exceedingly interesting
creatures are found in damp
situations, as in the humus of
gardens; as yet very little is
known of their geographical dis-
tribution, as almost all of the
studies of them have been made
by two Italian naturalists.
The first discovered species
was described in 1907 by Pro-
fessor F. Silvestri of Portici,
regarded it as the type of a
distinct order of insects, for which
he proposed the name Protura
Later Professor Antonio Berlese
of Florence described several
additional species, an»d published
an extended monograph of the
order (Berlese '09 6).
Professor Berlese concluded
that these arthropods are more
closely allied to the "Myriapoda"
and especially to the Pauropoda
than they are to the insects, and changed the name of the order, in
an arbitrary manner, to Myrientomata.
It seems clear to me that in either case whether the Border is
classed among the insects or assigned to some other position it should
be known by the name first given to it, that is, the Protura
Fig. 36. — Acerentomon doderoi: A, dor-
sal aspect; B, ventral aspect; 1, 1, 1,
vestigial abdominal legs (After
Berlese).
26 AN INTRODUCTION TO ENTOMOLOGY
In the present state of our knowledge of the affinities of the classes
of arthropods, it seems best to regard the Protura as representing a
separate class, of rank equal to that of the Pauropoda, Symphyla, etc. ;
and for this class I have adopted the name proposed for the group by
Berlese, that is the Myrientomata.
The class Myrientomata includes a single order.
ORDER PROTURA
As this is the only order of the class Myrientomata now known it
must be distinguished by the characteristics of the class given above.
Two families have been established : the Acerentomidae, charac-
terized by the absence of spiracles and tracheae ; and the Eosentomidae
the members of which possess two pairs of thoracic spiracles and
simple tracheae.
That the Protura are widely distributed is evident from the fact
that in addition to those found in Italy representatives of the order
have been found in peat in Hampshire, England, and others have been
taken near New York City.
CLASS HEXAPODA
The Insects
The members of this class are air-breathing arthropods, with distinct
head, thorax, and abdomen. They have one pair of antennas, three pairs
of legs, and usually one or two pairs of wings in the adult state. The
opening of the reproductive organs is near the caudal end of the body.
We have now reached in our hasty review of the classes of arthro-
pods the class of animals to which this book is chiefly devoted, the
Hexapoda,* or Insects, the study of which is termed entomology.
Insects are essentiaily terrestrial ; and in the struggle for existence
they are the most successful of all terrestrial animals, outnumbering
both in species and individuals all others together. On the land they
abound under the greatest variety of conditions, special forms having
been evolved fitted to live in each of the various situations where
other animals and plants can live; but insects are not restricted to
dry land, for many aquatic forms have been developed.
The aquatic insects are almost entirely restricted to small bodies
of fresh water, as streams and ponds, where they exist in great num-
bers. Larger bodies of fresh water and the seas are nearly destitute
of them except at the shores.
*Hexapoda: hex (?£), six; pous (irotfs), afoot.
CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 27
As might be inferred from a consideration of the immense number
of insects, the part they play in the economy of nature is an exceed-
ingly important one. Whether this part is to be considered a bene-
ficial or an injurious one when judged from the human standpoint
would be an exceedingly difficult question to determine. For if
insects were to be removed from the earth the whole face of nature
would be changed.
While the removal of insects from the earth would eliminate many
pests that prey on vegetation, would relieve many animals of annoying
parasites, and would remove some of th,e most 'terrible -diseases to
which our race is subject, it would result in the destruction of many
groups of animals that depend, either
directly or indirectly, upon insects for food,
and the destruction of many flowering
plants that depend upon insects for the
fertilization of their blossoms. Truly this
world would speedily become a very differ-
ent one if insects were exterminated.
It may seem idle to consider what
would be the result of the total destruction
of insects; but it is not wholly so. A care-
ful study of this question will do much
to open our eyes to an appreciation of the
wonderful "web of life" of which we are a
part.
Most adult insects can be readily dis-
tinguished from other arthropods .by the
form of the body, the segments being grouped into three distinct
regions, head, thorax, and abdomen (Fig. 37), by the possession of
only three pairs of legs, and in most cases by the presence of wings.
The head bears a single pair of
antennae, the organs of sight, and the
mouth-parts. To the thorax, are
articulated the organs of locomotion,
the legs and the wings when they are
present. The abdomen is usually
without organs of locomotion but
frequently bears other appendages at
the caudal end.
These characteristics are also possessed by the immature forms
of several of the orders of insects; although with these the wings are
Fig- 37- — Wasp with head,
thorax, and abdomen
separated.
Fig. 38. — Nymph of the red-
legged locust.
28
AN INTRODUCTION TO ENTOMOLOGY
rudimentary (Fig. 38). But in other orders of insects the immature
forms have been greatly modified to adapt them to special modes of
life, with the result that they depart widely from the insect type. For
example, the larvas of bees, wasps, flies, ,and many beetles are legless
and more or less worm-like in form (Fig. 4) ; while the larvas of butter-
flies and moths possess abdominal as well as thoracic legs (Fig. 39).
Fig. 39. — A larva of a handmaid moth, Datana.
Although the presence of wings in the adult state is characteristic
of most insects, .there are two orders of insects, the Thysanura and
the Collembola, in which wings are absent. These orders represent
a branch of the insect series that separated from the main stem before
the evolution of wings took place; their wing-
less condition is, therefore., a primitive one.
There are also certain other insects, as the lice
and bird-lice, that are wingless. But it is
believed that these have descended from
winged insects, and have been degraded by
their parasitic life; in these cases the wingless
condition is an acquired one. Beside these
there are many species belonging to orders in
which most of the species are winged that
have acquired a wingless condition in one or
both sexes. Familiar examples of these are the
females of the Coccidae (Fig. 40), and the
females of the canker-worm moths. In fact,
wingless forms occur in most of the orders of
winged insects.
As the structure and transformations of insects are described in
detail in the following chapters, it is unnecessary to dwell farther on
the characteristics of the Hexapoda in this place.
Fig. 40. — A mealy-bug,
Dactylopius.
CHAPTER II.
THE EXTERNAL ANATOMY OF INSECTS
I. THE STRUCTURE OF THE BODY-WALL
fcj a' THE THREE LAYERS OP THE BODY-WALL
THREE, more or less distinct, layers can be recognized in the body-
wall of an insect: first, the outer, protecting layer, the cuticula'
second, an intermediate, cellular layer, the hypodermis; and third, an
nner, delicate, membranous layer, the basement membrane. These
layers can be distinguished
only by a study of carefully
prepared, microscopic sec-
tions of the body-wall.
Figure 41 represents the ap-
pearance of such a section.
As the outer and inner layers
are derived from the hypo-
dermis, this layer will be
bm ^ described first.
The hypodermis. — The ac-
tive living part of the body-
wall consists of a layer of cells,
which is termed the hypo-
dermis (Fig. 41, h).
The hypodermis is a portion of one of the germ-layers, the ectoderm. In
other words, that portion of the ectoderm which in the course of the development
of the insect comes to form a part of the body- wall is termed the hypodermis;
while to invaginated portions of the ectoderm other terms are applied, as the
epithelial layer of the tracheae, the epithelial layer of the fore-intestine, and the
epithelial layer of the hind-intestine.
The cells of which the hypodermis is composed vary in shape; but
they are usually columnar in form, constituting what is known to
histologists as a columnar epithelium. Sometimes the cells are so
flattened that they form a simple pavement epithelium. I know of
no case in which the hypodermis consists of more than a single layer
of cells; although in wing-buds and buds of other appendages, where
the cells are fusiform, and are much crowded, it appears to be irregu-
(29)
Fig. 41.— A section of the body-wall of
an insect: c, cuticula; h, hypodermis;
bm, basement membrane; e, epidermis,
d, dermis; tr, trichogen; s, seta.
30 AN INTRODUCTION TO ENTOMOLOGY
larly stratified. This is due to the fact that the nuclei of different
cells are in different levels.
The trickogens. — Certain of the hypodermal cells become highly
specialized and produce hollow, hair-like organs, the setae, with which
they remain connected through pores in the cuticula. Such a hair-
forming cell is termed a trichogen (Fig. 41, tr); and the pore in the
cuticula is termed a trfahopore.
The cuticula. — Outside of the hypodermis there is a firm layer,
which protects the body and serves as a support for the internal
organs; this is the cuttcula (Fig. 41, c). The cuticula is produced by
the hypodermis ; the method of its production is discussed in a later
chapter where the molting of insects is treated. The cuticula is not
affected by caustic potash; it is easy, therefore, to separate it from
the tissues of the body by boiling or soaking it in an aqueous solution
of this substance.
Chitin. — The well-known firmness of the larger part of the cuticula
of adult insects is due to the presence in it of a substance which is
termed chitin. This substance bears some resemblance in its physical
properties to horn ; but is very different from horn in chemical com-
position. In thin sheets it is yellowish in color; thicker layers of it
are black. It is stained yellow by picric acid and pink by safranin.
Chitinized and non-ckitinized cuticula. — When freshly formed, the
cuticula is flexible and elastic, and certain portions of it, as at the
nodes of the body and of the appendages, remain so. But the greater
part of the cuticula, especially of adult insects, usually becomes firm
and inelastic; this is probably due to a che mical change resulting in
the production of chitin. What the natureof this change is or how it
is produced is not yet known, but it is evident that a change occurs ;
we may speak, therefore, of chitinized cuticula and non-chitinized
cuticula. This difference is well-shown in sections of the cuticula
stained by picro-carmine, which colors the chitinized portions yellow
and the non-chitinized parts pink; it can be shown also by other
double stains, as eosin-methylene-blue.
Chitinized cuticula is inelastic, while non-chitinized cuticula is
elastic. The elasticity of non-chitinized cuticula is well-shown by the
stretching of the body-wal after a molt and before the hardening of
the cuticula. It is also shown by the expanding of the abdomen of
females to accommodate the growing eggs, the stretching of the body-
wall taking place in the non-chitinized portions between the segments.
An extreme case of this is shown by the queens of Termites.
THE EXTERNAL ANATOMY OF INSECTS 31
The formation of chitin is not restricted to the hypodermis, but is
a roperty of the invaginated portions of the ectoderm ; the f ore-
int^stine, the hind-intestine, and the tracheae are all lined with a
cutecular layer, which is continuous with the cuticula of the body-wall
andi is chitinized. The most marked case of internal formation of
chitin is the development of large and powerful teeth in the proven-
tricujus of many insects.
The epidermis and the dermis. — Two quite distinct parts of the
cuticula are recognized by recent writers; these are distinguished as
the epidermis and the dermis respectively.
The epidermis is the external portion; in it are located all of the
cuticular pigments; and from it are formed all scales, hairs, and other
surface structures. It is designated by some writers as the primary
cuticula, (Fig. 41, .0).
The dermis is situated beneath the epidermis. It is formed in
layers, which gve sections of the cuticula the well-known laminate
appearance. Iti is sometimes termed the secondary cuticula (Fig. 41 ,d)
The basement membrane. — The inner ends of the hypodermal cells
are bounded by a more or less distinct membrane; th s is termed the
basement membrane (Fig. 41, bm). The basement meimbrane is most
easily seen in those places where the inner ends of the hypodermal cells
are much smaller than the outer ends; here it is a continuous sheet
connecting the tips of the hypodermal cells.
b. THE EXTERNAL APOPHYSES OF THE CUTICULA
The outer surface of the cuticula bears a wonderful variety of pro-
jections. These, however, can be grouped under two heads : first,
those that form an integral part of the cuticula; and second, those
that are connected with the cuticula b y a joint. Those that form an
integral part of the cuticula are termedaptiphyses ; those tha t are con-
nected by a joint are termed appendages of the cuticula.
The cuticular nodules. — The most frequently occurring out-
growths of the cuticula are small, more or less conical nodules.
These vary greatly in size, form, and distribution over the surface of
the body in different species of insects, and are frequently of
taxonomic value.
The fixed hairs. — On the wings of some insects, as the Trichoptera
and certain of the Lepidoptera, there is in addition to the more
obvious setae and scales many very small, hair-like structures, which
32
AN INTRODUCTION TO ENTOMOLOGY
differ from setae in being directly continuous with the cuticula, and
not connected with it by a joint; these are termed the fixed hairs,
The mode of origin and development of the fixed hairs has not
been studied; they may be merely elongated cuticular nodules.
The spines. — The term spine has been used loosely by writers on
entomology. Frequently large setae are termed spines. In this work
such setae are called spine-like setae; and the term spine is applied
only to outgrowths of the cuticula that are not separated from it by a
joint. Spines differ also from spine-like setae in being produced by
undifferentiated hypodermal cells and are usually if not always of
multicellular origin, while each seta is produced by a single trichogen
cell. The accompanying diagram (Fig. 42) illustrates this difference.
C. THE APPENDAGES OF THE CUTICULA
Under this head are included those outgrowths of the cuticula that
are connected with it by a joint. Of these there are two quite dis-
tinct types represented by the spurs and the setae respectively.
The spurs. — There exist upon the legs of many insects appendages
which on account of their form and position have been termed spurs.
Spurs resemble the true spines described above and differ from setae
in being of multicellular origin; they differ from spines in being
appendages, that is, in
being connected with the
body-wall by a joint.
The setae.— The setae
are what are commonly
called the hairs of in-
sects. Each seta (Fig.
42, s) is an appendage of
the body-wall, which
arises from a cup-like
cavity in the cuticula,
the alveolus, situated at
the outer end of a per-
foration of the cuticula,
Fig. 42. — Diagram illustrating the difference be-
tween a spine (sp) and a seta (s).
the trtchopore; and each
seta is united at its base with the wall of the trichopore by a ring of
thin membrane, the articular membrane of the seta.
The setae are hollow; each is the product of a single hypodermal
cell, a trichogen (Fig. 42), and is an extension of the epidermal
layer of the cuticula.
THE EXTERNAL ANATOMY OF INSECTS 33
In addition to the trichogen there may be a gland-cell opening into
the seta, thus forming a glandular hair, or a nerve may extend to the
seta, forming a sense-hair; each of these types is discussed later.
The most common type of seta is bristle-like in form; familiar
examples of this type are the hairs of many larvae. But numerous
modifications of this form exist. Frequently the setag are stout and
firm, such are the spine-like seta; others are .furnished with lateral
prolongations, these are the plumose hairs; and still others are flat,
wide, and comparatively short, examples of this form are the scales
of the Lepidoptera and of many other insects.
The taoconomic value of sei&. — In many cases the form of the setae
and in others their "arrangement on the cuticula afford useful charac-
teristics for the classification of insects. Thus the scale-like form of
the setae on the wing-veins of mosquitoes serves to distinguish these
insects from closely allied midges; and the clothing of scales is one
of the most striking of the characteristics of the Lepidoptera,
The arrangement of the setae upon the cuticula, in some cases at
least, is a very definite one. Thus Dyar ('94) was able to work out a
classification of lepidopterous larvae by a study of the setae with
which the body is clothed.
A classification of seta. — If only their function be considered the
hairs or setae of insects can fee grouped in the three following classes :
(1) The clothing hairs. — Under this head are grouped those hairs
and scales whose primary function appears to be merely the* protection
of the body or of its appendages. So far as is known, such hairs con-
tain only a prolongation of the trichogen cell that produced them. It
should be stated, however, that this group is merely a provisional one;
for as yet comparatively little is known regarding the relation of these
hairs to the activities of the insects possessing them.
In some cases the clothing hairs have a secondary function. Thus
the highly specialized overlapping scales of the wings of Lepidoptera,
which are modified setae, may serve to strengthen the wings; and the
markings of insects are due almost entirely to hairs and scales. The
fringes on the wings of many insects doubtless aid in flight, and the
fringes on the legs of certain aquatic insects also aid in locomotion.
(2) The glandular hairs. — Under this head are grouped those hairs
that serve as the outlets of gland cells. They are discussed in the next
chapter, under the head of hypodermal glands.
(3) The sense-hairs — In many case a seta, more or less modified
in form, constitutes a "part of a sense-organ, either of touch, taste, or
smell; examples of these are discussed in the next chapter.
34 AN INTRODUCTION TO ENTOMOLOGY
d. THE SEGMENTATION OF THE BODY
The cuticular layer of the body-wall, being more or less rigid,
forms an external skeleton; but- this skeleton is flexible along certain
transverse lines, thus admitting of the movements of the body, and
producing the jointed appearance characteristic of insects and of
other arthropods.
An examination of a longitudinal section of the body-wall shows
that it is a continuous layer and that the apparent segmentation is due
to infoldings of it (Fig. 43).
The body-seg-
ments, somites, or
metameres. — Each
section of the body p.^ 43<_Diagram of a longitudinal section of the
between two of the body- wall of an insect.
infoldings described
above is termed a body-segment, or somite, or me'tamere.
The transverse conjunctive. — The infolded portion of the body-
wall connecting two segments is termed a conjunctiva. These con-
junctivae may be distinguished from others described, later as the
transverse conjunctives.
The conjunctivas are less densely chitinized than the other portions
of the cuticula; their flexibility is due to this fact, rather than to a
comparative thinness as has been commonly described.
e. THE SEGMENTATION OF THE APPENDAGES
The segmentation of the legs and of certain other appendages is
produced in the same way as that of the body. At each node of an
appendage there is an infolded, flexible portion of the wall of the
appendage, a conjunctiva, which renders possible the movements of
the appendage.
/. THE DIVISIONS OF A BODY-SEGMENT
In many larvae, the cuticula of a large part of the body-wall is of
the non-chitinized type; in this case the wall of a segment may form
a ring which is not divided into parts. But in most nymphs, naiads,
and adult insects, there are several densely chitinized parts in the wall
of each segment; this enables us to separate it into well-defined
portions.
The tergum, the pleura, and the sternum. — The larger divisions of
a segment that are commonly recognized are a dorsal division, the
THE EXTERNAL ANATOMY OF INSECTS 35
tergum; two lateral divisions, one on each side of the body, the pleura;
and a ventral division, the sternum.
Each of these divisions may include several definite areas of'
chitinization. In this case the sclerites of the tergum are referred to
collectively as the tergites, those of each pleurum, as the pleurites, and
those constituting the sternum, as the sternites.
The division of a segment into a tergum, two pleura, and a sternum
are most easily seen in the wing-bearing segments, but it can be
recognized also in the prothorax of certain generalized insects. This
is especially the case in many Orthoptera, as cockroaches and walking-
sticks, where the pleura of the prothorax are distinct from the tergum
and the sternum. In the abdomen it is evident that correlated with
the loss of the abdominal appendages a reduction of the pleura has
taken place.
The lateral conjunctivas. — On each side of each abdominal segment
of adults the tergum and the sternum are united by a strip of non-
chitinized cuticula; these are the lateral conjunctivas. Like the
transverse conjunctivas, the lateral ones are more or less infolded.
The sclerites. — Each definite area of chitinization of the cuticula
is termed a sclerite.
The sutures. — The lines of separation between the sclerites are
termed sutures. Sutures vary greatly in form ; they may be infolded
conjunctivas ; or they may be mere lines indicating the place of union
between two sclerites. Frequently adjacent sclerites grow together
so completely that there is no indication of the suture; in such cases
the suture is said to be obsolete.
The median sutures. — On the middle line of the tergites and also of
the sternites there frequently exist longitudinal sutures. These are
termed "the median sutures. They represent the lines of the closure
of the embryo, and are not taken into account in determining the
number of the sclerites.
The dorsal median suture has been well-preserved in the head and
thorax, as it is the chief line of rupture of the cuticula at the time of
molting.
The pilif erous tubercles of larvae.— The setas of larvae are usually
borne on slightly elevated annular sclerites; these are termed pittf-
erous tubercles.
The homologizing of the sclerites. — While it is probable that the
more important sclerites of the body in winged insects have been
derived from a common winged ancestor and, therefore, can be
homologized, many secondary sclerites occur which can not be thus
homologized.
36 AN INTRODUCTION TO ENTOMOLOGY
g. THE REGIONS OF THE BODY
The segments of the body in an adult insect are grouped into three,
more or less well-marked regions: the head, the thorax, and the
abdomen. Each of these regions consists of several segments more or
less closely united.
The head is the first of these regions; it bears the mouth-parts,
the eyes, and the antennae. The thorax is the second region; it bears
the legs and the wings if they are present. The abdomen is the third
region; it may bear appendages connected with the organs of repro-
duction.
II. THE HEAD
The external skeleton of the head of an insect is composed of
several sclerites more or less closely united, forming a capsule, which
includ es a portion of the viscera, and to which are articulated certain
appendages.
a. THE CORNEAS OF THE EYES
The external layer of the organs of vision, the corneas of the eyes,
is, in each case, a translucent portion of the cuticula. It is a portion
of the skeleton of the head, which serves not merely for the admission
of light but also to support the more delicate parts of the visual
apparatus.
The corneas of the compound eyes. — The compound eyes are the
more commonly observed eyes of insects. They are situated one on
each side of the head, and are usually conspicuous. Sometimes, as in
dragon-flies, they occupy the larger part of the surface of the head.
The compound eyes are easily recognized as eyes ; but when one
of them is examined with a microscope it is found to present an
appearance very different from that of the eyes of higher animals, its
surface being divided into a large number of six-sided divisions (Fig.
44) ; hence the term compound eyes applied to them.
A study of the internal structure of this organ
has shown that each of these hexagonal divisions
is the outer end of a distinct element of the eye.
Each of these elements is termed an ommattdinm.
The number of ommatidia of which a compound
- a eye is comPosed varies greatly; there may be not
cornea of a com- more than fifty, as in certain ants, or there may
pound eye. foQ many thousand, as in a butterfly or a dragon-fly.
As a rule, the immature stages of insects with a gradual metamor-
phosis and also those of insects with an incomplete metamorphosis,
THE EXTERNAL ANATOMY OF INSECTS 37
that is to say nymphs and naiads possess compound eyes. But the
larvae of insects with a complete metamorphosis, except Corethra, do
not possess well-developed compound eyes; although there are fre-
quently a few separate ommatidia on each side of the head. These
are usually termed ocelli; but the ocelli of larvae should not be con-
fused with the ocelli of nymphs, naiads, and adults.
The corneas of the ocelli. — In addition to the compound eyes most
nymphs, naiads, and adult insects possess other eyes, which are
termed ocelli. The cornea of each ocellus is usually a more or less
nearly circular, convex area, which is not divided into facets. The
typical number of ocelli is four; but this number is rarely found.
The usual number is three, a median ocellus, which has been derived
from a pair of ocelli united, and a distinct pair of ocelli. Frequently
the median ocellus is lacking, and less frequently, all of the ocelli
have been lost. The position of the ocelli is discussed later.
b. THE AREAS OF THE SURFACE OF THE HEAD
In descriptions of insects it is frequently necessary to refer to the
different regions of the surface of the head. Most of these regions
were named by the early insect anatomists; and others have been
described by more recent writers.
This terminology is really of comparatively little morphological value; for
in some cases a named area includes several sclerites, while in others only a portion
of a sclerite is included. This is due to the fact that but few of the primitive
sclerites of the head have remained distinct, and some of them greatly over-
shadow others in their development. The terms used, however, are sufficiently
accurate to meet the needs of describers of species, and will doubtless continue in
use. It is necessary, therefore, that students of entomology become familiar
with them.
The best landmark from which to start in a study of the areas of
the surface of the head is the epicranial suture, the inverted Y-shaped
suture on the dorsal part of the head, in the more generalized insects
(Fig. 45, e. su.). Behind the arms of this
suture there is a series of paired sclerites, which
meet on the dorsal wall of the head, the line of
union being the stem of the Y, a median suture;
and between the arms of the Y and the mouth
there are typically three single sclerites (Fig. 45,
F, C, L). It is with these unpaired sclerites
that we will begin our definitions of the areas
of the head. Fig. ^S^-Head fa
The front. — The front is the unpaired
sclerite between the arms of the epicranial suture (Fig. 45, F).
38
AN INTRODUCTION TO ENTOMOLOGY
Fig. 46. — Head of
a cockroach.
In the more generalized insects at least, if not in all, the front
bears the median ocellus; and in the Plecoptera, the paired ocelli also.
Frequently the suture between the front and the following sclerite, the
clypeus, is obsolete; but as it ends on each side in the invagination
which forms an anterior arm of the tentorium or
endo-skeleton (Fig. 46, at), its former position can
be inferred, at least in the more generalized
insects, even when no other trace of it remains.
In Figure 46 this is indicated by a dotted line.
The clypeus. — The clypeus is the intermediate
of the three unpaired sclerites between the epi-
cranial suture and the mouth (fig. 46, c). To this
part one condyle of the mandible articulates.
Although the clypeus almost always appears
to be a single sclerite, except when divided trans-
versely as indicated below, it really consists of a
transverse row of three sclerites, one on the median line, and one on
each side articulating with the mandible. The median sclerite may
be designated the clypeus proper, and each lateral sclerite, the ante-
coxal piece of the mandible. Usually there are no indications of the
sutures separating the clypeus proper from the antecoxal pieces ; but
in some insects they are distinct. In the larva of Corydalus, the ante-
coxal pieces are not only distinct but are quite large (Fig. 47, ac, ac).
In some insects the clypeus is completely or partly divided by a
transverse suture into two parts (Fig. 45). These may be designated
as the first clypeus and the second clypeus, respectively; the first
clypeus being the part next the front (Fig.
45, Ci) and the second clypeus being that next
the labrum (Fig. 45, C2).
The suture between the clypeus and the
epicranium is termed the clypeal suture.
The labrum. — The labrum is the movable
flap which constitutes the upper lip of the
mouth (Fig. 45, L). The labrum is the last of
the series of unpaired sclerites between the
epicranial suture and the mouth. It has the
appearance of an appendage but is really a
portion of one of the head segments.
The .epicranium. — Under the term epi-
Fig. *5«. — Head of a
larva of Corydalus,
dorsal aspect
cranium are included all of the paired sclerites of the skull, and some-
times also the front. The paired sclerites constitute the sides of
THE EXTERNAL ANATOMY OF INSECTS
39
the head and that portion of the dorsal surface that is behind the
arms of the epicranial suture. The sclerites constituting this
region are so closely united that they were regarded as a single
piece by Straus-Durckheim (1828), who also inc uded the front in
this region, the epicranial suture being obsolete in the May beetle,
which he used as a type.
The vertex. — The dorsal portion of the epicranium; or, more
specifically, that portion which is next the front and between the
compound eyes is known as the vertex (Fig. 45, Vt V). In many
insects the vertex bears the paired ocelli. It is not a definite sclerite;
but the term vertex is a very useful one and will doubtless be retained.
The occiput. — The hind part of the dorsal surface of the head is the
occiput. When a distinct sclerite, it is formed
. from the tergal portion of the united postgenas
described below (Fig. 47, 0, 0).
The genae. — The genes are the lateral portions
of the epicranium. Each gena, in the sense in
which the word was used by the older writers,
includes a portion of several sclerites. Like
vertex, however, the term is a useful one.
The postgenae. — In many insects each gena is
divided by a well-marked suture. This led the
writer, in an earlier work ('95), to restrict the
term gena to the part in front of^the suture (Fig.
48, '£), and to propose the term postgena for the
part behind the suture (Fig. 48, Pg).
The gula. — The gula is a sclerite forming the ventral wall of the
hind part of the head in certain orders of insects,
and bearing the labium or second maxillae (Fig.
49, Gu). In Ijthe more generalized orders, the
sclerite corresponding to the gula does not form
a part of the skull. The sutures forming the
lateral boundaries of the gula are termed the
gular sutures.
The ocular sclerites. — In many insects each
compound eye is situated in the axis of an
annular sclerite; these sclerites bearing the
compound eyes are the ocular sclerites (Fig. 50, os).
The antenna! sclerites. — In some insects there
is at the base of each antenna an annular sclerite;
these are the antennal sclerites (Fig. 50, as). The antennal sclerites
are most distinct in the Plecoptera.
Fig. 48. — Head and
neck of a cock-
roach.
Fig. 49. — H-2ad of
Corydnlus, adult,
ventral aspect.
40 AN INTRODUCTION TO ENTOMOLOGY
The trochantin of the mandible. — In some insects, as Orthoptera
there is a distinct sclerite between each mandible and the gena ;
this is the trochantin of the mandible (Fig. 45, tr).
The maxillary pleurites.— In some of the more generalized insects,
as certain cockroaches and crickets, it can be seen that each maxilla
is articulated at the ventral end of a pair of sclerites, between which
is the invagination that forms the posterior arm of the tentorium;
these are the maxillary pleurites\ the pos-
terior member of this pair of sclerites can
be seen in the lateral view of the head of a
cockroach (Fig. 48, m. em).
The cervical sclerites.; — The cervical scler-
ites are the small sclerites found in the neck of
many insects. Of these there are dorsal,
lateral, and ventral sclerites. The cervical
sclerites were so named by Huxley ('78);
Fi so— Head of a Tecer]tty they have been termed the inter seg-
cricket, ental surface mental plates by Crampton ('17), who con-
of the dorsal wall. siders them to be homologous with sclerites
found in the intersegmental regions of the
thorax of some generalized insects.
The lateral cervical sclerites have long been known as the jugular
sclerites (pieces jugulaires, Straus Durckheim, 1828).
C. THE APPENDAGES OF THE HEAD
Under this category are classed a pair of jointed appendages
termed the antenna, and the organs known collectively as the mouth-
parts.
The antennae. — The antenna are a pair of jointed appendages
articulated with the head in front of the eyes or between them. The
antennae vary greatly in form; in some insects they are thread-like,
consisting of a series of similar segments; in others certain segments
are greatly modified. The thread-like form is the more generalized.
In descriptive works names have been given to particular parts of the antennae,
as follows (Fig. 51):
The Scape. — The first or proximal segment of an antenna is called the scape (a).
The proximal end of this segment is often subglobose, appearing like a distinct
segment; in such cases it is called the bulb (a1).
THE EXTERNAL ANATOMY OF INSECTS
41
a—-
The Pedicel. — The pedicel is the second segment of an antenna (7>). In
some insects it differs greatly in form from the other segments.
* The Cldvola. — The term cla-
vola is applied to that part of
the antenna distad of the pedi-
cel (c); in other words, to all
of the antenna except the first
and second segments. In some
insects certain parts of the cla-
vola are specialized and have
received particular names.
These are the ring- joints, the
funicle, and the club.
The Ring- joints. — In certain
Fig.si.-Antennaofachalcis-fly. ingects (^ Chalcidida) the
proximal segment or segments of the clavola are much shorter than the suc-
ceeding segments; in such cases they have received the name of ring-joints (c1).
The Club. — In many insects the distal seg-
ments of the antenna? are more or less enlarged.
In such cases they are termed the club (c3).
The Funicle. — The funicle (c2) is that part
of the clavola between the club and the ring-
joints; or, when the latter are not specialized,
between the club and the pedicel.
The various forms of antennae are designated
by special terms. The more common of these
forms are represented in Fig. 52. They are
as follows:
1. Setaceous or bristle-like, in which the
segments are successively smaller and smaller,
the whole organ tapering to a point.
2. Filiform or thread-like, in which the
segments are of nearly uniform thickness.
3. Momliform or necklace-form, in which
the segments are more or less globose, suggesting
a string of beads.
4. Serrate or saw-like, in which the segments
are triangular and project like the teeth of a saw.
5. Pectinate or comb-like, in which the seg-
ments have long processes on one side, like the
teeth of a comb.
6. Cldvate or club-shaped, in which the segments become gradually broader,
so that the whole organ assumes the form of a club.
7. Capitate or with a head, in which the terminal segment or segments form
a large knob.
8. Lamellate in which the segments that compose the knob are extended on
one side into broad plates.
When an antenna is bent abruptly at an angle like a bent knee (Fig. 51) it is
said to be geniculate.
Fig. 52. — Various forms of
antennas.
42
AN INTRODUCTION TO ENTOMOLOGY
The mouth-parts. — The mouth-parts consist typically of an upper
lip, labrum, an under lip, labium, and two pairs of jaws acting hori-
zontally between them. The upper jaws are called the mandibles;
the lower pair, the maxilla.
The maxillag and labium are
each furnished with a pair of
feelers, called respectively
the maxillary palpi, and
the labial palpi. There
may be also within the
mouth one or two tongue-
like organs, the epipkarynx
and the hypopkarynx. The
mouth-par ;s of a locust will
serve as an example of the
typ'cal form of the mouth-
parts (Fig. 53).
mx
The mouth-parts enumer-
ated in the preceding paragraph
are those commonly recognized
in insects; but in certain insects
there exist vestiges of a pair of
jaws between the mandibles and
the maxillae, these are the maxil-
Fig. 53. — Mouth-parts of a locust: la, lab-
rum ;md, mandible; mx, maxilla; h, hypo-
pharynx; /, labium.
No set of organs in the
body of an insect vary in
form to a greater degree than
do the mouth-parts. Thus
with some the mouth is
formed for biting, while with others it is formed for sucking. Among
the biting insects some are predaceous, and have jaws fitted for
seizing and tearing their prey; others feed upon vegetable matter,
and have jaws for chewing this kind of food. Among the sucking
insects the butterfly merely sips the nectar from flowers, while the
mosquito needs a powerful instrument for piercing its victim. In
this chapter the typical form of the mouth-parts as illustrated by the
biting insects is described. The various modifications of it presented
by the sucking insects are described later, in the discussions of the
characters of those insects.
THE EXTERNAL ANATOMY OF INSECTS
43
The labrum. — The labrum or upper lip (Fig. 53), is a more or less
flap-like organ above the opening of the mouth. As it is often freely
movable, it has the appearance of an appendage of the body ; but it
is not a true appendage, being a part of one of the body segments that
enter into the composition of the head.
The mandibles. — The mandibles are the upper pair of jaws (Fig.
53). They represent the appendages of one of the segments of the
head. In most cases they are reduced to a single segment; but in
some insects, as in certain beetles of the family Scarabaeidae, each
mandible consists of several more or less distinct sclerites.
The majtittulcz. — The maxtllula are /a pair of appendages, which
when present are situated between the mandibles and the maxillae.
With most insects they are "either absent or are so slightly developed
that they do not ftave the appearance of appendages, and have been
considered as merelwateral lobes of the hypopharnyx. Borner ('04)
finds that the hypopharvnx of nearly all insects having an incomplete
metamorphosis bears a ^air ofr' vestigial maxillulae; maxillulae have
been found in the Thysanura/ Dermaptera, Orthoptera, Corrodenti;
the naiads of EphemeridsAand the larvae of
Coleoptera. >
In certain Thysanidra the\ maxillulae are
well-preserved; figure/54 represents a maxillula
of Machilis maritima. These appendages are
the " paraglossa" of writers on theVrhysanura
and Collembola ^d the superlingu<z\i Folsom
Coo).
la-
The term m^xillulas, a diminutive of maxmsi, was
proposed by Hansen ('93), who regards them as Jftomo-
logous withyftie first maxillae of the Crustaceae. \hey
are the ao|4ndages of a segment of the head which\is
very sligiuly developed in most insects.
The maxilla. — The maotilla are the second
pair of jaws of most insects, of all insects except - 54-—
lula of Machilis man-
those in which the maxillulae are retained. Like tima; la, lacinia ; ga ,
the mandibles they are the appendages of one fjlea; ' p, palpus
/ (After Carpenter),
of the segments of the head.
The maxillae are much more complicated than the mandibles, each maxilla
consisting, when all of the parts are present, of five primary parts and three
appendages. The primary parts are the cardo or hinge, the stipes or foot-
stalk, the palpifer or palpus-bearer, the subgalea or helmet-bearer, and the
lacinia or blade. The appendages are the maxillary palpus or feeler, the galea
44
AN INTRODUCTION TO ENTOMOLOGY
or superior lobe, and the digitus or finger. The maxilla may also bear claw-like
or tooth-like projections, spines, bristles, and hairs.
In the following description of the parts of the maxillae, only very general
statements can be made. Not only is there an infinite variation in the form of
these parts, but the same part may have a very different outline on the dorsal
aspect of the maxilla from what it has on the ventral. Compare Fig. 55 and Fig.
56, which represent the two aspects of the maxilla of Hydrophilus. Excepting
Fig. 56, the figures of maxillas represent the ventral aspect of this organ.
The cardo or hinge (a) is the first or proximal part of the maxilla. It is usually
more or less triangular in outline, and is the part upon which nearly all of the
motions of this organ depend In many cases, however, it is not the only part
directly joined to the body ; for frequently muscles extend direct to the aibgalea,
without passing through the cardo.
The stipes or footstalk (&) is the part next in order proceeding distad. It is
usually triangular, and articulates with the cardo by its base, with the palpifer
by its lateral margin, and with the subgalea by its mesal side. In many insects
the stipes is united with the subgalea, and the two form the larger portion of the
body of the maxilla (Fig. 53). The stipes has no appendages; but the palipfer
on the one ide, and the subgalea on the other, may become united to the stipes
without anys trace of suture remaining, and their appendages will then appear
to be borne by the stipes. Thus in Fig. 53 it appears to be the stipes that bears
the galea, and that receives muscles from the body.
The palpifer or palpus-bearer (c} is situated upon the lateral (outer) side
of the stipes; it does not,
however, extend to the base
of this organ, and frequently
projects distad beyond it.
It is often much more
developed on the dorsal
side of the maxilla than on
the ventral (Figs. 55 and 56).
It can bereadily distinguished
when it is distinct by the
insertion upon it of the ap-
pendage which gives to it
its name.
The maxillary palpus or
feeler (<f) is the most conspicuous of the appendages of the maxilla. It is an
organ composed of from one to six freely movable segments, and is articulated
to the palpifer on the latero-distal angle of the body of the maxilla.
The subgalea or helmet-bearer (e) when developed as a distinct sclerite is most
easily distinguished as the one that bears the galea. It bounds the stipes more
or less completely on its mesal (inner) side, and is often directly connected
with the body by muscles. In many Coleoptera it is closely united to the
lacinia; this gives the lacinia the appearance of bearing the galea, and of being
connected with the body (Fig. 56). In several orders the subgalea is united to
the stipes; consequently in these orders the stipes appears to bear the galea,
and to be joined directly to the body if any part besides the cardo is so
connected.
Fig. 55- — Ventral as-
pect of a maxilla of
Hydrophilus.
Fig. 56. — Dorsal as-
pect of a maxilla of
Hydrophilus.
THE EXTERNAL ANATOMY OF INSECTS
45
The gdlea or helmet (/) is the second in prominence of the appendages
of the maxilla. It consists of one or two segments, and is joined to the maxilla
mesad of the palpus. The galea varies greatly
in form : it is often more or less flattened, with
the distal segment concave, and overlapping
the lacinia like a hood. It was this form that
suggested the name galea or helmet. In other
cases the galea resembles a palpus in form (Fig.
57). The galea is also known as the outer lobe,
the upper lobe, or the superior lobe.
The lacinia or blade (g) is borne on the mesal
(inner) margin of the subgalea. It is the cutting
or chewing part of the maxilla, and is often
furnished with teeth and spines. The lacinia is
also known as the inner lobe, or the inferior lobe.
The digitus or finger (Ji) is a small appendage
sometimes borne by the lacinia at its distal end.
In the Cicindelidae it is in the form of an articu-
lated claw (Fig. 57) ; but in certain other beetles
it is more obviously one of the segments of the
Fig. 57.-Maxilla otdcindela. maxiUa (pigs> 55 and 56)>
The labium or second maxilla. — The labium or under lip (Fig. 53),
is attached to the cephalic border of the gula, and is the most ventral
of the mouth-parts. It appears to be a single organ, although some-
times cleft at its distal extremity; it is, however, composed of a pair
of appendages grown together on the middle $ne of the body. In the
Crustacea the parts corresponding to the labium of insects consists of
two distinct organs,
resembling the
maxillae; and in the
embryos of insects
the labium arises as
a pair of append-
ages.
In naming the parts
of the labium, entomo-
logists have usually
taken some form of it
in which the two parts
are completely grown
together, that is, one
which is not cleft on
the middle line (Fig.
58). I will first describe such a labium, and later one in which the division
into two parts is carried as far as we find it in insects.
Fig. 58. — Labium of Harpalus.
46 _ AN INTRODUCTION TO ENTOMOLOGY
The labium is usually described as consisting of three principal parts and a
pair of appendages. The principal parts are the submenlum, the mentum, and
the ligula; the appendages are the labial palpi.
The submentum. The basal part of the labium consists of two transverse
sclerites; the proximal one, which is attached to the cephalic border of the gula,
is the submentum (a). This is often the most prominent part of the body of
the labium.
The mentum is the more distal of the two primary parts of the labium (6).
It is articulated to the cephalic border of the submentum, and is often so
slightly developed that it is concealed by the submentum.
The ligula, includes the remaining parts of the labium except the labial palpi.
It is a compound organ; but in the higher insects the sutures between the
different sclerites of which it is composed are usually obsolete. Three parts,
however, are commonly distinguished (Fig. 58), a central part, often greatly
prolonged, the glossa (c2) and two parts, usually small membranous projections,
one on each side of the base of the glossa, the paraglossa (c3) . Sometimes, how-
ever, the paraglossae are large, exceeding the glossa in size.
Th'e labial palpi. From the base of the ligula arise a pair of appendages, the
labial palpi. Each labial palpus consists of from one to four freely movable
segments.
In the forms of the labium just described, the correspondence of its parts to
the parts of the maxillae is not easily seen; but this is much more evident in the
labium of some of the lower insects, as for example a cockroach (Fig. 59). Here
the organ is very deeply cleft; only the submentum
and mentum remain united on the median line; while
the ligula consists of two distinct maxilla-like parts.
It is easy in this case to trace the correspondence
referred to above. Each lateral half of the submentum
corresponds to the cardo of a maxilla; each half of the
mentum, to the stipes; while the remaining parts of a
maxilla are represented by each half of the ligula, as
follows: near the base of the ligula there is a part (cl)
which bears the labial palpus; this appears in the
figure like a basal segment of the palpus; but in many
insects it is easily seen that it is undoubtedly one of
the primary parts of the organ; it has been named
< 59. — Labium of a ^le palpiger, and is the homologue of the palpifer of
cockroach. a maxilla. The trunk of each half of the ligula is
formed by a large sclerite (c4) ; this evidently corres-
ponds to the subgalea. At the distal extremity of this subgalea of the labium
there are two appendages. The lateral one of these (c3) is the paraglossa,
and obviously corresponds to the galea. The mesal one (62) corresponds to the
lacinia or inner lobe. This part is probably wanting in those insects in which
the glossa consists of an undivided part; and in this case the glossa probably
represents the united and more or less elongated subgaleae.
The epipharynx. — In some insects there is borne on the ental sur-
face of the labtum, within the cavity of the mouth, an unpaired fold,
which is membranous and more or less chitinized; this is the epi-
ph&rynx.
THE EXTERNAL ANATOMY OF INSECTS 47
The hypopharynx. — The hypopharynx is usually a tongue-like
organ borne on the floor of the mouth cavity. This more simple form
of it is well-shown in the Orthoptera (Fig. 53). To the hypopharnyx
are articulated the maxillula? when they are present. The hypo-
pharynx is termed the lingua by some writers.
d. THE SEGMENTS OF THE HEAD
The determination of the number of segments in the head of an insect is a
problem that has been much discussed since the early days of entomology. The
first important step towards its solution was made by Savigny (1816), who sug-
gested that the movable appendages of the head were homodyanmous with legs.
This conclusion has been accepted by all; and as each segment in the body of an
insect bears only a single pair of appendages, there are at least four segments
in the head; i.e., the antennal, the mandibular, the maxillary, and the second
maxillary or labial.
In more recent times workers on the embryology of insects have demonstrated
the presence of three additional segments. First, there has been found in the
embryos of many insects a pair of evanescent appendages situated between the.
antennae and the mandibles. These evidently correspond to the second antennae
of Crustacea, and indicate the presence of a second antennal segment in the head
of an insect. This conclusion is confirmed by a study of the development of the
nervous system. And in the Thysanura and Collembola vestiges of the second
antennae persist in the adults of certain members of these orders.
Second, as the compound eyes are borne on movable stalks in certain Crusta-
cea, it was held by Milne-Edwards that they represent another pair of appendages;
but this view has not been generally accepted. It is not necessary, however, to
discuss whether the eyes represent appendages or not ; the existence of an ocalar
segment has been demonstrated by a study of the development of the nervous
system.
It has been shown that the brain of an insect is formed from three pairs of
primary ganglia, which correspond to the three principal divisions of the brain,
the protecerebrum, the deutocerebrum, and the tritucerebrum. And it has also been
shown that the protocerebrum innervates the compound eyes and ocelli; the
deutocerebrum, the antennae; and the tritocerebrum, the labrum. —3Phis demon-
strates the existence of three premandibular segments: an ocular segment or
protocerebral segment, without appendages, unless the compound eyes repre-
sent them; an antennal or deutocerebral segment, bearing antennae; and a
second antennal or tritocerebral segment, of which the labrum is a part, and to
which the evanescent appendages between the antennae and the mandibles doubt-
less belong. As \ftallanes has shown that the tritocerebrum of Crustacea inner-
vates the second antennae, we are warranted in considering the tritocerebral
segment of insects to be the second antennal segment.
Third, the presence of a pair of jaws, the maxillulae, between the mandibles
and the maxillae has been demonstrated in several widely separated insects. These
are doubtless the appendages of a segment, which is so reduced in most insects
that it has been overlooked until comparatively recently. Folsom (foo) in his
work on the development of the mouth-parts of A nurida demonstrated the exist-
ence1 of the pair of primary ganglia pertaining to this segment.
48
AN INTRODUCTION TO ENTOMOLOGY
In addition to the maxillular ganglia, which have been almost universally
overlooked, and the existence of which has been denied bv some writers, the sub-
cesophageal ganglion is formed by the union of three pairs of primitive ganglia,
pertaining respectively to the mandibular, the maxillary-, and the labial segments
of the embryo.
LIST OF THE SEGMENTS OF THE HEAD
First, ocular, or protocerebral.
Second, antennal, or deutocerebral.
Third, second antennal, or tritocerebral.
Fourth, mandibular.
Fifth, maxillular.
Sixth, maxillary.
Seventh, labial, or second maxillary.
III. THE THORAX
a. THE SEGMENTS OF THE THORAX
The prothorax, the mesothorax, and the metathorax. — The thorax
is the second or intermediate region of the body ; it is the region that
in nymphs, naiads, an i adults" bears the organs of locomotion, the legs,
and the wings when they are present. This region is composed of
three of the body-segments more or less firmly joined together; the
segments are most readily distinguished by the
fact" that each bears a pair of legs. In winged
insects, the wings are borne by the second and
third segments. The first segment of the thorax,
the one next the head, is named the prothorax;
the second thoracic segment is the mesothorax',
and the third, the metathorax.
f-~ The simplest form of the thorax in adult
JK IL insects occurs in the Apterygota (the Thysanura
1 7\ Fv and the Collembola) where although the seg-
ments differ in size and proportions, they are
distinct and quite similar (Fig. 60).
In the Pterygota, or wirg3d insects, the
prothorax is either free or closely united to the
mesothorax ; in many cases it is greatly reduced in
size; it bears the first pair of legs. The meso-
thorax and the metathorax are more or less closely
united, forming a box, which bears the wings and
the second and third pairs of legs. This union of
these two segments is often so close that it is very difficult to distin-
guish their limits. Sometimes the matter is farther complicated by
a union with the thorax of a part or of the whole of the first
Fig. 60. — Lepisma
saccharina (After
Lubbock).
THE EXTERNAL ANATOMY OF INSECTS 49
abdominal segment. In the Acridiidag, for example, the sternum of
the first abdominal segment forms a part of the intermediate region
of the body, and in the Hymenoptera the entire first abdominal
segment pertains to this region.
The alitrunk. — When, as in the Hymenoptera, the intermediate
region of the body includes more than the three true thoracic seg-
ments it is designated the alitrunk.
The propodeum or the median segment. — When the alitrunk con-
sists of four segments the abdominal segment that forms a part of it is
termed the propodeum or the median segment. In such cases the true
second abdominal segment is termed the first.
. 6: THE SCLERITES OF A THORACIC SEGMENT
The parts of the thorax most generally recognized by entomologists
were described nearly a century ago by Audouin (1824) ; some addi-
tional parts not observed by Audouin have been described in recent
times, by the writer ('02), Verhoeff ('03), Crampton ('09), and
Snodgrass ('09, '10 a, and '10 b). The following account is based on
all of these works.
In designating the parts of the thorax the prefixes pro, meso, and
meta are used for designating the three thoracic segments or corres-
ponding parts of them; and the prefixes pre and post are used to
designate parts of any one of the segments. Thus the scutum of th 3
prothorax is designated the proscutum; while the term prescutum is
applied to the sclerite immediately in front of the scutum in each of
the thoracic segments. This system leads to the use of a number of
hybrid combinations of Latin and Greek terms, but it is<so firmly
established that it would not be wise to attempt to change it en this
account.
Reference has already been made to the division of a body-segment
into a tergum, two pleura, and a sternum ; each of these divisions will
be considered separately; and as the maximum number of parts are
found in the wing-bearing segments, one of these will be taken as an
illustration.
The sclerites of a tergum. — In this discussion of the external ana-
tomy of the thorax reference is made only to those parts that form
the external covering of this region of the body. The infoldings of
the body-wall that constitute the internal skeleton are discussed in the
next chapter.
The notum. — In nymphs and in the adults of certain generalized
insects the tergum of each wing-bearing segment contains a single
50
AN INTRODUCTION TO ENTOMOLOGY
chitinized plate; this sclerite is designated the notum. The term
notum is also applied to the tergal plate of the prothorax and to that
of each abdominal segment. The three thoracic nota are designated
as the pronotum, the mesonotum, and the metanotmn respectively.
The notum of a wing-bearing segment is the part that bears the
wings of that segment, even when the tergum contains more than one
sclerite. Each wing is attached to two processes of the notum, the
anterior notal process (Fig. 61, a n p) and the posterior notal process
(Fig. 61, p n p); and the posterior angles of the notum are produced
into the axillary cords, which form the posterior margins of the basal
membranes of the wings (Fig. 61, Ax C).
The postnotum or postscutellum. — In the wing-bearing segments of
most adult insects the tergum consists of two principal sclerites ; the
notum already described, and behind this a narrower, transverse
sclerite which is commonly known as the postscutellum, and to which
Snodgrass has applied the term postnotum (Fig. 61, P N}.
The divisions of the notum. — In most specialized insects the notum
of each wing-bearing segment is more or less distinctly divided by
transverse lines or sutures into three parts; these are known as the
prescutum (Fig. 61, Psc), the scutum (Fig. 61, Set), and the scutellum
(Fig. 61, Scl).
It has been commonly held, since the
days of Audouin, that the tergum of each
thoracic segment is composed typically of four
sclerites, the prescutum, scutum, scutellum,
and postscutellum. But the investigations of
Snodgrass indicate that in its more genera-
lized form the tergum contains a single
sclerite, the notum; that the postscutellum
or postnotum is a secondary tergal chitini-
zation in the dorsal membrane behind the
notum, in more specialized insects; and that
the separation of the notum into three parts,
the prescutum, scutum, and scutellum, is a
still later specialization that has arisen
independently in difterent orders, and does
not indicate a division into homologous
parts in all orders where it exists.
The patagia. — In many of the more
specialized Lepidoptera the pronotum Fig. 6l. -Diagram of a generalized
IS produced on each side into a flat thoracic segment (From Snod-
lobe, which in some cases is even con- ^
stricted at the base so as to become a stalked plate, these lobes are
the patagia.
hi
THE EXTERNAL ANATOMY OF INSECTS
51
The par apsides. — In some Hymenoptera the scutum of the meso-
thorax is divided into three parts by tv o longitudinal sutures. The
lateral portions cf the scutum thus separated from the mesal part are
termed the pa: df sides.
The sclerites of the pleura. — In the accompanying figure (Fig. 61)
the sclerites of the left pleurum of a wing-bearing segment are repre-
sented diagramrratically; these sclerites are the following:
The episternum. — Each pleurum is composed chiefly of two
sclerites, which typically occupy a nearly vertical position, but
usually are more cr kss oblique. In most insects the dorsal end of
these sclerites extends farther forward than the ventral end, but in
the Odonata the reverse may be true. The more anterior in position
of these two sclerites is the episternum (Fig. 61, Eps),
In several of the orders of insects one or more of the episterna are
divided by a distinct suture into an upper and a lower part. These
two parts have been designated by Crampton ('09) as the an&pist&r-
num and the katepisternum respectively (Fig. 62).
The epimerum. — The epimerum is the more posterior of the two
principal sclerites of a pleurum (Fig. 61). It is separated from the
episternum by the pleural suture (Fig. 61 , PS) which extends from the
pleural wing prccess above (Fig. 61, Wp) to the pleural coxal process
below (Fig. 61, CxP).
In some of the orders of insects one or more of the epimera are
divided by a distinct suture into an upper and a .
lower part. These two parts have been desig-
nated by Crampton ('09) as the antpimerum
and the katepimerum respectively (Fig. 62).
The preepisternum. — In some of the more
generalized insects there is a sclerite situated
in front of the episternum; this is the pre-
episternum (Fig. 61, Peps).
The paraptera. — In many insects there is on
each side a small sclerite between the upper
end of the episternum and the base of the wing ;
these have long been known as the pardptera.
Fig. 6.?.— Lateral aspect Sncdgrass (10 a) has shown that there are in
of the meso- and meta- . 1 . . -u- -u
thorax of Mantispa some insects two sclentes in this region, which,
rugicoUis; i, i, anepis- he designates the ep Internal paraptera or
ternum; 2,2,katepister- ._. _, .. „. - ,,
num; j, ?, anepimer- preparaptera (Fig. 61, iP and 2P); and that
um; 4, 4, katepimerum; one or occasionally two are similarly situated
between the epimerun and the base of the wing,
the epimeral paraptera or postparaptera (Fig. 61, $P and
52
AN INTRODUCTION TO ENTOMOLOGY
The spiracles. — The external openings of the respiratory system
are termed spiracles. Of these there are two pairs in the thorax.
The first pair of thoracic spiracles open, typically, one on each side in
the transverse conjunctiva between the prothorax and the meso-
thorax ; the second pair open in similar positions between the meso-
thorax and the me'athorax. In some cases the spiracles have
migrated either forward or backward upon the adjacent segment.
For a discussion of the number and distribution of the spiracles, see
the next chapter.
The peritremes. — In many cases a spiracle is surrounded by a cir-
cular sclerite; such a sclerite is termed a peritreme.
The acetabula or coxal cavities. — In some of the more specialized
insects, as many beetles for example, the basal segment of the legs is
inserted in a distinct cavity; such a cavity is termed an acetabulum or
coxal cavity. When the epimera of the prothorax extend behind the
coxae and reach the prosternum, the coxal cavities are said to be
closed (Fig. 63) ; when the epimera do not extend behind the coxae
to the prosterum, the coxal cavities are described as open (Fig. 64) .
The sclerites of a sternum. — In the more generalized insects the
sternum of a wing-bearing segment may consist of three or four
sclerites. These have been designated, beginning with the anterior
one, the presternum (Fig.
61, Ps), the sternum or
eusternum (Fig. 61, S),
the sternellum (Fig. 61,
SI) , and the poststernellum
(Fig. 61, Psl).
In the more special-
ized insects only one of
these, the sternum, re-
mains distinctly visible.
It is an interesting fact
that while in the speciali-
zation of the tergum
there is an increase in
the number of the scleri-
tes in this division of a
segment, in the specialization of the sternum there is a reduction.
It is a somewhat unfortunate fact that the term sternum has been
used in two senses : first, it is applied to the entire ventral division of
a segment ; and second, it is applied to one of the sclerites entering
Fig. 63. — Prothorax of Harpalus, ventral aspect;
c, coxa; em, epimerum; es, episternum; /,
femur; n, pronotum; s, s, s, prosternum.
THE EXTERNAL ANATOMY OF INSECTS
53
tr
Fig. 64. — Prothorax of Penthe; c, coxa; cc, coxal
cavity ;/, femur ; s, prosternum; tr, trochanter.
into the composition of this division when it consists of more than
a single sclerite. To meet this difficulty Snodgrass has proposed
that the term eusternum
be applied to the sclerite
that has been known as
the sternum; and that
the word sternum be
used only to designate
the entire ventral divi-
sion of a segment.
C. THE ARTICULAR
SCLERITES OF THE
APPENDAGES
At the base of each leg
and of each wing there
are typically several
sclerites between the appendage proper and the sclerites of the trunk
of the segment ; these sclerites, which occupy an intermediate position
between the body and its appendage, are termed the articular sclerites.
Frequently one or more of the articular sclerites become consoli-
dated with sclerites of the trunk so as to appear to form a part of its
wall ; this is especially true of those at the base of the legs.
The articular sclerites of the legs. — The proximal segment of the leg,
the coxa, articulates with the body by means of two distinct articula-
tions, which may be termed the pleural articulation of the coxa and the
ventral articulation of the coxa respectively. The pleural articulation
is with the ventral end of the foot of the lateral apodeme of the seg-
ment, i. e. with the pleural coxal process, which is at
the ventral end of the suture between the episternum
and the epimerun (Fig. 61, CxP). The ventral arti-
culation is with a sclerite situated between the coxa
and the episternum; this sclerite and others asso-
ciated with it may be termed the articular sclerites
of the legs. The articular sclerites of the legs to
which distinctive names have been applied are the
following :
The trochantin. — The maximum number of
articular sclerites of the legs are found in the more
generalized insects; in the more specialized insects
the number is reduced by a consolidation of some of them with
Fig. 65.— The
base of a leg
of a cock-
roach.
54 4 AT INTRODUCTION TO ENTOMOLOGY
adjacent parts. The condition found in a cockroach may be taken
as typical. In this insect the trcchdntin (Fig. 65, t) is a triangular
sclerite, the apex of which points towards the middle line of the body,
and is near the ventral articulation of the coxa (Fig. 65, y). In most
specialized insects the trochantin is consolidated with the antecoxal
piece, and the combined sclerites, which appear as one, are termed
the trochantin.
The antecoxal piece. ^ Between the trochantin and the episternum
there are, in the cockroach studied, two sclerites; the one next the
trochantin is the antecoxal piece. This is the articular sclerite that
articulates directly with the coxa (Fig. 65, ac). As stated above, the
antecoxal piece is usually consolidated with the trochantin, and the
term trochantin is applied to the combined sclerites. Using the term
trochantin in this sense, the statement commonly made that the
ventral articulation of the coxa is with the trochantin is true.
The second antecoxal piece. — The sclerite situated between the
antecoxal piece and the episternum is the second antecoxal piece (Fig.
65, 2dac). This is quite distinct in certain generalized insects; but it
is usually lacking as a distinct sclerite.
The articular sclerites of the wings. — In the Ephemerida and Odo-
nata the chitinous wing-base is directly continuous with the walls of
the thorax. In all other orders there are at the base of each wing
several sclerites which enter into the composition of the joint by which
the wring is articulated to the thorax ; these may be termed collectively
the articular sclerites of ike w'ngs. Beginning with the front edge
of this joint and passing backward these sclerites are as follows:
The tegula. — In several orders of insects there is at the base of the
costal vein a small, hairy, slightly chitinized pad; this is the tegula
(Fig. 66 , Tg) . In the more highly specialized orders, the Lepidoptera,
the Hymenoptera, and the Diptera, the tegula is largely developed
so as to form a scale-like plate overlapping the base of the wing.
The tegula3 of the front wings of Lepidoptera are specially large
and are carried by special tegular plates of the notum. These, in turn,
are supported by special internal tegular arms from the bases of the
pleural wing-processes (Snodgrass, '09)
The axillaries. — Excepting the tegula, which is at the front edg3
of the wing-joint, the articular sclerites of the wings have been termed
collectively the axillaries. Much has been written about these
sclerites, and many names have been applied to them. The simplest
terminology is that of Snodgrass ('09 and '10 a) which I here adopt.
THE EXTERNAL ANATOMY OF INSECTS 55
The first axillary. — This sclerite (Fig. 66, i Ax) articulates with
the anterior notal wing-process and is specially connected with the
base of the subcostal vein of the wing. In rare cases it is divided
into two.
The second axillary. — The second axillary (Fig. 66, 2 Ax) articulates
with the first axillary proximally and usually with the base of the
radius distally; it also articulates below with the wing-process of the
pleurum, constituting thus a sort of pivotal element.
The third axillary. — The third axillary (Fig. 66, 3 Ax) is interposed
between the bases of the anal veins and the fourth axillary when this
sclerite is present. When the fourth axillary is absent, as it is in
Fig. 66. — Diagram of a generalized wing and its articular sclerites (From
Snodgrass).
nearly all insects except Orthoptera and Hymenoptera, the third
axillary articulates directly with the posterior notal wing-process.
The fourth axillary. — When this sclerite is present it articulates
with the posterior notal wing -process proximally and with the third
axillary distally (Fig. 66, 4 Ax). Usually this sclerite is absent; it
occurs principally in Orthoptera and Hymenoptera.
The median plates. — The median plates of the -wing-joint are not
of constant shape and occurrence; when present, these plates are
associated with the bases of the media, the cubitus, and the first anal
vein when the latter is separated from the other anals. Often one of
them is fused with the third axillary and sometimes none of them are
present.
d. THE APPENDAGES OF THE THORAX
The appendages of the thorax are the organs of locomotion.
They consist of the legs and the wings. Of the former there are three
56
AN INTRODUCTION TO ENTOMOLOGY
,- c
pairs, a pair borne by each of the three thoracic segments'; of the
latter there are never more than two pairs, a pair borne by the meso-
thorax and a pair borne by the metathorax. One or both pairs of
wings may be wanting.
The legs. — Each leg consists of the following named parts and
their appendages: coxa, trochanter, femur, tibia, and tarsus.
The coxa. — The coxa is the proximal segment of the leg ; it is the
one by which the leg is articulated to the body (Fig. 67). The coxa
varies much in form, but it is usually a truncated cone or nearly
globular. In some insects the coxae of the third pair of legs are more
or less flattened and immovably attached to the metasternum; this
is the case in beetles of ths family Carabidae for example. In such
cases the coxae really form a part of the body-wall, and are liable to be
mistaken for primary parts of the metathorax instead of the proximal
segments of appendages.
In several of the orders of in ects the coxa is apparently composed
of two, more
or less dis-
tinct, parallel
parts; this is
the case, for
example, in in-
sects of the
trichopterous
genus Neuro-
nia (Fig. 68,
Cx and epm).
But it has
been shown
by Snodgrass
('09) that the
posterior part
of the sup-
posed double
coxa (Fig. 68,
epm) is a de-
tached por-
tion of the
cpimerum,the
katepimerum.
The styli — In certain generalized insects, as. Machilis of the order
B C
|Fig. 67. — Legs of insects: A, wasp; B , ichneumon-fly ; C,
bee; c, coxa; tr, trochanter; /, femur; ti, tibia; ta,
tarsus; m, metatarsus.
THE EXTERNAL ANATOMY OF INSECTS
57
Thysanura, the coxa of each middle and hind leg bears a small
appendage, the stylus (Fig. 69). The styli are of great interest as
they are believed to correspond to one of the two branches of the legs
of Crustacea; thus indicating that insects have descended from
forms in which the legs were biramous.
In several genera of the Thysanura one or more of the abdominal
segments bear each a pair of styli ; in Machilis they are found on the
second to the ninth abdominal segments. These styli are regarded as
vestiges of abdominal legs.
The trochanter. — The trochanter is the second part of the leg. It
consists usually of a very short, triangular or quadrangular segment,
between the coxa and the femur. Sometimes the femur appears to
articulate directly with the coxa ; and the trochanter to be merely an
appendage of the proximal end of the femur (e. g. Carabidae) . But
the fact is that in these insects, although the femur may touch the
coxa, it does not articulate with it; and the
organs that pass from the cavity of the coxa
to that of the femur must pass through the
trochanter. In some Hymenoptera the tro-
chanter consists of two segments (67, B).
The femur. — The femur is the third part of
the leg; and is usually the largest part. It
consists of a single segment.
The tibia. — The tibia is the fourth part of
the leg. It consists of a single segment; and
Fig. 68.— Lateral aspect is usually a little more slender than the femur,
of the mesothorax of although it often equals or exceeds it in length.
distal extremity is greatly broadened and
shaped more or less like a hand. Near the distal end of the tibia
there are in most insects one or more spurs, which are much larger
than the hairs and spines which arm the
leg; these are called the tibial spurs, and
are much used in classification.
The tarsus. — The tarsus is the fifth and
most distal part of the leg, that which is
popularly called the foot. It consists of a
series of segments, varying in number
from one to six. The most common num-
ber of segments in the tarsus is five.
In many insects, the first segment of the tarsus is much longer,
Fig. 69. — A leg of Machilis;
s, stylus.
58 AN INTRODUCTION TO ENTOMOLOGY
and sometimes much broader, than the other segments. In such
cases this segment is frequently designated as the metatarsus (Fig.
67, C, w).
In some insects the claws borne by the distal end of the tarsus are
outgrowths of a small terminal portion of the leg, the sixth segment
of the tarsus of some authors. This terminal part with its appendages
has received the name pr&tarsus (De Meijere '01). As a rule the
praetarsus is withdrawn into the fifth segment of the tarsus or is not
present as a distinct segment.
On the ventral surface of the segments of the tarsus in many
insects are cushion-like structures; these are called pulvtlli. The
cuticula of the pulvilli is traversed by numerous pores which open
either at the surface of the cuticula or through hollow hairs, the
tenent-hairs , and from which exudes an adhesive fluid that enables the
insect to walk on the lower surface of objects.
.With many insects (e. g. most Diptera) the distal segment of the
tarsus bears a pair of pulvilli, one beneath each claw. In sudh cases
there is frequently between these pulvilli a third single appendage of
similar structure; this is called the empodium; writers on the Orthop-
tera commonly called the appendage between the claws the arolium.
In other insects the empodium is bristle-like or altogether wanting.
In many insects the pulvillus of the distal segment of the tarsus
is a circular pad projecting between the tarsal claws. In many
descriptive works this is referred to as ike pulvillus , even though the
other pulvilli are well-developed. The pulvilli are called the onyckii
by some writers.
The claws borne at the tip of the tarsus are termed the tarsal claws
or ungues; they vary much in form; they are usually two in number,
but sometimes there is only one on each tarsus.
The wings. — The wings of insects are typically two pairs of mem-
branous appendages, one pair borne by the mesothorax and one pair
by the metathorax; prothoracic wings are unknown in living insects
but they existed in certain paleozoic forms.
Excepting in the subclass Apterygota which includes the
orders Thysanura and Collembola, wings are usually present in adult
insects. Their absence in the Apterygota is due to the fact that
they have not been evolved in this division of the class Hexapoda;
but when they are absent in adult members of the subclass Pterygo-
ta, which includes the other orders of insects, their absence is due
to a degradation, which has resulted in their loss.
THE EXTERNAL ANATOMY OF INSECTS 59
The loss of wings is often confined to one sex of a species; thus
with the canker-worm moths, for example, the females are wingless,
while the males have well-developed wings; on the other hand, with
the fig-insects, Blastophaga, the female is winged and the male
wingless.
Studies of the development of wings have shown that each wing is
a saclike fold of the body-wall; but in the fully developed wing, its
saclike nature is not obvious; the upper and lower walls become
closely applied throughout the greater part of their extent ; and since
they become very thin, they present the appearance of a single delicate
membrane. Along certain lines, however, the walls remain separate,
and are thickened, forming the firmer framework of the wing. These
thickened and hollow lines are termed the veins of the wing ; and their
arrangement is described as the venation of the wing.
The thin spaces of the wings which are bounded by veins are
called cells. When a cell is completely surrounded by veins it is said
to be closed; and when it extends to the margin of the wing it is said
to be open.
The different types of insect wings. — What may be regarded as the
typical form of insect wing is a nearly flat, delicate, membranous
appendage of the body, which is stiffened by the so-called wing-veins ;
but striking modifications of this form exist ; and to certain of them
distinctive names have been applied, as follows :
In the Coleoptera and in the Dermaptera, the front wings are
thickened and serve chiefly to protect the dorsal wall of the body and
the membranous hind wings, which are folded beneath them when
not in use. Front wings of this type are termed wing-covers or elytra.
The front wings of the Heteroptera, which are thickened at; the
base like elytra, are often desig-
nated the hemelytra.
The thickened .fore wings of
Qrthoptera are termed tezmina by
many wntersT"~
The hind wings of Diptera,
which are knobbed, thread-like
organs, are termed haltcres. The
hind wings of the males of the
family Coccidae are also thread-
Fig. 70. — Diagram of a wing showing like.
margins and angles. -_. ., - . . f , ,
The reduced front wings of the
Strepsiptera are known as the pseudo-halter es.
60
AN INTRODUCTION TO ENTOMOLOGY
The margins of wings. — Most insect wings- are more or less
triangular in outline; they, therefore, present three margins: the
costal margin or costa (Fig. 70, a-6); the outer margin (Fig. 70,
b-c)\ and the inner margin (Fig. 70, c-d).
The angles of wings. — The angle at the base of the costal margin
of a wing is the humeral angle (Fig. 70, a); that between the costal
margin and the outer margin is the apex of the wing (Fig. 70, 6);
Fig. 71. — Wing of Conopx; as, axillary excision; /, posterior lobe.
and that between the outer margin and the inner margin is the anal
angle (Fig. 70, c).
The axillary cord. — The posterior margin of the membrane at the
base of the wing is usually thickened and corrugated; this cord-like
structure is termed the axillary cord. The axillary cord normally
arises, on each side, from the posterior lateral angle of the notum, and
thus serves as a mark for determining the posterior limits of the
notum.
The axillary membrane. — The
membrane of the wing base is
termed the axillary membrane;
it extends from the tegula at the
base of the costal margin to the
axillary cord ; in it are found the
axillary sclerites.
The alula. — In certain families
of the Diptera and of the Coleop-
tera the axillary membrane is
expanded so as to form a lobe or
lobes which fold beneath the base of the wing when the wings are
closed; this part of the wing is the alula or alulet. The alulae are
termed the squama by some writers, and the calypteres by others.
Fig. 72. — Wings of the honeybee;
h, hamuli.
THE EXTERNAL ANATOMY OF INSECTS
61
The axillary excision. — In the wings of most Diptera and in the
wings of many other insects there is a notch in the inner margin of
the wing near its base (Fig. 71, ae), this is the axillary excision.
The posterior lobe of the wing. — That part of the wing lying between
the axillary excision when it exists, and the axillary membrane is the
posterior lobe of the wing. The posterior lobe of the wing and an alula
are easily differentiated as the alula is margined by the axillary cord.
The methods of uniting the two wings of each side. — It is obvious
that a provision for ensuring the synchronous action of the fore and
hind wings adds to their efficiency; it is as important that the two
pairs of wings should act as a unit as it is that the members of a boat's
crew should pull together. In many insects the synchronous action
of the wings is ensured by the fore wing overlapping the hind wing.
But in other insects special structures have been developed which
fasten together the two wings of each side. The different types of
these structures have received special names as follows:
The hamuli. — With certain insects the costal margin of the hind
wings bears a row of hooks, which fasten into a fold on the inner
margin of the fore wings (Fig. 72) ; these hooks are named the hamuli.
The frenulum and the frenulum hook. — In most moths there is a
strong spine-like organ or a
bunch of bristles borne by the
hind wing at the humeral
angle (Fig. 73,7); this is the
frenulum or little bridle. As a
rule the frenulum of the female
consists of several bristles ; that
of the male, of a single, strong,
spine-like organ. In the males
of certain moths, where the
frenulum is highly developed,
there is a membranous fold on
the fore wing for receiving the
end of the frenulum, this is the
frenulum hook (Fig. 73, fh).
The jugum. — In one family
of moths, the Hepialidas, the
posterior lobe of the fore wing ,
1 ., r. ... Fig. 73. — Wings of Thyndopteryx epSiemera-
1S a Slender, finger-like organ formis; /, frenulum; fh, frenulum hook.
which is stiffened by a branch
of the third anal vein, and which projects beneath the costal margin
of the hind wing. As the greater part of the inner margin of the fore
62
AN INTRODUCTION TO ENTOMOLOGY
wing overlaps the hind wing, the hind wing is held between the two
(Fig. 74). This type of the posterior lobe of the fore wing is termed
the jugum or yoke. The structure of the jugum is shown in Figure 7 5 .
The fibula. — In several groups of insects an organ has been
developed that serves to unite the fore and hind wings, but which
functions in a way quite different from that of the jugum. Like the
jugum it is found at the base of the fore wing; but unlike the jugum
it extends back above the base of the hind wing and is clasped over an
elevated part of the hind wing; this organ is the fibula or clasp.
In some insects, as in the Trichoptera, the fibula consists only of
a specialized posterior lobe of the fore wing; in others, as in the
genus Corydalus of the order Neuroptera, the proximal part of the
fibula is margined by the axillary cord, showing that the axillary
membrane enters into the composition of this organ (Fig. 76).
The hypothetical type of the primitive wing-venation. — A careful
study of the wings of many insects has shown that the fundamental
type of venation is the same in all of the orders of winged "insects.
But this fact is evident only when the more primitive or generalized
members of different orders are compared with each other. In most
of the orders of insects the greater number of species have become so
modified or specialized as
regards the structure of
their wings that it is diffi-
cult at first to trace out the
primitive type.
This agreement in the
important features of the
venation of the wings of
the generalized members of
the different orders of insects
is still more evident when
the wings of nymphs, naiads,
and pupae are studied. It
has been demonstrated that
in the development of wings
of generalized insects the
longitudinal wing-veins are
formed about preexisting
tracheae. In the develop-
ment of the wing, these
tracheae grow out into the
Fig. 74. — Wings of a hepialid, seen from
below; a, accessory vein.
wing-bud, and later the. wing-veins are formed about them.
THE EXTERNAL ANATOMY OF INSECTS
63
The wings of nymphs, naiads, and pupae are broad at the base,
and consequently the tracheae that precede the wing-veins are not
crowded together as are the wing-veins at the base of the wings of
Fig. 75- — Jugum of a hepialid.
Pig. 76. — Fibula of Corydalus.
adults. For this reason the identity of the wing- veins can be deter-
mined more surely in the wings of immature insects than they can be
in the wings of adults. This is especially true where two or more
veins coalesce in the adult wing while the tracheae that precede these
veins are distinctly separate in the immature wing.
A study was made of the tracheation of the wings of immature
insects of representatives of most of the orders of insects, and, assum-
ing that those features that are possessed by all of them must have
been inherited from a common ancestor, a diagram was made repre-
senting the hypothetical tracheation of a nymph of the primitive
winged insect (Fig. 77). In this diagram the tracheae are lettered
Fig> 77.— Hypothetical tracheation of a wing of the primitive nymph.
with the abbreviations used in designating the veins that are formed
about them in the course of the development of the wing. The dia-
gram will serve, therefore, to indicate the typical venation of an insect
64 AN INTRODUCTION TO ENTOMOLOGY
wing, except that the tracheae are not crowded together at the base of
the wing as are the veins in the wings of adults.*
Longitudinal veins and cross-veins. — The veins of the wing can be
grouped under two heads: first, longitudinal veins, those that
normally extend lengthwise the wing; and second, cross-veins, those
that normally extend in a transverse direction.
The insertion of the word normally in the above definitions is
important ; for it is only in comparatively generalized wings that the
direction of a vein can be depended upon for determining to which of
these two classes it belongs.
The principal wing-veins. — The longitudinalwing-vein s constitute
the principal framework of the wings. In the diagram representing
the typical venation of an insect wing (Fig. 77), only longitudinal
veins are indicated ; this is due to the fact that the diagram was based
on a study of the tracheation of wings, and in the more generalized
wings the cross-veins are not preceded by tracheae; moreover in the
wings of more generalized paleozoic insects there were no definite
cross-veins, but merely an irregular network of thickened lines
between the longitudinal veins.
There are eight principal veins; and of these the second, third,
fourth, and fifth are branched. The names of these veins and the
abbreviations by which they are known are as follows, beginning with
the one nearest the costal margin of the wing:
Names of veins Abbreviations
Costa . C
Subcosta Sc
Radius R
Media M
Cubitus Cu
First Anal ist A
Second Anal 2d A
Third Anal 3dA
The chief branches of the wing-veins. — The chief branches of the
principal veins are numbered, beginning with the branch nearest to
the costal margin of the wing. The term used to designate a branch
of a vein is formed by compounding the name of the vein with a
*For many details regarding the development of the wings of insects, their
structure, and the terminology of the wing-veins, that can not be included in
this work, see a volume by the writer entitled The Wings of Insects. This is
published by The Comstock Publishing Company, Ithaca, N. Y.
THE EXTERNAL ANATOMY OF INSECTS 65
numeral indicating the number of the branch ; thus, for example, the
first branch of the radius is radius-one or vein Ri.
In the case of radius and media, each of which has more than two
branches, each division of the vein that bears two or more branches
has received a special name. Thus after the separation of radius-one
from the main stem of radius there remains a division which is typi-
cally four-branched; this division is termed the radial sector, or
vein Rs; the first division of the radial sector, which later separates
into radius-two and radius-three, is designated as radius-two-plus-
three or vein 'R.z+s', and the second division is termed radius-four-
plus-five or vein R4+5- Media is typically separated into two divi-
sions, each of which is two-branched ; the first division is media-one-
plus two or vein Mi+2, the second is media-three-plus-four or vein
M3+4.
The veins of the anal area. — The three anal veins exhibit a wide
range of variation both as to their persistence and to_their form when
id A
Fig. 78. — A wing^f Rhyphus.
present. In those cases where the anal veins are branched there is
no indication that the branching has been derived from a uniform
primitive type of branching. For this reason in describing a branched
anal vein merely the number of branches is indicated.
In some cases, as in the Odonata, there is a single anal vein the
identity of which can not be determined. In such cases this vein is
designated merely as the anal vein or vein A, and its branches as AI,
Az, A3, etc.
The reduction of the number of wing-veins. — In many wings the
number of the veins is less than it is in the hypothetical type. In
some cases this is due to the fact that one or more veins have faded
out in the course of the evolution of the insects showing this deficiency;
frequently in such wings vestiges of the lacking veins remain, either
as faint lines in the positions formerly occupied by the veins or as
66 AN INTRODUCTION TO ENTOMOLOGY
short fragments of the veins. A much more common way in which
the number of veins has been reduced is by the coalescence of adja-
cent veins. In many wings the basal parts of two or more principal
veins are united so as to appear as a single vein ; and the number of
the branches of a vein has been reduced in very many cases by two or
more branches becoming united throughout their entire length.
When a vein consists of two or more of the primitive veins united,
the name applied to the compound vein should indicate this fact. In
the wing of Rhyphus (Fig. 78), for example, radius is only three-
branched; but it would be misleading to designate these branches as
Ri, R2, and R3, for this would indicate that veins R4 and R5 are lacking.
The first branch is evidently RI ; the second branch is composed of the
Fig- 79- — A wing of Tabanus.
coalesced R2 and Ra, it is, therefore, designated as R2+3; and the
third branch, which consists of the coalesced R4 and R5, is designated
as R4+5.
A second method of coalescence of veins is illustrated by a wing of
Tabanus (Fig. 79). In this wing the tips of cubitus-two and the
second anal vein are united ; here the coalescence began at the margin
of the wing and is progressing towards the base. The united portions
of the two veins are designated as 2d A+Cu2.
When it is desired to indicate the composition of a compound
vein it can be readily done by combining the terms indicating its
elements. But in descriptions of hymenopterous wings where a
compound vein may be formed by the coalescence of several veins the
logical carrying out of this plan would result in a very cumbersome
terminology, one that it is impracticable to use in ordinary descrip-
tions. In such cases the compound vein is designated by the term
indicating its most obvious element. Thus, for example, in the fore
THE EXTERNAL ANATOMY OF INSECTS
67
wing of Pamphilius, where veins M4, Cui, and Cu2 coalesce with the
first anal vein, the united tips of these veins is designated as vein ist A,
the first anal vein being its most obvious element (Fig. 80), although
it is really vein M4+Cui+Cti2+ist A.
Serial veins. — In the wings of some insects, where the wing-vena
tion has been greatly modified, as in certain Hymenoptera, there exist
what appears to be simple veins that in reality are compound veins
composed of sections of two or more veins joined end to end with no
indication of the point of union. Compound veins formed in this
Fig. 80. — Wings of Pamphilius.
manner are termed serial veins. Examples of wings in which there ar e
serial veins are figured in the chapter treating of the Hymenoptera.
In designating serial veins either the sign & or a dash is used
between the terms indicating the elements of the vein, instead of the
sign + as the latter is used in designating compound veins formed by
the coalescence of veins side by side. If the serial vein consists of
only two elements the sign & is used; thus the serial vein in the wings
of braconids, which consists of the medial cross-vein and vein M2, is
designated as m & M2.
In those cases where sections of several veins enter into the com-
position of a serial vein, the serial vein is designated by the abbrevia-
tion of the name of the basal element connected by a dash with the
68
AN INTRODUCTION TO ENTOMOLOGY
abbreviation of the name of the terminal element. Thus a serial
vein, the basal element of which is the cubitus and the terminal ele-
ment vein MI, is designated as vein Cu — MI. A serial vein thus
formed exists in the hind wings of certain ichneumon flies.
The increase of the number of wing-veins. In the wings of many
insects the number of veins is greater than it is in the hypothetical
type. This multiplication of veins is due either to an increase in the
Fig. 81. — Wings of Osmylus hyalinatus.
number of the branches of the principal veins by the addition of
secondary branches, termed accessory veins, or to the development of
secondary longitudinal veins between these branches, termed inter-
calary veins. In no case is there an increase in the number of principal
veins.
The accessory veins. — The wings of Osmylus (Fig. 81) are an exam-
ple of wings in which accessory veins have been developed; here the
radial sector bears many more branches than the typical number;
those branches that are regarded as the primitive branches are
lettered Rb R2, R3, R4, and R5 respectively (Fig. 82); the other
THE EXTERNAL ANATOMY OF INSECTS 69
branches are the secondarily developed accessory veins. Two types
of accessory veins are recognized the marginal accessory veins and
the definitive accessory veins.
The marginal accessory veins are twig-like branches that are the
result of bifurcations of veins that have not extended far back from
the margin of the wing; many such short branches of veins exist in
the -wings of Osmylus (Fig. 81). The number and position of the
marginal accessory veins are not constant, differing in the wings of
the two sides of the same individual.
The definitive accessory veins differ from the marginal accessory
Fig. 82. — Base of fore wing shown in Figure 81.
veins in having attained a position that is comparable in stability to
that of the primitive branches of the principal veins.
In those cases where the accessory veins are believed to have been
developed in regular order they are designated by the addition of a
letter to the abbreviation of the name of the vein that bears them;
thus if vein R2 bears three accessory veins they are designated as
veins R2a, R2b, and R2c, respectively.
The intercalary veins. — The intercalary veins are secondarily
developed longitudinal veins that did not arise as branches of the
primitive veins, but were developed in each case as a thickened fold in
a corrugated wing, more or less nearly midway between two pre-
existing veins, with which primarily it was connected only by cross-
veins. Excellent examples of unmodified intercalary veins are com-
70
AN INTRODUCTION TO ENTOMOLOGY
Fig. 83.— Wing of a May-fly (After Morgan).
mon in the Ephemerida, where most of the intercalary veins remain
distinct from the veins between which they were developed, being
connected with
them only by
cross-veins, the
proximal end of
the intercalary
vein being free
(Fig. 83).
When it is
desirable to re-
fer to a parti-
cular interca-
lary vein it can
be done by combining the initial /, indicating intercalary, with the
designation of the area of the wing in which the intercalary vein occurs.
For example, in the wings of most May-flies there is an intercalary
vein between veins Cui and Cu2, i e. in the area Cui ; this intercalary
vein is desig-
nated as ICui.
The adven-
titious veins. —
In certain in-
sects there are
secon dary
veins that are
neither acces-
sory veins nor
intercalary
veins as de-
fined above;
these are
termed adven-
titious veins.
Examples of
these are the
supplements of
the wings of
Fig. 84. — Wings of Prionoxystus.
certain Odonata and the spurious vein of the Syrphidas.
The anastomosis' of veins. — The typical arrangement of wing-veins
is of tea modified by an anastomosis of adjacent veins; that is, two
.
THE EXTERNAL ANATOMY OF INSECTS
71
veins will come together at some point more or less remote from their
extremities and merge into one for a greater or less distance, while
their extremities remain separate. In the fore wing of Prionoxystus
(Fig. 84), for example, there is an anastomosis of veins R3 and R4+5.
The named cross-veins. — In the wings of certain insects, as the
dragon-flies, May-flies, and others, there are many cross- veins; it is
impracticable in cases of this kind to name them. But in several of
the orders of insects there are only a few cross-veins, and these have
been named. Figure 85 represents the hypothetical primitive type
2dA
Fig. 85. — The hypothetical primitive type of wing- venation with the named
cross-veins added.
of wing-venation with the named cross-veins added in the positions in
which they normally occur ; these are the following :
' The humeral cross-vein (h) extends from the subcosta to costa near
the humeral angle of the wing.
^The radial cross-vein (r) extends between the two principal divi-
sions of radius, i. e. from vein RI to vein Rs.
*} The sectorial cross-vein (s) extends between the principal divisions
of the radial sector — i. e., from vein R2+3 to vein R4+5 or from vein
RS to vein R4.
d The radio-medial cross-vein (r — m) extends from radius to media,
usually near the center of the wing. When in its typical position
this cross- vein extends from vein R4+5 to vein MI +2.
The medial cross-vein (m) extends from vein M2 to vein M3. This
cross-vein divides cell M2 into cells, ist M2 and 26. M2; see Figure 87
where the cells are lettered.
The medio-cubital cross-vein (m — cu) extends from media to
cubit us.
72
AN INTRODUCTION TO ENTOMOLOGY
The arculus. — In many insects there is what appears to be a cross-
vein extending from the radius to the cubitus near the base of the
wing; this is the arculus. The arculus is designated in figures of
wings by the abbreviation ar. Usually when the arculus is present
the media appears to arise from it; the fact is, the arculus is com-
pound, being composed of a section of media and a cross-vein.
Figure 86 is a dia-
gram representing
the typical struc-
ture of the arculus.
That part of the
arculus which is a
R+M
Fig. 86. — Diagram of an arculus of a dragon-fly.
section of media is
designated as the
anterior arculus (aa)
and that part formed by a cross- vein, the posterior arculus (pa) .
The terminology of the cells of the wing. — Each cell of the wing is
designated by the name of the vein that normally forms its front
margin when the wings are spread. See Figure 87 where both the
veins and the cells of the wing are lettered.
The cells of the wing fall naturally into two groups: first, those
on the basal part of the wing ; and second, those nearer the distal end
of the wing. The former are bounded by the stems of the principal
veins, the latter, by the branches of these veins; a corresponding
distinction is made in designating the cells. Thus a cell lying behind
the main stem of radius and in the basal part of the wing is designated
as cell R; while a cell lying behind radius-one is designated as cell J?i.
Fig. 87. — A wing of Rhyphus.
It should be remembered that the coalescence of two veins results
in the obliteration of the cell that was between them. Thus when
THE EXTERNAL ANATOMY OF INSECTS 73
veins R<L and ^3 coalesce, as in the wings of Rhyphus (Fig. 87), the cell
lying behind vein 7?2+3 is cell ^3, and not cell ^2+3, cell R2 having been
obliterated.
When one of the principal cells is divided into two or more parts by
one or more cross-veins, the parts may be numbered, beginning with
the proximal one. Thus in Rhyphus (Fig. 87), cell M2 is divided by
the medial cross-vein into cell istMz and cell 2dM2.
When two or more cells are united by the atrophy of the vein or
veins separating them, the compound cell thus formed is designated
by a combination of the terms applied to the elements of the com-
pound cell. When, for example, the stem of media is atrophied, the
cell resulting from the combination of cells R and M is designated as
cell R+M.
The application of this systenTof naming the cells of the wing is an
easy matter in those orders where there are but few cross- veins ; but
in those orders where there are many cross-veins it is not practicable
to apply it. In the latter case we have to do with areas of the wing
rather than with separate cells. These areas are designated as are the
cells of the few- veined wings with which they correspond; thus the
area immediately behind vein R2 is area R2.
The corrugations of the wings. — The wings of comparatively few
insects present a flat surface ; in most cases the membrane is thrown
into a series of folds or corrugations. This corrugating of the wing in
some cases adds greatly to its strength, as in the wings of dragon-flies;
in other cases the corrugations are the result of a folding of the wing
when not in use, as in the anal area when this part is broadly ex-
panded.
It rarely happens that there is occasion to refer to individual
members of either of these classes of folds, except perhaps the one
between the costa and the radius, which is the subcostal fold and that
which is normally between the cubitus and the first anal vein, the
'cubito-anal fold.
Convex and concave veins. — When the wings are corrugated, the
wing-veins that follow the crests of ridges are termed convex veins;
and those that follow the furrows, concave veins.
The furrows of the wing. — There are found in the wings of many
insects one or more suture-like grooves in the membrane of the wing;
these are termed the furrows of the wing. The more important of
these furrows are the four following:
The anal furrow when present is usually developed in the cubito
anal fold; but in the Heteroptera it is found in front of the cubitus.
74 AN INTRODUCTION TO ENTOMOLOGY
The median furrow is usually between radius and media.
The nodal furrow is a transverse suture beginning at a point in the
costal margin of the wing corresponding to the nodus of the Odonata
and extending towards the inner margin of the wing across a varying
number of veins in the different orders of insects.
The axillary furrow is a line that serves as a hinge which facilitates
the folding of the posterior lobe of the wing of many insects under that
part of the wing
in front of it.
The bulkz. —
The bulla are
weakened places
in veins of the
wing where they
are crossed by
furrows.. The
bullae are usually
Fig. 88. — Wings of Myrmecia: b, b. b. bullae. « . *
paler in color
than the other portions of the wing; they are common in the wings
of the Hymenoptera (Fig. 88), and of some other insects.
The ambient vein. — Sometimes the entire margin of the wing is
stiffened by a vein-like structure; this is known as the ambient vein.
The humeral veins. — In certain Lepidoptera and especially in the
Lasiocampidae, the humeral area of the hind wings is greatly expanded
and in many cases is strengthened by the development of secondary
veins. These are termed the humeral veins.
The pterostigma or stigma. — A thickened, opaque spot which
exists near the costalmargin of the outer part of the wing^jn many
insects is known as the pterostigma or stigma.
The epipleurcs. — A part of the outer margin of the elytra of beetles
when turned down on the side of the thorax is termed the epipleura.
The discal cell and the discal vein. — The term discal cell is applied
to a large cell which is situated near the center of the wing; and the
term discal vein, to the vein or series of veins that limits the outer end
of the discal cell. These terms are not a part of the uniform terminol-
ogy used in this book, and can not be made so, being applied to
different parts of the wing by writers on different orders of insects.
They are included here as they are frequently used, as a matter of
convenience, by those who have adopted the uniform terminology.
The discal cell of the Lepidoptera is cell R+M+lstM2; that of the
Dipcera is cell ist M2; and that of the Trichoptera is cell R2+3-
THE EXTERNAL ANATOMY OF INSECTS 75
The anal area and the preanal area of the wing. — In descriptions of
wings it is frequently necessary to refer to that part of the wing
supported by the anal veins; this is designated as the anal area of the
wing; and that part lying in front of the anal area, including all of
the wing except the anal area, is termed the preanal area.
IV. THE ABDOMEN
a. THE SEGMENTS OF THE ABDOMEN
The third and terminal region of the body, the abdomen, consists
of a series of approximately similar segments, which as a rule are
without appendages excepting certain segments near the caudal end
of the body.
The body-wall of an abdominal segment is usually comparatively
simple, consisting in adults of a tergum and a sternum, united by
lateral conjunctivas. Sometimes there are one or two small sclerites
on each lateral aspect of a segment; these are probably reduced
pleura.
The number of segments of which the abdomen appears to be
composed- varies greatly in different insects. In the cuckoo-flies
(Chrysididae) there are usually only three or four visible; while in
many insects ten or eleven can be distinguished. All intergrades
between these extremes occur.
The apparent variation in the number of abdominal segments is
due to two causes: in some cases, some of the segments are tele-
scoped ; and in others, adjacent segments coalesce, so that two or more
segments appear as one.
A study of embryos of insects has shown that the abdomen con-
sists typically of eleven segments; although this number may be
reduced during the development of the insect by the coalescence of
adjacent segments.
In some insects there is what appears to be a segment caudad of
the eleventh segment; this is termed the telson. The telson differs
from the segments preceding it in that it never bears appendages.
Special terms have been applied, especially by writers on the
Coleoptera, to the caudal segments of the abdomen. Thus the
terminal segment of a beetle's abdomen when exposed beyond the
elytra is termed the pygidium; the tergite cephalad of the pygidium,
especially in beetles with short elytra, the propygidium; and the last
abdominal sternite, the hypopygium. The term hypopygium is also
applied to the genitalia of male Diptera by writers on that onler of
insects.
76
AN INTRODUCTION TO ENTOMOLOGY
b. THE APPENDAGES OF THE ABDOMEN
In the early embryonic stages of insects, each segment of the
abdomen, except the telson,. bears a pair of appendages (Fig. 89) . This
indicates that the primitive ancestor of insects possessed many legs,
like a centipede. But the appendages of the first
seven abdominal segments are usually lost during
embryonic life, these segments being without appen-
dages in postembryonic stages, except in certain
Thysanura and Collembola, and in some larvae.
Reference is made here merely to the primary
appendages of the segments, those that are homodyna-
mous with the thoracic legs; secondarily developed
appendages, as for example, the tracheal gills, are
present in the immature instars of many insects.
The styli or vestigial legs of certain Thysanura. — In
certain Thysanura the coxa of each middle and hind
thoracic leg bears a small appendage, the stylus (Fig. 90) ;
and on from one to nine abdominal segments there is
a pair of similar styli. These abdominal • styli are
believed to be homodynamous with those of the thoracic
legs, and must, therefore, be regarded as vestiges of
abdominal legs.
The collophore of the Collembola. — Although in the
postembryonic stages of Collembola the collophore is
an unpaired organ on the middle line of the ventral aspect of the first
abdominal segment, the fact that it arises in the embryo as a pair of
appendages comparable in position to the thoracic legs, has led to the
belief that it represents the legs of this segment. The structure of
the collophore is described more fully later in the chapter treating of
the Collembola.
The spring of the Collembola. — The spring of the Collembola,
like the collophore, is believed to represent a pair of primary append-
ages. This organ is discussed in the chapter treating of the Col-
lembola.
The genitalia. — In most insects there are more or less prominent
appendages connected with the reproductive organs. These append-
ages constitute in males the genital claspers and in females the ovi-
positor; to them have been applied the general term genitalia, they
are also known as the gonapophyses.
The genitalia, when all are developed consist of three pairs of
appendages. Writers vary greatly in their views regarding the seg-
Fig. Sg.-Era-
bryo of Hy-
drophilus
showing ab-
dominal ap-
pendages.
THE EXTERNAL ANATOMY OF INSECTS
77
ments of the abdomen to which these appendages belong. One cause
of difference is that some writers regard the last segment of the abdo-
men as the tenth abdominal
segment while others believe it
to.be the eleventh. This seg-
ment bears the cerci when they
are present. The genitalia are
borne either by the two or the
three segments immediately
preceding the last. If the last
segment is the eleventh the
genitalia are, according to one
view, the appendages of the
eighth, ninth, and tenth seg-
ments; according to another
view, they are tjie appendages
of the ninth and tenth seg-
ments, those of the tenth seg-
ment being doubled.
The genitalia of many in-
sects have been carefully fig-'
ured and described and special
terms have been applied to
each of the parts. But as most
of these descriptions have been
based upon studies of repre-
sentatives of a single order of
insects or even of some smaller
group, there is a great lack
-m/
Fig. 90. — Ventral aspect of Machilis; c.cer- of uniformity in the terms
filament; mp, maxillary palpus; o, oviposi- aPPlied to homologous parts
tor; s, s, styli. That part of the figure
representing the abdomen is after Oude-
mans.
cus; Ip, labial palpus; mf, median caudal
in the different orders of in-
sects; such of these terms as
are commonly used are defined
later in the characterizations of the several orders of insects.
The cerci. — In many insects there is a pair of caudal appendages
which are known as the cerci; these are the appendages of the
eleventh abdominal segment, the last segment of the body except in
the few cases where a telson is present.
The cerci vary greatly in form; in some insects, as in most Thy-
sanura, in the Plecoptera, and in the Ephermerida, they are long and
78
AN INTRODUCTION TO ENTOMOLOGY
many jointed; while in others they are short and not segmented.
The function of the cerci is different in different insects; they are
believed to be tactile in some, olfactory in others,
and in some males they aid in holding the female
during copulation.
The median caudal filament. — In many of the
Ephemerida and in some of the Thysanura, the last
abdominal segment bears a long, median filament,
which resembles the many-jointed cerci of these
insects (Fig. 91); this filament is believed to be a
prolongation of the tergum of this segment and not a
true appendage like the cerci.
The prolegs of larvae. — The question whether the
prolegs of larvae represent true appendages or are
merely hypodermal outgrowths has been much dis-
cussed. Several embryologists have shown that in
embryos of Lepidoptera and of saw-flies limb-rudi-
ments appear on all or most of the abdominal seg-
ments ; and that they very soon disappear on those
segments which in the larva have no legs while on other segments
they are transferred into functional prolegs. If this view is estab-
lished we must regard such prolegs as representing primitive abdo-
minal appendages, that is as true abdominal legs.
Fig. 91. — Lepis-
ma saccharina.
V. THE MUSIC AND THE MUSICAL ORGANS
OF INSECTS
Much has been written about music; but the greater part of this
literature refers to music made by man for human ears. Man, how-
ever, is only one of many musical animals; and, although he excels
all others in musical accomplishments, a study of what is done by our
humbler relatives is not without interest.
The songs of birds command the attention of all observers. But
there is a great orchestra which is performing constantly through the
warmer portions of the year, which is almost unnoticed by man.
Occasionally there is a performer that cannot be ignored, as: —
"The shy Cicada, whose noon-voice rings
So piercing shrill that it almost stings
The sense of hearing." (ELIZABETH AKERS.)
But the great majority fiddle or drum away unnoticed by human ears.
THE EXTERNAL ANATOMY OF INSECTS 79
Musical sounds are produced by many different insects, and in
various ways. These sounds are commonly referred to as the songs of
insects ; but properly speaking few if any insects sing ; for, with some
possible exceptions, the note of an insect is always at one pitch, lacking
musical modulations like those of the songs of man and of birds.
The sound produced by an insect may be a prolonged note, or it
may consist of a series of short notes of varying length, with intervals
of rest of varying lengths. These variations with differences in pitch
give the wide range of insect calls that exists.
In some cicadas where the chambers containing the musical organs
are covered by opercula, the insect can give its call a rhythmic
increase and decrease of loudness, by opening and closing these
chambers. <
As most insect calls are strident, organs specialized for the pro-
duction of these calls are commonly known as stridulating organs.
But many sounds of insects are produced without the aid of organs
specialized for the production of sound. The various ways in which
insects produce sounds can be grouped under the following heads :
First. — By striking blows with some part of the body upon sur-
rounding objects.
Second. — By rapid movements of the wings. In this way is
produced what may be termed the music of flight.
Third. — By rasping one hard part of the body upon another.
Under this head fall the greater number of stridulating organs.
Fourth. — By the rapid vibration of a membrane moved by a muscle
attached to it. This is the type found in the cicadas.
Fifth. — By the vibration of membranes set in motion by th-^ rush
of air through spiracles. The reality of this method has been ques-
tioned.
Sixth. — By rapid changes of the outline of the thorax due to the
action of the wing muscles.
a. SOUNDS PRODUCED BY STRIKING OBJECTS OUTSIDE THE BODY
Although the sounds produced by insects by striking blows with
some part of the body upon surrounding objects are not rapid enough
to give a musical note, they are referred to here for the sake of
completeness.
The most familiar sounds of this kind are those produced by the
insects known as the death-watch. These are small beetles of the
family Ptinidas, and especially those of the genus Anobium. These
are wood-boring insects, frequently found in the woodwork of old
80 • AN INTRODUCTION TO ENTOMOLOGY
houses and in furniture, where they make a ticking sound by striking
their heads against the walls of their burrows. The sound consists of
several, sharp, distinct ticks, followed by an interval of silence, and is
believed to be a sexual call.
The name death-watch was applied to these insects by supersti-
tious people who believed that it presaged the death of some person
in the house where it is heard. This belief probably arose from the
fact that the sound is most likely to be heard in the quiet of the night,
and would consequently be observed by watchers by sick-beds.
The name death-watch has also been applied to some species of the
Psocidae, Clothilla pulsatoria and Atropos dimnatoria, which have been
believed to make a ticking sound. This, however, is doubted by
some writers, who urge that it is difficult to believe that such minute
and soft insects can produce sounds audible to human ears.
The death-watches produce their sounds individually ; , but an
interesting example of an insect chorus is cited by Sharp ('99, p. 156),
who, quoting a Mr. Peal, states that an ant, presumably an Assamese
species, "makes a concerted noise loud enough to be heard by a human
being at twenty or thirty feet distance, the sound being produced by
each ant scraping the horny apex of the abdomen three times in rapid
succession on the dry, crisp leaves of which the nest is usually com-
posed."
b. THE MUSIC OF FLIGHT
The most obvious method by which insects produce sounds is by
beating the air with their wings during flight. It can be readily seen
that if the wing-strokes are sufficiently rapid and are uniform, they
will produce, like the flapping reeds of a mouth organ, a musical note.
When, however, we take into account the fact that to produce the
lowest note regularly employed in music, the C of the lowest octave,
requires 32 vibrations a second, i. e,, nearly 2,000 vibrations per'
minute, it will seem marvellous that muscular action can be rapid
enough to produce musical notes. Nevertheless, it is a fact that
many insects sing in this way; and too their notes are not confined to
the lower octaves. For example, the common house fly hums F of
the middle octave, to produce which, it must vibrate its wings 345
times per second or 20,700 times per minute.
As a rule, the note produced by the wings is constant in each
species of insect. Still with insects, as with us, the physical condition
of the singer has its influence. The vigorous honey-bee makes the A
of 435 vibrations, while the tired one hums on the E of 326 vibrations.
THE EXTERNAL ANATOMY OF INSECTS 81
While it is only necessary to determine the note produced by
vibrating wings to ascertain the rate of vibration, a graphical demon-
stration of the rate is more convincing. Such a demonstration has
been made by Marey ('69) who fixed a fly so that the tip of the wing
just touched the smoked surface of a revolving cylinder, and thus
obtained a wavy line, showing that there were actually 320 strokes in
a second. This agrees almost exactly with the number inferred from
the note produced.
The music of flight may be, in many cases, a mere accidental result
of the rapid movement, and in no sense the object of that movement,
like the hum of a trolley car ; but there are cases where the song seems
to be the object of the movement. The honeybee produces different
sounds, which can be understood by man, and probably by bees, as
indicating different conditions. The contented hum of the worker
collecting nectar may be a song, like the well-known song of a hen
wandering about on a pleasant day, or may be an accidental sound.
But the honeybee produces other sounds that communicate ideas.
The swarming sound, the hum of the queenless colony, and the note
of anger of a belligerent bee can be easily distinguished by the experi-
enced beekeeper, and doubtless also by the bee colony. It seems
probable, therefore, that in each of these cases the rate of vibration of
the wings is adjusted so as to produce a desired note. This is also
probably true of the song of the female mosquito, which is pitched so
as to set the antennal hairs of the male in vibration.
While the music of flight is a common phenomenon, many insects
have a silent flight on account of the slowness of the wing-movement.
C. STRIDULATING ORGANS OF THE RASPING TYPE
The greater number of the insect sounds that attract our attention
are produced by the friction of hard parts of the cuticula by which a
vibrating surface is set in motion. In some eases, as in many of the
Orthoptera, the vibrating surface is a part of the wings that is special-
ized for this purpose; but in other cases, a specialized vibrating sur-
face has not been observed.
Stridulating organs of the rasping type are possessed by represen-
tatives of several of the orders of insects ; but they are most common
in the order Orthoptera, and especially in the families Acridiidae,
Locustidas, and Gryllidae, where the males of very many species
possess them. Very few other Orthoptera stridulate; and with few
exceptions it is only the males that sing.
82
AN INTRODUCTION TO ENTOMOLOGY
In each of these families the vibrating element of the stridulating
organ is a portion of one or of both of the fore wings ; but this is set in
motion in several different ways. In some exotic Acridiidae abdominal
stridulating organs exist.
The stridulating organs of the Acridiidae. — With many species of
the Acridiidse we find the males furnished with stridulating organs;
but these are comparatively simple, and are used only in the day time.
Two methods of stridulation are used by members of this family.
The simpler of these two methods is employed by several common
species belonging to the (Edipodinae; one of which is the Carolina
locust, Dissosteira Carolina, whose crackling flight is a common feature
of country roadsides. These locusts, as they fly, rub the upper sur-
face of the costal margin of the hind wings upon the lower surface of
the thickened veins of the fore wings, and thus produce a loud but not
musical sound.
The second method of stridulation practiced by locusts consists
in rubbing the inner surface of the hind femora, upon each* 'of which
there is a series of bead-like prominences (Fig. 92), against the outer
surface of the fore wings.
With these insects, there is a
thickening of the radius in the
basal third of each fore wing,
and a widening of the two
areas between this vein and
the costal margin of the wing,
which serves as a sounding
board (Fig. 93). The two
wings and femora constitute a
pair of violin-like organs; the thickened radius in each case cor-
responding to the strings; the membrane of the wing, to the body
of the instrument ; and the file of the femur, to the bow. These two
organs are used simultaneously. When about to stridulate, the insect
Fig. 92. — A, hind femora of Stenobothrus;
B, file greatly enlarged.
ig- 93- — Fore wing of a male of Stenobothrus. R, radius; Sc, subcosta;
C, costa.
THE EXTERNAL ANATOMY OF INSECTS
83
places itself in a nearly horizontal position, and raising both hind legs
at once rasps the femora against the outer surface of the wings. The
most common representatives of insects that
stridulate in this way belong to the genus Steno-
bothrus.
The stridulating organs of the Gryllidae and
the Locustidae. — The stridulating organs of the
Gryllidae and the Locustidas are of the same type,
and are the most highly specialized found in the
Orthoptera. They consist of modified portions of
the fore wings ; both the vibrating and the rasping
elements of the organs pertaining to the wings.
It is by rubbing the two fore wings together
that sound is produced.
In what is probably the more generalized con-
dition of the organs, as seen in Gryllus, each
fore wing bears a rasping organ, the file (Fig.
94, /) a hardened area, the scraper (Fig. 94, s),
against which the file of the other wing acts, and
vibrating areas, the tympana (Fig. 94, t, t). As
the file of either wing can be used to set the
tympana of the wings in vibration, we may say
Fig. 94.— Fore wing of that Gryllus is ambidextrous.
ftSm^above?9 that When the cricket wishes to make his call, he
part of the wing elevates his fore wings so that they make an angle
oVCthe1Sside1 of °the of about forty-five degrees with the body; then
abdomen is not holding them in such a position that the scraper
? t°y^ana?CrB!base of one rests on the file of the other, he moves the
of wing seen 'from wings back and forth laterally, so that the file and
JUST' C.'fileTreat- scraper rasp upon each other. This throws the
Ly enlarged. wings into vibration and produces the call.
It is easy to observe the chirping of crickets. If one will move
slowly towards a cricket that is making his call, and stop when the
cricket stops chirping until he gains confidence and begins again,
one can get sufficiently near to see the operation clearly. This can
be done either in the day time or at night with the aid of a light.
The songs of the different genera of crickets can be easily dis-
tinguished, and that of each species, with more care. Writers on the
Orthoptera have carefully described the songs of our more common
crickets, and especially those of the tree crickets . The rate of chirping
84
AN INTRODUCTION TO ENTOMOLOGY
is often influenced by temperature, being slower in cool nights than
in warm ones; and becoming slower towards morning if the tem-
perature falls.
In certain genera of crickets as Nemobius and (Ecanthus, while
each fore wing is furnished with a file and tympana, the scraper of the
right wing is poorly formed and evidently not functional. As these
insects use only the file of the right wing to set the tympana of the
wings in vibration, they may be said to be right-handed.
Fig. 95- — Wings of a female nymph of (Ecanthus (From Comstock and
Needham).
In the Locustidae a similar modification of the function of the
stridulating organs has taken place. In all of our common represen-
tatives of the family, at least, only one of the files is used. But in
these cases it is the file of the left wing that is functional ; we may say,
therefore, that so far as observed the Locustidae are left-handed.
Different genera exhibit great differences as to the extent of the reduc-
tion of the unused parts of the stridulating organs. The file is
present in both wings of all of the forms that I have studied; but the
unused file is sometimes in a vestigial condition. The scraper is less
persistent, being frequently entirely lacking in one of the wings. In
some cases, the tympana of one wing have been lost; but in others
the tympana of both wings are well preserved, although only one file
THE EXTERNAL ANATOMY OF INSECTS 86
is used. In these cases it is probable that the tympana of both wings
are set in vibration by the action of the single functional file.
The determination of the homologies of the parts of the wing that
enter into the composition of the stridulating organs was accomplished
by a study of the tracheation of the wings of nymphs (Comstock and
Needham, *98-'99) . The results obtained by a study of the wings of
CEcanthus will serve as an illustration.
Figure 95 represents the wings of a female nymph of this genus,
with the tracheae lettered. The only parts to which we need to give
attention in this discussion are the cubital and anal areas of the fore
wing; for it is this part of the wing that is modified in the male to
form the musical organ. Both branches of cubitus are present, and
Cui bears three accessory branches. The three anal tracheae are
present and are quite simple.
Fig. 96. — Fore wing of a male nymph of CEcanthus (From Comstock and
Needham).
The homologies of the tracheae of the fore wing of a male nymph,
Figure 96, were easily determined by a comparison with the tracheae
of the female. The most striking difference between the two sexes
is a great expanding of the area between the two branches of cubitus
in the male, brought about by the bending back of the basal part
of Cu2.
The next step in this study was to compare the wing of an adult
male, Figure 97, with that of the nymph of the same sex; and the
solution of the problem was soon reached. It can be easily seen that
the file is on that part of Cu2 that is bent back toward the inner mar-
gin of the wing (Fig. 97, /); the tympana are formed between the
branches of cubitus (Fig. 97, /, t)\ and the scraper is formed at the
outer end of the anal area (Fig. 97,5).
86
AN INTRODUCTION TO ENTOMOLOGY
A similar study was made of the wings of Conocephalus, as an
example of the Locustidse. Figure 98 represents the wings of a male
nymph ; and
Figure 99 the
fore wing of
an adult. The
most striking
feature, and
one character-
istic of the
family, is that
the musical
organ occupies
an area near
the base of
the wing which
Fig. 97. — Fore wing of an adult male of (Ecanthus; /, vein :0 c 'Oii ^^
bearing the file; s, scraper; t,t, tympana.
pared with
the area occupied by the musical organs of the Gryllidse. But
here, as in the Gryllidas, the file is borne by the basal part of Cu2, the
Fig. 98. — Wings of a male nymph of Conocephalus, (From Comstock and
Needham).
tympana are formed between the branches of cubitus, and the scraper
is formed at the outer end of the anal area.
THE EXTERNAL ANATOMY OF INSECTS
87
Rasping organs of other than orthopterous insects. — Rasping
organs are found in many other than orthopterous insects and vary
M
Fig- 99- — Right fore wing of an adult male of Conocephalus, seen
from below; /, file; s, scraper.
greatly in form and in their location on the body. Lack of space for-
bids any attempt to enumerate these variations here ; but examples of
various types of stridulating organs will be described in later chapters
when treating of the insects that possess them. As in the Orthoptera,
they consist of a rasp and a scraper. The rasp is a file-like area of the
surface of a segment of the body or of an appendage; and the scraper
is a hard ridge or point so situated that it can be drawn across the rasp
by movements
of the body or
of an append-
age. In some
cases the ap-
paratus con-
sists of two
rasps so situ-
ated that they
can be rubbed
together.
With many
beetles one of
Fig. 100. — Stridulating organ of an ant, Myrmica rubra
-(From Sharp after Janet); d, scraper; e, file.
the two parts of the stridulating organ is situated upon the elytra ;
and it is quite probable that in these cases the elytra acts as vibrating
surfaces, as do the wings of locusts and crickets. But in many
•cases as where a part of a leg is rubbed against a portion of a
thoracic segment, there appears to be no vibrating surface unless it is
the wall of the body or of the appendage that acts as a sounding
board. In the stridulating organ of Myrmica rubra, var. Icevinodis,
figured by Janet (Fig. 100), the scraper is the posterior border of
one abdominal segment, and the -file is situated on the dor sum of
the following segment. It is quite conceivable that in this case
88 AN INTRODUCTION TO ENTOMOLOGY
the dorsal wall of the segment bearing the file is made to vibrate
by the successive impacts of the scraper upon the ridges of the
file. In fact this seems to me more probable than that the
sound produced is merely that of the scraper striking against the
successive ridges of the file. There is at least one recorded case
where the body wall is specialized to act as a sounding board.
According to Sharp ('95, p. 200), in the males of the Pneumorides,
a tribe of South African Acridiidas, where the phonetic organ is
situated on the abdomen, this part is inflated and tense, no
doubt with the result of increasing the volume and quality of the
sound.
Ordinarily the stridulating organs of insects are fitted to produce
notes of a single degree of pitch; but Gahan ('oo) figures those of
some beetles that are evidently fitted to produce sounds of more than
one degree of pitch; the file of Hispopria foveicollis, consists of three
parts, one very finely striated, followed by one in which the striae are
much coarser, and this in turn followed by one in which the striation
is intermediate in character between the other two.
While the stridulating organs of the Orthoptera are possessed
almost exclusively by the males, in the Coleoptera, very many species
of which stridulate, the phonetic organs are very commonly possessed
by both sexes, and serve as a mutual call. In one genus of beetles,
Phonapate, stridulating organs have been found only in the females
(Gahan, 'oo).
It seems evident that in the great majority of cases the sounds
produced by insects are sexual calls; but this is not always so. It
was pointed out long ago by Charles Darwin that " beetles stridulate
under various emotions, in the same manner as birds use their voices
for many purposes besides singing to their mates. The great Chiasog-
nathus stridulates in anger or defiance ; many species do the same from
distress or fear, if held so that they cannot escape; by striking the
hollow stems of trees in the Canary Islands, Messrs. Wollaston and
Crotch were able to discover the presence of beetles belonging to the
genus Acalles by their stridulation. Lastly the male Ateuchus
stridulates to encourage the female in her work and from distress
when she is removed" (The Descent of Man).
The most remarkable case where stridulating organs have been
developed for other than sexual pusposes is that of the larvae of certain
Lucanidse and Scarabaeidae described by Schiodte ('74). In these
larvae there is a file on the coxa of each middle leg, and the hind legs
are shortened and modified so as to act as scrapers. The most highly
THE EXTERNAL ANATOMY OF INSECTS
89
specialized example of this type of stridulating organ is possessed by
the larvae of Passalus, in which the legs of the third pair are so much
shortened that the
""^ i larvae appear to
have only four legs;
each hind leg is a
paw-like structure
fitted for rasping
the file (Fig. 101).
No satisfactory
explanation of the
advantage to these
larvae of the posses-
sion of stridulating
organs has been
offered; we can
only say that the
sound produced by
them is obviously
not a sexual call.
d. THE MUSICAL
ORGANS OF A CICADA
With the cica-
das there exists a
type of stridulating
organ peculiar to
them, and one that is the most complicated organ of sound
found in the animal kingdom. Yet, while the cicadas are the
most noisy of the insect world, the results obtained by their com-
plicated musical apparatus are not comparable with those pro-
duced by the comparatively simple vocal organs of birds and of
man.
It is said that in some species of Cicada both sexes stridulate ; but
as a rule the females are mute, possessing only vestiges of the musical
apparatus.
The structure of the stridulating organs varies somewhat in
details in different species of Cicada; but those of Cicada plebeia,
which were described and figured by Carle t ('77), may be taken as an
example of the more perfect form. In the male of this species there is
a pair of large plates, on the ventral side of the body, that extend back
Fig. 1 01. —Stridulating organ of a larva of Passalus;
a, b, portions of the metathorax; c, coxa of the
second leg; d, file; e, basal part of femur of middle
leg; /, hairs with chitinous process at base of each;
g , the diminutive third leg modified for scratching
the file (From Sharp).
90
AN INTRODUCTION TO ENTOMOLOGY
from the hind border of the thorax and overlap the basal part of the
abdomen; these are the opercula (Fig. 102, o). The opercula are
expansions of the ster-
nellum of the
thorax, and
-sp
Fig. 102. — The musical apparatus of a cicada; fm,
folded membrane; /, base of leg; Ic, lateral cavity;
m, mirror; o, operculum, that of the opposite
side removed; sp, spiracle; /, timbal; vc, ventral
cavity (After Carlet).
meta-
each
serves as a lid covering
a pair of cavities, con-
taining the external
parts of the musical
apparatus of one side
of the body.
The two cavities
covered by a single
operculum may be de-
signated as the ventral
cavity (Fig. 102, v. c.)
and the lateral cavity
(Fig. 102, 1. c.) respec-
tively. Each cavity is formed by an infolding of the body- wall.
In the walls of these cavities are three membranous areas; these
are known as the timbal, the folded membrane, and the mirror.
The timbal is in the lateral cavity on the lateral wall of the parti-
tion separating the two cavities (Fig. 102, t); the other two mem-
branes are in the ventral cavity. The fo ded membrane is in the
anterior wall of the ventral cavity (Fig. 10 2, /. m.); and the mirror
is in the posterior wall of the same cavity (Fig. 102, m). Within the
body, there is in the region of the musical apparatus a large thoraco-
abdomnal air chamber, which co,mmunicates with the exterior
througih a pair of spiracles (Fig. 102 sp); and a large muscle, which
extends from the furca of the second abdominal segment to the inner
face of the timbal.
By the contraction of this muscle the timbal is pulled towards the
center of the body; and when the muscle is relaxed, the elasticity of
the chitinous ring supporting the timbal causes it to regain its form er
position. By a very rapid repetition of these movements of the timbal
the sound is produced.
It is probable that the vibrations of the timbal are transmitted to
the folded membrane and to the mirror by the air contained in the
large air chamber mentioned above; as the strings of a piano are
made to vibrate by the notes of a near-by violin. The sound, how-
ever, is produced primarily by the timbal, the destruction of which
THE EXTERNAL ANATOMY OF INSECTS 91
renders the insect a mute; while the destruction of the other mem-
branes, the timbal remaining intact, simply reduces the sound.
The chief function of the opercula is doubtless the protecting of
the delicate parts of the musical organ; but as they can be lifted
slightly and as the abdomen can be moved away from them to some
extent, the chambers containing the vibrating parts of the organ can
be opened and closed, thus giving a rhythmic increase and decrease of
the loudness of the call.
6. THE SPIRACULAR MUSICAL ORGANS
There has been much discussion of the question whether insects,
and especially Diptera and Hymenoptera, possess a sound -producing
organ connected with the spiracles or not. Landois ('67) believed
that he found such an organ and figures and describes it in several
insects. It varies greatly in form in different insects. In the Diptera
it consists of a series of leaf-like folds of the intima of the trachea;
these are held against each other by a special humming ring, which
lies close under the opening of the spiracle; and is found within two
or all four of the thoracic spiracles. These membranous folds of the
intima are set in vibration by the rush of air through the spiracles.
In the May-beetle, according to Landois, a buzzing organ is found
near each of the fourteen abdominal spiracles. It is a tongue-like
fold projecting into the lumen of the trachea under the base of the
closing apparatus. On its upper surface it is marked with very fine
arched furrows. He concludes that this tongue is put in vibration by
the breathing of the insect, and hence the buzzing of the flying beetle.
If insects produce sounds in the way described by Landois. they
have a voice quite analogous to our own. But the validity of the
conclusions of Landois has been seriously questioned; the subject,
therefore, demands further investigation.
/. THE ACUTE BUZZING OF FLIES AND BEES
Many observers have found that when the wings of a fly or of a bee
are removed or held so that they can not vibrate the insect can still
produce a sound. The sound produced under these circumstances is
higher, usually an octave higher, than that produced by the wings.
It is evident, therefore, that these insects can produce sounds in two
ways; 'and an extended search has been made for the organ or organj
producing the higher note.
92 AN INTRODUCTION TO ENTOMOLOGY
Landois believed that the spiracular organs referred to above were
the source of the acute sound. But more recently Perez ('78) and
Bellesme ('78) have shown that when the spiracles are closed artifi-
cially the insect can still produce the high tone. Perez attributes the
sound to the vibrations of the stumps of the wings against the solid
parts which surround them or of the sclerites of the base of the wing
against each other. But Bellesme -main tains that the sound is pro-
duced by changes in the form of the thorax due to the action of the
wing-muscles.* When the wing-muscles are at rest the section of this
region, according to this writer, represent an ellipse elongated ver-
tically; the contraction of the muscles transforms it to an ellipse
elongated laterally; the thorax, therefore, constitutes a vibrating
body which moves the air like a tine of a tuning fork. Bellesme
states that by fastening a style to the dorsal wall of the thorax he
obtained a record of the rate of its vibrations, the number of which
corresponded exactly to that required to produce the acute sound
which the ear perceives.
The fact that the note produced when the wings are removed is
higher than that produced by the wings is supposed by Bellesme to be
due to the absence of the resistance of air against the wings, which
admits of the maximum rate of contraction of the wing-muscles.
g. MUSICAL NOTATION OF THE SONGS OF INSECTS
Mr. S. H. Scudder ('93) devised a musical notation by which the
songs of stridulating insects can be recorded. As the notes are always
at one pitch the staff in this notation consists of a single horizontal
line, the pitch being indicated by a separate statement. Each bar
represents a second of time, and is occupied by the equivalent of a
semibreve; consequently a quarter note f, or a quarter rest 1, repre-
sents a quarter of a second ; a sixteenth note t, or a sixteenth rest "1
a sixteenth of a second and so on. For convenience's sake he intro-
duced a new form of rest, shown in the second example given below,
which indicates silence through the remainder of a measure; this
differs from the whole rest commonly employed in musical notation
by being cut off obliquely at one end.
*This view was maintained by Siebold at a much earlier date in his Anatomy
of the Invertebrates.
THE EXTERNAL ANATOMY OF INSECTS 93
The following examples taken from his paper on "The Songs of
our Grasshoppers and Crickets" will serve to illustrate this method
of notation.
The chirp of Gryllotalpa borealis (Fig. 103) "is a guttural^ sort of
sound, like gru or greeu, repeated in a trill indefinitely, but seldom
Fig. 103. — The chirp of Gryllotalpa borealis (From Scudder).
for more than two or three minutes, and often for less time. It is
pitched at two octaves above middle C."
xr! *r! xr! XT!
Fig. 104.— The chirp of the katydid (From Scudder).
The note of the true katydid, Cyrtophyllus concavus, (Fig. 104)
"which sounds like xr, has a shocking lack of melody; the poets who
have sung its praises must have heard it at a distance that lends
enchantment." "They ordinarily call 'Katy' or say 'She did' rather
than 'Katy did' ; that is they rasp their fore wings twice more fre-
quently than thrice." Mr. Scudder in his account of this song fails
to indicate its pitch.
h. INSECT CHORUSES
Most insect singers are soloists, singing without reference to other
singers or in rivalry with them. But. there are a few species the
members of which sing in unison with others of their kind that are
near them. The most familiar sound of autumn evenings in rural
places in this country is a chorus of the snowy tree cricket, CEcanthus
niveus. Very many individuals of this species, in fact all that are
chirping in any locality, chirp in unison. Early in the evening, when
the chirping first begins, there may be a lack of unanimity in keeping
time ; but this lasts only for a short period, soon all chirp in unison,
and the monotonous beat of their call is kept up uninterrupted
throughout the night. Individual singers will stop to rest, but when
they start again they keep time with those that have continued the
chorus.
Other instances of insect choruses have been recorded. Sharp
('99, 156) quotes accounts of two produced by ants; one of these is
given on an earlier page (p. 80).
CHAPTER III
THE INTERNAL ANATOMY OF INSECTS
BEFORE making a more detailed study of the internal anatomy of
insects, it is well to take a glance at the relative positions of the differ-
ent systems of organs within the body of insects and other arthropods.
One of the most striking features in the structure of these animals
is that the body-wall serves as a skeleton, being hard, and giving sup-
port to the other organs of the body. This skeleton may be repre-
sented, therefore, as a hollow cylinder. We have now to consider the
arrangement and the general form of the organs contained in this
cylinder.
The accompanying diagram (Fig. 105), which represents a. vertical,
longitudinal section of the body, will enable the student to gain an
Fig. 105. — Diagram showing the relations of the internal organs;
a, alimentary canal; h, heart; ra, muscle; n, nervous system;
r, reproductive organs.
idea of- the relative positions of some of the more important organs.
The parts shown in the diagram are as follows: The body-wall, or
skeleton; this is made up of a series of overlapping segments; that
part of it between the segments is not hardened with chitine, thus
remaining flexible and allowing for the movements of the body. Just
within the body-wall, and attached to it, are represented a few of the
muscles (m) ; it will be seen that these muscles are so arranged that the
contraction of those on the lower side of the body would bend it down,
while the contraction of those on the opposite side would act in the
opposite direction, other muscles not shown in the figure provide
for movements in other directions. The alimentary canal (a) occupies
the centre of the body, and extends from one end to the other. The
heart (h) is a tube open at both ends, and lying between the alimentary
canal and the muscles of the back. The central part of the nervous
system (n) is a series of small masses of nervous matter connected by
(94)
THE INTERNAL ANATOMY OF INSECTS 95
two longitudinal cords: one of these masses, the brain, lies in the
head above the alimentary canal ; the others are situated, one in each
segment, between the alimentary canal and the layer of muscles of the
ventral side of the body; the two cords connecting these masses, or
ganglia, pass one on each side of the oesophagus to the brain. The
reproductive organs (r) lie in the cavity of the abdomen and open near
the caudal end of the body. The respiratory organs are omitted from
this diagram for the sake of simplicity. We will now pass to a more
detailed study of the different systems of organs.
I. THE HYPODERMAL STRUCTURES
The active living part of the body-wall is the hypodermis, already
described in the discussion of the external anatomy of insects. In
addition to the external skeleton, there are derived from the hypo-
dermis an internal skeleton and several types of glands.
a, THE INTERNAL SKELETON
Although the skeleton of an insect is chiefly an external one, there
are prolongations of it extending into the body-cavity. These
inwardly directed processes, which serve for the attachment of
muscles and for the support of other viscera are termed collectively
the internal skeleton or endo-skeleton. The internal skeleton is much
more highly developed in adult insects than it is in the immature
ins tars.
Sources of the internal skeleton. — The parts of the internal skele-
ton are formed in two ways : first by the chitinization of tendons of
muscles; and second, by invaginations of the body- wall.
Chitinized tendons.— Chitinized tendons of the muscles that move
the mouth-parts, of muscles that move the legs, and of other muscles
are of frequent occurrence. As these chitinized tendons help support
the internal organs they are considered as a part of the internal
skeleton.
Invaginations of the body-wall or apodemes. — The second and more
important source of the parts of the internal skeleton consists of
invaginations of the body- wall. Such an invagination is termed an
dpodeme. The more important apodemes, if not all, arise as invagina-
tions of the body-wall between sclerites or at the edge of a sclerite on
the margin of a body-segment; although by the fusion of sclerites
about an apodeme, it may appear to arise from the disc of a sclerite.
96
AN INTRODUCTION TO ENTOMOLOGY
and metathorax of Melano-
*" lateral
Frequently, in the more generalized insects, the mouth of an apodeme
remains open in the adult insects. In Figure 106 are represented two
apodemes that exist in the thorax of a
locust, Melanoplus. Each of these (ap
and ap) is an invagination of the body-
wall, between the episternum and the
epimeron of a segment, immediately
above the base of a leg. These are known
as the lateral apodemes of the thorax and
serve as points of attachment of muscles.
The number of apodemes may be very
large, and it varies greatly in different
insects. ' Among the more important apo-
demes are the following: —
The tentorium. — The chief part of the internal skeleton of the
head is termed the tentorium. This was studied by Comstock
and Kochi ('02). We found that in the generalized insects studied
by us it is composed of two or three pairs of apodemes that, extend-
ing far into the head, meet and coalesce. The three pairs of
apodemes that may enter into the formation of the tentorium
were termed the anterior, the posterior, and the dorsal arms of the
tentorium respectively. The coalesced and more or less expanded
tips of these apodemes Constitute the body of the tentorium. From
the body of the tentorium there extend a variable number of processes
or chitinized tendons.
The posterior arms of the tentorium. — The posterior arms of the
tentorium (Fig. 107, 109, no, pt) are the lateral apodemes of the
.it
Fig. 1 07 . — Tentorium
of a cockroach, dor-
sal aspect.
Fig. 1 08. — Part of the
tentorium of a cric-
ket, ventral aspect.
maxillary segment. In many Orthoptera the open mouth of the
apodeme can be seen on the lateral aspect of the head, just above the
THE INTERNAL ANATOMY OF INSECTS
97
Fi?. 109. — Head of
Melanvplus, cau-
dal aspect.
articulation of the maxilla (Fig. 48). In the Acridiidse (Fig. 109)
these apodemes bear a striking resemblance to the lateral apodemes
of the thorax (Fig. 106), except that the ventral process of the maxil-
lary apodeme is much more prominent, and the two from the opposite
sides of the head meet and coalesce, thus forming
the caudal part of the body of the tentorium.
The anterior arms of the tentorium. — Each anterior
arm of the tentorium (Fig.'ioy, 108, no, at) is an
imagination of the body-wall which opens on the
margin of the antecoxal piece of the mandible
when it is distinct ; if this part is not distinct the
apodeme opens between the clypeus and the front
(Fig. 46, at).
The dorsal arms of the tentorium. — Each dorsal
arm of the tentorium arises from the side of the
body of the tentorium between the anterior and posterior arms
and extends either to the front or to the margin of the antennal
sclerite (Fig. 107, 108, no, dt).
The frontal plate of the tentorium. — In the cockroaches the anterior
arms of the tentorium meet and fuse, forming a broad plate situated
between the crura cerebri and the mouth ; this plate was termed by
us the frontal plate of the tentorium (Fig. 107, fp). On each side, an
extension of this plate connects it with the body of the tentorium;
these enclose a circular opening through which pass
the crura cerebri.
Other cervical apodemes and some chitinized
tendons are described in the paper cited above.
The endothorax. — The internal skeleton of the
thorax is commonly termed the endothorax; under
this head are not included the internal processes of
the appendages.
The endothorax is composed of invaginations of
each of the sections of a thoracic ring. Those por-
tions that are derived from tergites are termed
phragmas; those derived from the pleurites, lateral
- , , £ ., x
apodemes; and those, from the stermtes, jure®.
The phragmas. — A phragma is a transverse partition extending
entad from the front or the hind margin of a tergite; three of them
are commonly recognized; these were designated by Kirby and
Spence (1826) the prophragma, the mesophragma, and the meta-
phragma; but, as they do not arise one from each segment of the
IIO _ Ten-
torium of Mela-
aspect! The distal
end of the dorsal
arms detached.
AN INTRODUCTION TO ENTOMOLOGY
thorax, and arise differently in different insects, these terms are mis-
leading. No phragma is borne by the pro thorax; the mesothorax
may bear two and the metathorax one, or the mesothorax one and the
metathorax two. A more definite
terminology is that used by Snod-
grass ('09) by which the anterior
phragma of any segment is termed
the prephragma of that segment,
and the posterior phragma of any
segment is termed the postphragma
of that segment.
The lateral apodemes. — Each lat-
eral apodeme is an invagination of
the body-wall between the epister-
The lateral apodemes are referred to above
Fig. in . — Ventral aspect of the
metathorax of Stenopelmatus.
The position of the furca
within the body is represented
by a dotted line.
num and the epimeron.
(Fig. 106).
The f urea. — Each furca is an invagination of the body- wall arising
between the sternum and the sternellum (Fig. in); when the sternel-
lum is obsolete, as it is in most insects, the furca arises at the caudal
margin of the segment (Fig. 112).
b. THE HYPODERMAL GLANDS
A gland is an organ that possesses the function of either trans-
forming nutritive substances, which it, derives from the blood, into
some useful substance, as mucus, wax, or venom, or of assimilating
and removing from the body waste
material.
The different glands vary greatly in
structure; many are unicellular, the
gland consisting of a single cell, which
differs from the other cells of the epithe-
lium of which it is a part in being larger
and in possessing the secreting and ex-
creting functions; others are multicel-
lular, consisting of more' than one cell, Fig. 112.— Ventral aspect of the
usually of many cells. In these cases S5S^fhe^ti6^"the
the glandular area usually becomes furcae within the body are
invaginated, and provided with an
ibydott(
efferent duct ; and often the invagination is much branched.
The glands found in the body of an insect can be grouped under
three heads; the hypodermal glands, the glands of the alimentary
THE INTERNAL ANATOMY OF INSECTS
99
qanal, and the glands of the reproductive organs. In this place
reference is made only to the hypodermal glands, those developed
from the hypodermis.
The Molting-fluid glands. — Under this
head are classed those unicellular, hypo-
dermal glands that secrete a fluid that
facilitates the process of molting, as des-
cribed in the next chapter (Fig. 113).
While molting-fluid glands are very
numerous and conspicuous in certain
insects, those living freely exposed where
there exists the greatest liability to rapid
Fig. 113. — Molting-fluid glands desiccation, Tower ('06) states that he
of the last larval instar of has never foum} these glands in larva?
Leptinotarsadectml-meata,just . .
b)fore pupation; le, larval that live in burro ws, or in the soil, or in
epidermis; Id, larval dermis; cells; in these cases the molting fluid is
mf, molting fluid; pe, forming
pupal epidermis; h, hypoder- apparently secreted by the entire hypo-
mis; g molting fluid gland dermal layer.
(After Tower).
Glands connected with setae. — There
are in insects several kinds of glands in which the outlet of the gland
is through the lumen of a seta. The function of the excretions of
these glands is various as indicated
below. There are also differences in
the manner of issuance of the excre-
tion from the aeta. In some cases, as
in the tenent hairs on the feet of certain
insects, the excretion can be seen to
issue through a pore at the tip of the
seta. In some kinds of venomous setae
the tip of the seta breaks off in the
wound made by it and thus sets free
the venom. But in most cases the
manner of issuance has not been deter-
mined, although it is commonly believed
to be by means of a minute pore or
pores in the seta, the thickness of the
wall of the seta making it improbable
that the excretion passes from the seta
by osmosis.
The structure of a glandular seta
is illustrated by Figure 114; the
essential difference between such a seta and an ordinary one, that is a
Fig. 1 14. — Glandular s? ta; s, seta;
c, cuticula; h, hypodermis; bm,
basement membrane; tr, tricho-
gen; g, gland (After Holmgren).
100
AN INTRODUCTION TO ENTOMOLOGY
clothing hair, is tLat there is connected with it, in addition to the
trichogen cell which produced it, the gland cell which opens through it.
In most of the published figures of glandular setae there is no indi-
cation that these organs are supplied with nerves ; but in some cases
a nerve extending to the gland cell is clearly shown. This condition
may be found to be general when more extended investigations of
glandular cells have been made. The best known kinds of glandular
setae are the following :
Venomous seta and spines. — These are best known in larvae of
Lepidoptera, several common species of which possess stinging hairs;
among these are Lagoa crispata, Sibine stimulea, Automeris io, and
the brown-tail moth, Euproctis chrysorrhcea.
Androconia. — The term androconia* is applied to some peculiarly
modified scales on the wings of certain male butterflies. These are
the outlets of glands, which secrete a fluid with an agreeable odor;
the supposed function of which is to attract the opposite sex, like the
beautiful plumage and songs of male birds. The androconia differ
marvelously from ordinary scales in the
variety of their forms (Fig. 115). They
usually occur in patches on the upper sur-
face Of the fore wings; and are usually
concealed by other scales; but they are
scattered in some butterflies. The most
familiar examples of grouped androconia
are those that occur in the discal stigma of
the hair-streaks, in the brand of certain
skippers and in the costal fold of others,
and in the scent-pouch of the male of the
monarch butterfly
The specific scent-glands of females*. —
TThe well-known fact that if an unfertilized
female moth be confined in a cage or
otherwise in the open many males of the
Fig. 115.— Androconia from the samespeciesas the female will be attracted
wmgsof male butterflies (After . . • •«••«*
Kellogg-) . to it , and sometimes evidently irom a great
distance, leads to the conclusion that there
must emanate from the female a specific odor. The special glands
producing this odor have not been recognized.
Tenent hairs. — In many insects the pulvilli or the empodia are
clothed with numerous hairs that are the outlets of glands which
* Androconia: andro-
, male; conia (/co^io), dust.
THE INTERNAL ANATOMY OF INSECTS
101
secrete an adhesive fluid; this enables the insect to walk on the lower
surface of objects (Fig. 116).
ky
Fig. 116. — A, terminal part of a tenent hair from Eupolus, showing canal in the
hair and opening near the tip; B, cross-section through a tarsal segment of
Telephorus; c, cuticula; g, gland of tenent hair; h, h, tactile hairs; hy, hypo-
dermis; n, nerve; s, sense-cell of tactile hair; t, t, tenent hairs (After Dewitz).
The osmeteria. — -In many insects there are hypodermal glands that
o£>en into sac-like invaginations of the body-wall which can be
evaginated when the insect wishes to make use of the secretion pro-
duced by these glands ; such an organ i§ termed an osmeterium. The
invagination of the osmeterium admits of an accumulation of the
products of the gland within the cavity of the sac thus formed; when
the osmeteriurn is evaginated the secretion becomes exposed to the air,
being then on. the outside of the osmeterium, and rapid diffusion, of
the secretion results.
The most familiar examples of osmeteria are those of the larvae
of the swallow-tailed butterflies, which are forked, and are thrust out
from the upper part of the prothorax when the caterpillar is disturbed,
and which
diffuse a dis-
agreeable odor
(Fig. 117).
They are ob-
viously organs
of defense.
Osmeteria
Fig. 117 — Larva of Papilio thoas; o, osmeterium expanded.
are present in the larvae of certain blue butterflies, Lycsenidae. These
are in the seventh and eighth abdominal segments, and secrete a
honey-dew, which attracts ants that attend and probably protect
the larvae. The osmeteria of many other caterpillars have been
described.
102
AN INTRODUCTION TO ENTOMOLOGY
Fig. 1 1 8. — Wax-plates of the honeybee
(After Cheshire).
Glands opening on the surface of the body.— There are several
kinds of hypodermal glands, differing widely in function, that open
on the surface of the body; among the best known of these are the
following :
Wax-glands. — The worker honeybee has four pairs of wax-glands;
these are situated on the ventral wall of the second, third, fourth, and
fifth abdominal segments, and on that part of the segment which is
overlapped by the preceding segment; each gland is simply a disc-
like area of the hypodermis
(Fig. 1 1 8). The cuticle
covering each gland is
smooth and delicate, and is
known as a wax plate.
The wax exudes through
these plates and accumu-
lates, forming little scales,
which are used in making
the honey -comb.
Wax -glands exist in
many of the Homoptera. In some of these the unicellular wax-
glands are distributed nearly all over the body; and the product
of these glands forms, in some, a po,wdery covering; in others,
a clothing of threads; and in still others a series of plates (Fig. 119).'
Certain coccids excrete wax in con-
siderable quantities. China wax, which
was formerly an article of commerce,
is the excretion of a coccid known as
Pe-la (Ericerus Pe-la).
Froth-glands of spittle-insects. — In
the spittle-insects (Cercopidae) there
are large hypodermal glands in the
pleural regions of the seventh and eighth
abdominal segments, which open
through numerous minute pores in the
cuticula. These glands secrete a muci-
laginous substance, which is mixed with
a fluid excreted from the anus, and thus
fits it for the retention of bubbles of air
included in it b}^ means of abdominal appendages (Guilbeau '08) .
Stink-glands. — Glands that secrete a liquid having a fetid odor and
that are doubtless defensive exist in many insects. In the stink-bugs
Fig. 119. — Orthesia, greatly en-
larged.
THE INTERNAL ANATOMY OF INSECTS
103
(Pentatomidae) the fluid is excreted through two openings, one on each
side of the lower side of the body near the middle coxae; in the bed-
bug (Cimex) , the stink-glands open in the dorsal wall of the first three
abdominal segments ; mDytiscus, the glands open on the pro thorax;
and in certain Coleoptera they open near the caudal end of the body.
These are merely a few examples of the many glands of this type that
are known.
The cephalic silk-glands. — In the Lepidoptera, Trichoptera, and
Hymenoptera, there is a pair of glands that secrete silk, and which
open through the lower lip. These glands are designated as the
cephalic silk-glands to distinguish them from the silk-glands of certain
Neuroptera and Coleoptera in which the silk is produced by modified
Malpighian vessels and is spun from the anus.
The cephalic silk-glands are elongate and coiled; they often
extend nearly the whole length of the body; the two ducts unite and
the single terminal duct opens through the lower lip, and is not
connected with the mouth cavity. These glands are a pair of
salivary glands which have
been transformed into silk
organs. According to Carriere
Fig. 120. — The salivary glands
of the honeybee (After
Cheshire).
Fig. 121. — Theman-
dibular gland of a
honeybee.
and Burger ('97), who studied their development in the embryo
of a bee, they are developed from the rudiments of the spiracles
of the first thoracic segment. In the later development they move
104 AN INTRODUCTION TO ENTOMOLOGY
cephalad and the paired openings become a single one. This is the
reason that in the adult there are no spiracles in the prothorax.
The Salivary glands. — The term salivary glands is a general one,
applied to various glands opening in the vicinity of the mouth. The
number of these varies greatly in different insects; the maximum
number is found in the Hymenoptera. In the adult worker honey-
bee, for example, there are four pairs of glands opening into the
mouth; three of these are represented in Figure 120 and the fourth
in Figure 121. These are designated as the supracerebral glands
(Fig. 1 20, i), the postcerebral glands (Fig. 120, 2), the thoracic
glands (Fig. 120, j), and the mandibulary glands (Fig. 121),
respectively.
II. THE MUSCLES
There exist in insects a wonderfully large number of .muscles ;
some of these move the segments of the body, others move the appen-
dages of the body, and still others are found in the viscera. Those
of the viscera are described later in the accounts of the organs in
which they occur.
The muscles that move the segments of the body form several
layers just within the body-wall, to which they are attached. The
inner layer of these is well shown in Figure 122, which is a copy of
one of the plates in the great work by Lyonet (1762) on the anatomy
of a caterpillar, Cossus ligniperda. The two figures on this plate
represent two larvae which have been split open lengthwise, one on the
middle line of the back (Fig. 5), and one on the middle line of the
ventral surface (Fig. 4) ; in each case the alimentary canal has been
removed, so that only those organs that are attached quite closely to
the body-wall are left. The bands of parallel fibers are the muscles
that move the segments. It should be borne in mind, however, that
only a single layer of muscles is represented in these figures, the layer
that would be seen if a caterpillar were opened in the way indicated.
When these muscles are cut away many other muscles are found
extending obliquely in various directions between these muscles and
the body- wall.
In the head and thorax of adult insects the arrangement of the
muscles is even more complicated ; for here the muscles that move the
appendages add to the complexity of the muscular system.
As a rule, the muscles of insects are composed of many distinct
fibers, which are not enclosed in tendinous sheaths as with Verte-
THE INTERNAL ANATOMY OF INSECTS
105
Fig. 122. — Internal anatomy of a caterpillar, Cossus ligniperda; i, principal
longitudinal trachae; 2, central nervous system; j, aorta; 4, longitudinal
dorsal muscles; 5, longtiudinal ventral muscles; 6, wings of the hearty 7,
tracheal trunks arising near the spiracles; 8, reproductive organs; 9, vertical
muscles; 10, last abdominal ganglion (From Lyonet).
106 AN INTRODUCTION TO ENTOMOLOGY
brates. But the muscles that move the appendages of the body
are furnished . with a tendon at the end farthest from the body
(Fig. 123).
The muscles of in-
sects appear very differ-
ently from those of Ver-
tebrates. In insects, the
Fig. 123.— A leg of a May-beetle (After Straus- muscles are either color-
less and transparent, or
yellowish white; and they are soft, almost of a gelatinous consistency;
notwithstanding this they are very efficient. The fibers of insect
muscles are usually, if not always, of the striated type.
Much has been written regarding the muscular power of insects,
which has been supposed to be extraordinarily great; the power of
leaping possessed by many and the great loads, compared to the
weight of the body of the insect, that insects have drawn when
harnessed to them by experimenters, have been cited as illustrating
this. But it has been pointed out that these conclusions are not
warranted; that -the comparative contractile force of muscles of the
same kind depends on the number and thickness of the £ bers, that is,
on the comparative areas of the cross-sections of the muscles ccm-
pared; that this sectional area increases as the square of any linear
dimension, while the weight of similar bodies increases as the cube of
any linear dimension; and consequently, that the iruscles of the legs
of an insect one fourth inch long and supporting a load 399 times its
own weight, would be subjected to the same stress, per square inch of
cross-section, as they would be in an insect 100 inches long of precisely
similar shape, that carried only its own weight. We thus see that it is
the small size of insects rather than an unusual strength of their
muscles, that makes possible the apparently marvelous exhibitions of
muscular power.
Detailed accounts of the arrangement of the muscles in particular
insects have been published by various writers; among the more
important of these monographs are the following: Lyonet (1762),
on the larva of a cossid moth; Straus-Durckheim (1828), on a May-
beetle; Newport (1839), on the larva of a Sphinx moth; Lubbo^k
(1858), on the larva Pyg&ra bucephala; and Berlese ('ooa), on
several insects.
THE INTERNAL ANATOMY OF INSECTS
107
III. THE ALIMENTARY CANAL AND ITS APPENDAGES
a. THE MORE GENERAL FEATURES
The alimentary canal is a tube extending from one end of the body
to the other. In some larvag, its length is about the same as that of
the body; in this case it extends in a nearly straight line, occupying
OMRTTJM
Fig. 124. — Internal anatomy of a cockroach, Periplaneta orientalis; a, antennae;
bi, 62, 63, first, second, and third legs; ct cerci: d, ventricular ganglion; e,
salivary duct: /, salivary bladder, g, gizzard or proventriculus : h, hepatic
cceca; it mid-intestine; j, Malpighian vessels; k, small intestine; /, large
intestine: w, rectum; n, first abdominal ganglion; o, ovary; p, sebaceous
glands (From Rolleston).
108 AN INTRODUCTION TO ENTOMOLOGY
the longitudinal axis of the body, as is represented in the diagram
given above (Fig. 105). In most insects, however, it is longer than
the body, and is consequently more or less convoluted (Fig. 124);
great variations exist in the length of the alimentary canal as' com-
pared to the length of the body; it is longer in herbivorous insects
than it is in those that are carnivorous.
The principal divisions. — Three chief divisions of the alimentary
canal are recognized ; these are termed the fore-intestine', the mid-
intestine, and the hind-intestine, respectively. In the embryological
development of the alimentary canal, the fore-intestine and the hind-
intestine each arises as an invagination of the ectoderm, the germ
layer from which the hypodermis of the body-wall is derived (p. 29).
The invagination at the anterior end of the body, which develops
into the fore-intestine, is termed the stomod&um; that at the posterior
end, which develops into the hind-intestine, the proctod&um. Between
these two deep invaginations of the outer germ layer of the "embryo,
the stomodaeum and the proctodeeum, and ultimately connecting
them, there is developed an entodermal tube, the mesenteron, which
becomes the mid-intestine.
These embryological facts are briefly stated here merely to
elucidate two important features of the alimentary canal: first, the
fore-intestine and the hind-intestine are invaginations of the body
wall and consequently resemble it in structure, the chitinous lining of
these two parts of the alimentary canal is directly continuous with
the cuticula of the body wall, and the epithelium of these two parts
and the hypodermis are also directly continuous; and second, the
striking differences, pointed out later, in the structure of the mid-
intestine from that of the fore- and hind-intestines are not surprising
when the differences in origin are considered.
Imperf orate intestines in the larvae of certain insects. — In the larvae
of certain insects the lumen of the alimentary canal is not a continuous
passage; in these larvae, while food passes freely from the fore-
intestine to the mid-intestine, there is no passage of the waste from
the mid-intestine to the hind-intestine; there being a construction at
the point where the mid-intestine and hind-intestine join, which
closes the passage during a part or the whole of the larval life. This
condition has been observed in the following families: —
(a) Hymenoptera. — Proctotrypidae (in the first larval instar),
Ichneumonidag, Formicidag, Vespidas, and Apidae.
(b) Diptera. — Hippoboscidae.
THE INTERNAL ANATOMY OF INSECTS
109
(c) Neuroptera. — Myrmeleonidae, Osmylidae, Sisyridae, and
Chrysopidae. In these families the larvae spin silk from the anus.
(d) Coleoptera. — In the Campodeiform larvae of Stylopidae and
Meloidas.
b. THE FORE-INTESTINE
The layers of the fore-intestine. — The following layers have been
recognized in the fore-intestine :
The intima. — This is a chitinous layer which lines the cavity of
the fore-intestine; it is directly continuous with the cuticula of the
body-wall ; and is molted with the cuticula when this is molted.
The epithelium. — This is a cell layer which is continuous with the
hypodermis; it is sometimes quite delicate so that it is difficult to
demonstrate it.
The basement membrane. — Like the hypodermis the epithelium is
bounded on one side by a chitinous layer and on the other by a base-
ment membrane.
The longitudinal muscles. — Next to the basement membrane there
is a layer of longitudinal muscles.
The circular muscles. — Out-
side of the longitudinal muscles
there is a layer of circular
muscles.
The peritoneal membrane. —
Surrounding the alimentary
canal there is a coat of con-
nective tissue, which, is termed
the peritoneal membrane. This
is one of a few places in which
connective tissue, so abundant
in Vertebrates, is found in in-
sects.
The regions of the fore-
intestine. — Several distinct reg-
ions of the fore-intestine are
recognized; but the extent of
these regions differ greatly in
different insects.
The pharynx. — The pharynx
is not a well-defined region of the
intestine; the term pharynx is commonly applied to a region between
the mouth and the oesophagus; in mandibulate insects the pharynx
sd
Fig. 125. — Longitudinal section through
the head of Anosa plexippus, showing
the interior of the left half; mx, left
maxilla, the canal of which leads into the
pharynx; ph, pharynx; o, oesophagus;
m, m, muscles of the pharynx; sd,
salivary duct (After Burges).
110
AN INTRODUCTION TO ENTOMOLOGY
is not distinct from the mouth-cavity; but in sucking insects the
pharynx is a highly specialized organ, being greatly enlarged, muscu-
lar, and attached to the wall of the head by muscles. It is the pump-
ing organ by which the liquid food is drawn into the alimentary canal.
The pharnyx of the milkweed butterfly (Fig. 125) is a good example
of this type of pharynx.
The cesophagus. — The oeso-
phagus is a simple tube which
traverses the caudal part of the
head and the cephalic part of the
thorax. There are variations in
the application of the term
oesophagus depending on the
presence or absence of a crop
and of a proventriculus, which
are modified portions of the
oesophagus; when either or both
of these are present, the term
oesophagus is commonly restricted
to the unmodified part of the
fore-intestine.
The crop. — In many insects a
portion of the oesophagus is dilated
and serves as a reservoir of food;
this expanded part, when present,
is termed the crop. In the cock-
roach (Fig. 124) it is very large,
comprising the greater part of the
fore-intestine ; in the ground-beetle
Carabus (Fig. 126, c), it is much
more restricted; this is the case
also in the honeybee, where it is
a nearly spherical sac in which
,i n -I ... , Fig. 126. — Alimentary canal of Carabus
the nectar is stored as it is col- luratus; h> head^e, oesophagus; c,
crop; pv, proventriculus; mi, mid-
intestine covered with viiliform gastric
cceca; mv, Malpighian vessels; hi, part
of hind- intestine; r, rectum; ag, anal
glands; mr, muscular reservoir (After
ag—
lected from flowers and carried to
the hive. In some insects the
crop is a lateral dilatation of the
oesophagus, and in some of these
Dufour).
it is stalked.
The proventriculus. — In certain insects that feed on hard sub-
stances, the terminal portion of the fore-intestine, that part im-
THE INTERNAL ANATOMY OF INSECTS
111
Fig. 127. — Cross-section of the
proventrioulus of a larva of
Corydalus.
mediately in front of the mid-intestine or ventriculus, is a highly
specialized organ in which the food is prepared for entrance into
the more delicate ventriculus; such an
organ is termed the proventriculus (Fig.
126, pv). The characteristic features
of a proventriculus are a remarkable
development of the chitinous intima
into folds and teeth and a great in-
crease in the size of the muscles of this
region. The details of the structure
of this organ vary greatly in different
insects; a cross-section of the proven-
triculus of the larva of Corydalus (Fig.
127) will serve to illustrate its form.
In the proventriculus, the food is both
masticated and more thoroughly
mixed with the digestive fluids.
The cesophageal valve, — When the
fore-intestine projects into the mid-
intestine, as shown in Figure 128,
the folded end of the fore-intestine
is termed the cesophageal valve.
C. THE MID-INTESTINE
The mid-intestine is the inter-
mediate of the three principal
divisions of the alimentary canal,
which are distinguished by differ-
ences in their embryological origins,
as stated above. The mid-intestine
is termed by different writers the
mesenteron, the stomach, the chylific
ventricle, the chylestomach, and the
ventriculus.
The layers of the mid-intestine. —
The structure of the mid-intestine
differs markedly from that of the
fore-intestine. In the mid-intestine
there is no chitinous intima, and the
relative positions of the circular and
longitudinal muscles are reversed.
Fig. 128. — The cesophageal valve of a
larva of Simulium; F, fore-intestine:
M, mid-intestine; u, point^ of union
of fore-intestine and mid-intestine;
p, peritoneal membrane; i,
intima of fore-intestine; e, epithe-
lium of fore-intestine; pt, peritrophic
membrane; m, muscles
112
AN INTRODUCTION TO ENTOMOLOGY
The sequence of the different layers is as follows : a lining epithelium,
which is supported by a basement membrane, a layer of circular
muscles, a layer of longitudinal muscles, and a peritoneal membrane.
The epithelium. — The epithelium of the mid-intestine is very con-
spicuous, being composed of large cells, which secrete a digestive fluid.
These cells break when they discharge their secretion and are replaced
by new cells, which are developed in centers termed nidi (Fig. 129, n).
The extent of the digestive epithelium is increased in many insects
by the development of pouch-like diverticula of the mid-intestine,
these are the gastric cceca (Fig. 124, h). These differ greatly in num-
ber in different insects and are wanting in some. In some predaceous
beetles they are villiform and very numerous (Fig. 126, mi).
The peritrophic membrane.
— In many insects there is a
membranous tube which is form-
ed at or near the point of union of
the fore-intestine and the mid-
intestine and which incloses the
food so that it does not come in
contact with the delicate epithe-
lium of the mid-intestine ; this is
known as the peritrophic mem-
brane (Fig. 128, pt). As a rule
this membrane is found in insects
that eat solid food and is lacking
in those that eat liquid food. It
is obvious that the digestive fluid
and the products of digestion
pass through this membrane. It
is continuously formed at its
point of origin and passes from
the body inclosing the excrement.
d. THE HIND-INTESTINE
The layers of the hind-intes-
tine.— The layers of the hind-in-
testine are the same as those of
the fore-intestine described
above, except that a greater or
less number of circular muscles exist between the basement membrane
of the epithelial layer and the layer of longitudinal muscles. The
Fig. 129. — Resting epithelium of mid-
intestine of a dragon-fly naiad; b,
bases of large cells filled with digestive
fluid; cm, space filled by circular mus-
cles; Im, longitudinal muscles; n, nidus
in which new cells are developing (From
Needham).
THE INTERNAL ANATOMY OF INSECTS 113
sequence of the layers of .the hind-intestine is, therefore, as follows:
the intima, ihe^ epithelium, the basement membrane, the ental circular
muscles, the longitudinal muscles, the ectal circular muscles, and the
peritoneal membrane.
The regions of the hind-intestine. — Three distinct regions are
commonly recognized in the hind-intestine, these are the small intestine
(Fig. 124, k), the large intestine (Fig. 124, /), and the rectum (Fig.
124, m).
The Malpighian vessels. — There open into the beginning of the
hind-intestine two or more simple or branched tubes (Fig. 124, ;'),
these are the Malpighian vessels. The number of these vessels varies
in different insects but is very constant within groups; there are
either two, four, or six of them; but, as a result of branching, there
may appear to be one hundred or more. The function of the Mal-
pighian vessels has been much discussed ; it was formerly believed to
be hepatic, but now it is known that normally it is urinary.
The Malpighian vessels as silk- glands. — There are certain larvae
that in making their cocoons spin the silk used from the anus. These
larvae are chiefly found among those in which the passage from the
mid-intestine to the hind-intestine is closed. The silk spun from the
anus is secreted by the Malpighian vessels.
Among the larvae in which the Malpighian vessels are known to
secrete silk are those of the Myrmeleonidae, Osmylus (Hagen 1852),
Sisyra (Anthony '02), Lebia scapularis (Silvestri '05), and the
Coccidae (Berlese '96). Berlese states that the Malpighian vessels
secrete the woof of the. scale of the Coccidae.
The caecum. — In some insects there is a pouch-like diverticulum
of the rectum, this is the ccecum.
The anus. — The posterior opening of the alimentary canal, the
anus, is situated at the caudal end of the abdomen.
IV. THE RESPIRATORY SYSTEM
Insects breathe by means of a system of air-tubes, which ramify
in all parts of the body and its appendages; these air-tubes are of two
kinds, which are termed trochees and tracheoles, respectively. In
adult insects and in most nymphs and larvae, the air is received
through openings in the sides of the segments of the body, which are
known as spiracles or stigmata.
Many insects that live in water are furnished with special devices
for obtaining air from above the water; but with naiads and a few
114
AN INTRODUCTION TO ENTOMOLOGY
aquatic larvae the spiracles are closed; in these insects the air is
purified by means of gill-like organs, termed tracheal gills. A few
insects have blood-gills.
Two types of respiratory systems, therefore, can be recognized:
first, the open type, in which the air is received through spiracles; and
second, the closed type, in which the spiracles are not functional.
a. THE OPEN OR HOLOPNEUSTIC TYPE OR RESPIRATORY ORGANS
That form of respiratory organs in which the tracheae communicate
freely with the air outside the body through open spiracles is termed
the open or holopneustic type.*
As the open type of respiratory organs is the most common one,
those features that are common to both types will be discussed under
this head as well as those that are peculiar to this type. Under the
head of closed respiratory organs will be discussed only those features
distinctly characteristic of that type.
7. The Spiracles
The position of the spiracles. — The spiracles are situated one on
each side of the segments that bear them or are situated on the lateral
aspects of the body in the transverse conjunctiva?.
The question of the position of the spiracles has not been thor-
oughly investigated; but I believe that normally the tracheae, of
Fig. 130. — Lateral view of a silk worm thowing the spiracles
(After Verson)
which the spiracles are the mouths, are invaginatiors of the transverse
conjunctivae between segments. From this normal position a spiracle
may migrate either forward or backward upon an adjacent segment
(Fig. 130).
The number of spiracles. — The normal number of spiracles is ten
pairs ; when in their normal position, there is a pair in front of the
*H61opneustic: holo (5Xos), whole; pneuma
, breath.
THE INTERNAL ANATOMY OF INSECTS 115
second and third thoracic segments and the first to the eighth abdom-
inal segments, respectively. There are none in the corresponding
position in front of the first thoracic segment. See account of
cephalic silk-glands p. 103.
The two pairs of thoracic spiracles are commonly distinguished as
the mesothoracic apd the metathoracic spiracles ; that is each pair of
spiracles is attributed to the segment in front of which it is normally
situated. Following this terminology there are no prothoracic
spiracles ; although sometimes the first pair of spiracles is situated in
the hind margin of the prothorax, having migrated forward from its
normal position. It Wfpuld be better to designate the thoracic
spiracles as the first and second pairs of thoracic spiracles, respec-
tively; in this way the same term would be applied to a pair of
spiracles whatever its position. There are many references in
entomological works to "prothoracic spiracles," but these refer to the
pair of spiracles that are more commonly designated the mesothoracic
spiracles.
In many cases the abdominal spiracles have migrated back upon
the segment in front of which they are normally situated, being fre-
quently situated upon the middle of the segment.
The statements made above refer to the normal number and dis-
tribution of spiracles; but a very wide range of variations from this
type exists. Perhaps the most abnormal condition is that found in
the genus Smynthurus of the Collembola, where there is a single pair
of spiracles which is borne by the neck. In the Poduridae, also of the
Collembola, the respiratory system has been lost, there being neither
tracheae nor spiracles.
Terms indicating the distribution of the spiracles. — The following
terms are used for indicating the distribution of the spiracles ; they
have been used most frequently in descriptions of larvae of Diptera.
These terms were formed by combining with pneustic (from pneo, to
breathe) the following prefixes: peri-, around, about; pro-, before';
meta- after; and amphi, both.
Peripneustic. — Having spiracles in a row on each side of the body,
the normal type.
Propmustic. — With only the first pair of spiracles.
Metapneustic. — With only the last pair of spiracles.
Amphipneustic. — With a pair of spiracles at each end of the body.
116
AN INTRODUCTION TO ENTOMOLOGY
Fig. 131. — Spiracles; a, of the larva of
Corydalus; b, of the larva of Droso-
phila amcena.
The structure of spiracles. — In their simplest form the spiracles or
stigmata are small round or oval openings in the body-wall. In many
cases they are provided with hairs to exclude dust ; in some, as in the
larva of Corydalus, each spiracle is
furnished with a lid (Fig. 131, a);
in fact, very many forms of
spiracles exist. Usually each spir-
acle opens by a single aperture;
but in some larva? and pupae of
Diptera they have several openings
(Fig. 131, 6).
The closing apparatus of the
tracheae. — Within the body, a
short distance back of the spiracle,
there is an apparatus consisting of
several chitinous parts, surrounding the trachea, and moved by a
muscle, by which the trachea can be closed by compression (Fig. 132).
This is the closing apparatus of the trachea. The closing of this appara-
tus and the
contraction of
the body by
the respiratory
muscles is sup-
posed to force
the air into
thetracheoles,
which are the
essential res-
piratory or-
gans.
Fig. 1 32. — Diagrams representing the closing apparatus of the
tracheae; a, b,c, chitinous patts of the apparatus; m, muscle;
A, apparatus open; B, apparatus closed; C, spiracle and
trunk of trachea showing the position of the apparatus.
(From Judeich and Nitsche).
2. THE TRACHEAE
Each spiracle is the opening of an air-tube or trachea. The main
tracheal trunk which arises from the spiracle soon divides into several
branches, these in turn divide, and by repeated divisions an immense
number of branches are formed. Every part of the body is supplied
with tracheae.
In a few insects the group of tracheae arising from a spiracle is not
connected with the groups arising from other spiracles; this is the
case in Machilis (Fig. 133). In most insects, however, each group of
tracheae is connected with the corresponding groups in adjacent seg-
THE INTERNAL ANATOMY OF INSECTS 111
ments by one or more longitudinal tracheae, and is also connected
Fig. 133. — The tracheae of Machilis (From Oudemans).
with the group on the opposite side of the same segment by one or
more transverse tracheae (Fig. 134).
The structure of the tracheae. — The fact that
in their embryological development the tracheae
arise as invaginations of the body-wall, makes it
easy to understand the structure of the tracheae.
The three layers of the body-wall are directly-
continuous with corresponding layers in the wall
of a trachea (Fig. 135). These layers of -a
trachea are designated as the intima, the epithe^
Hum, and the basement membrane.
The intima is the chitinous inner layer of the
tracheae. It is directly continuous with the
cuticula of the body-
wall, and like the
cuticula is molted at
each ecdysis.
A peculiar feature
of the intima of
tracheae is the fact
that it is furnished
with thickenings
which extend spirally^
These give the
Fig. 134. — Larva of tracheae their charac- Fig. 135. — Section of a trachea
Cantharis vesicatoria, t • .,.• f q i and the body-wall; c, cuti-
showing the distribu- l cula; h, hypodermis; bm,
t.irm of trflr.he.ap! CFrom striated appearance, basement membrane; sp%
Henneguy after jf • f f spiral thickening of the ih-
Beauregard). tima, the taenidium. .
the larger tracheae be
pulled apart the intima will tear between the folds of the spiral
thickening, and the latter will uncoil from within the trachea like a
118 AN INTRODUCTION TO ENTOMOLOGY
thread (Fig. 135). The spiral thickening of the intima of a trachea is
termed the t&mdium. In some insects there are several parallel
taenidia; so that when an attempt is made to uncoil the thread a
ribbon-like band is produced, composed of several parallel threads.
This condition exists in the larger trachea? of the larva Corydalus.
The epithelium of the trachea is a cellular layer, which is directly
continuous with the hypodermis of the body -wall.
The basement membrane is a delicate layer, which supports the
epithelium, as the basement membrane of the body-wall supports the
hypodermis.
3. The Tracheoles
The tracheoles are minute tubes that are connected with the tips
of trachea? or arise from their sides, but which differ from tracheae in
their appearance, structure, and mode of origin; they are not small
trachea?, but structures that differ both histologically ancl in their
origin from trachea?.
The tracheoles are exceedingly slender, measuring less than one
micron in diameter; ordinarily they do not taper as do trachea?;
they contain no ta?nidia; and they rarely branch, but often anasto-
mose which gives them a branched appearance (Fig. 136, t and
138 B, *).
Each tracheole is of unicellular origin, and is, at first, intracellular
in position, being developed coiled within a single cell of the epithelium
of a trachea. In this stage of its development it has no connection
with the lumen of the trachea in the wall of which it is developing,
being separated from it by the intima of the trachea. A subsequent
molting of the intima of the trachea opens a connection between the
lumen of the tracheole and the trachea. At the same time or a little
later the tracheole breaks forth from its mother cell, uncoils, and
extends far beyond the cell in which, it was developed.
The tracheoles are probably the essential organs of respiration, the
tracheae acting merely as conduits of air to the tracheoles.
4. The Air-Sacs
In many winged insects there are expansions of the tracheae,
which are termed air -sacs. These vary greatly in number and size.
In the honeybee there are two large air-sacs which occupy a consider-
able part of the abdominal cavity; while in a May-beetle there are
hundreds of small air-sacs. The air-sacs differ from tracheae in
lacking taenidia.
THE INTERNAL ANATOMY OF INSECTS
119
As the air-sacs lessen the specific gravity of the insect they proba-
bly aid in flight; as filling the lungs with air makes it easier for a man
to float in water ; in each case there is a greater volume for the same
weight.
5. Modifications of the open type of respiratory organs in
aquatic insects
There are many insects in which the spiracles are open that live in
water ; these insects breathe air obtained from above the surface of
the water. Some of these insects breathe at the surface of the water,
Fig. 136. — Part of a trachcal gill of the larva of Corydalus; T, trachea; /,
tracheoles.
as the larvas and pupae of mosquitoes, the larvas of Eristalis, and the
Nepidas; others get a supply of air and carry it about with them
beneath the surface of the water, as the Dytiscidae, the Notonectidae
and the Corisidae. The methods of respiration of these and of other
aquatic insects with open spiracles are described in the accounts of
these insects given later.
b. THE CLOSED OR APNEUSTIC TYPE OF RESPIRATORY ORGANS
That type of respiratory organs in which the spiracles do not
function is termed
the closed or
apneustic* type; it
exists in naiads and
in a few aquatic
larvae.
i. The Tracheal
Gills
In the immature
«Sects mentioned
above, the air in
the body is purified by means of organs known as tracheal gills.
* Apneustic: apneustos (&Trvev<rTos), without breath.
120
AN INTRODUCTION TO ENTOMOLOGY
These are hair-like or more or less plate-like expansions of the body-
wall, abundantly supplied with tracheae and tracheoles. Figures 136
and 137 represents a part of a tuft of hair-like tracheal gills of a
larva of Corydalus and figure 138 a plate-like tracheal gill of
a naiad of a damsel-fly. In these tracheal gills the tracheoles
are separated from the air in the water only by the delicate
wall of the tracheal
gill which admits of
the transfer of gases
between the air in the
tracheoles and the air
in the ^ water.
Tracheal gills are
usually borne by the
abdomen, sometimes
by the thorax, and in
case of one "genus of
stone-flies by the head .
They pertain almost
exclusively to the immature stages of insects ; but stone-
flies of the genus Pteronarcys retain them throughout their
existence. In the naiads of the Odonata the rectum is
supplied with many tracheae and functions as a tracheal gill.
138. — Tracheal gill of a damsel-
ly: A, entire gill showing the
tracheae; B, part of gill more
magnified showing both tracheae (T)
and tracheoles (t).
2. Respiration of Parasites
It is believed that internal parasitic larvae derive their air 'from air
that is contained in the blood of their hosts, and that this is done by
osmosis through the cuticula of the larva, the skin of the larva being
furnished with a network of fine tracheae (Seurat '99).
3. The blood-gills
Certain aquatic larvae possess thin transparent extensions of the
body wall, which are filled with blood, and serve as respiratory organs.
These are termed blood-gills.
Blood-gills have been observed in comparatively few insects;
among them are certain trichopterous larvae; the larva of an exotic
beetle, Pelobius; and a few aquatic dipterous larvae, Chironomus and
Simulium. It is probable that the ventra sacs of the Thysanura,
described in the account of that order, are also blood-gills.
THE INTERNAL ANATOMY OF INSECTS
121
V. THE CIRCULATORY SYSTEM
The general features of the circulatory system. — In insects the cir-
culatory system is not a dosed one, the blood flowing in vessels during
only a part of its course. The greater part of the circulation of this
fluid takes place in the cavities of the body and of its appendages,
where it fills the space not occupied by the internal organs.
Almost the only blood-vessel that exists in insects lies just beneath
the body- wall, above the alimentary canal (Fig. 105, h). It extends
from near the caudal end of the abdomen through the thorax into the
head. That part of it that lies in the abdomen is the heart; the more
slender portion, which traverses the thorax and extends into the head
is the apxia.
On each side of the heart, there is a series
of triangular muscles extending from the heart
to the lateral wall of the body. . These con-
stitute the dorsal diaphragm or the wing's of the
heart. They are discussed later under the
ff cl 13 head: Suspensoria of the Viscera.
11 The heart.— The heart is a tube, which is
usually closed at its posterior end; at its
anterior end it is continuous with the aorta.
The heart is divided by constrictions into
chambers which are separated by valves (Fig.
139). The number of these chambers varies
greatly in different insects; in some, as in
Phasma and in the larva of Corethra, there is
only one, in others, as in the cockroach, there
are as many as thirteen, but usually there are
Fig. 139.— Heart of a not more than eight. The blood is admitted to
May -beetle; a, lateral the heart through slit-like openings, the ostia of
aspect 01 the aorta; o, . . . . 1
interior of the heart the heart; usually there is a pair of ostia in the
showing valves; c, IntrrnJ wnlte r>f pa^ Hi am NT Each ostium is
ventral aspect of the **~x*^*- ^
heart and wing-mus- furnished with a valve which closes it when the
cles, the muscles are chamber contracts.
represented as cut away . ...
from the caudal part of The wall of the heart is composed of two dis-
the heart; d, dorsal tinctlavers: an inner muscular layer ; and an
aspect of the heart •* . -11
(After Straus-Durck- outer, connective tissue or peritoneal layer.
heim)- The muscular layer consists chiefly of annular
muscles; but longitudinal fibers have also been observed.
122 AN INTRODUCTION TO ENTOMOLOGY
The pulsations of the heart. — When a heart consists of several
chambers, they contract one after another, the wave of contraction
passing from the caudal end of the heart forwards. As the valves
between the chambers permit the blood to move forward but not
in the opposite direction, the successive contraction of the chambers
causes the blood received through the ostia to flow toward the head, into
the aorta.
The aorta. — The cephalic prolongation of the heart, the aorta
(Fig. 139, a), is a simple tube, which extends through the thorax into
the head, where it opens in the vicinity of the brain. In some cases,
at least, there are valves in the aorta.
The circulation of the blood. — The circulation of the blood can be
observed in certain transparent insects, as in young naiads, in larvae
of Trichoptera, and in insects that have just molted. The blood flows
from the open, cephalic end of the aorta and passes in quite definite
streams to the various parts of the body-cavity and into the cavities
of the appendages. These streams, like the ocean currents, have no
walls but flow in the spaces between the internal organs. After
bathing these organs, the blood returns to the sides of the heart,
which it enters through the ostia.
Accessory circulatory organs. — Accessory pulsating circulatory
organs have been described in several insects. These are sac-like
structures which contract independently of the contractions of the
heart. They have been found in the head in several Orthoptera; in
the legs of Hemiptera, and in the caudal filaments of Ephemerida.
VI. THE BLOOD
The blood of insects is a fluid, which fills the perivisceral cavity,
bathing all of the internal organs of the body, and flowing out into the
cavities of the appendages of the body. In only a comparatively
small portion of its course, is the blood enclosed in definite blood-
vessels; these, the heart and the aorta are described above. The
blood consists of two elements, a fluid plasma and cells similar to the
white corpuscles of the blood of vertebrates, the leucocytes.
The blood of insects differs greatly in appearance from the blood
of vertebrates, on account of the absence of ^ed blood-corpuscles. In
most insects the blood is colorless ; but in many species it has a yellow-
ish, greenish, or reddish color. In the latter case, however, the color
is not due to corpuscles of the type which gives the characteristic
color to the blood of vertebrates.
THE INTERNAL ANATOMY OF INSECTS 123
The leucocytes are nucleated, colorless, amoeboid cells similar to
the white corpuscles of vertebrates, in appearance and function; they
take up and destroy foreign bodies and feed upon disintegrating tissue.
It is believed that the products of digestion of disintegrating tissue by
the leucocytes pass into the blood and serve to nourish new tissue.
The blood receives the products of digestion of food, which pass
in a liquid form, by osmosis, through the walls of the alimentary canal.
On the other hand it gives up to the tissues which it bathes the
materials needed for their growth. In insects oxygen is supplied to
the tissues and gaseous wastes are removed chiefly by the respiratory
system and not by means of the blood as in vertebrates.
VII. THE ADIPOSE TISSUE
On opening the body of an insect, especially of a larva, one of the
most conspicuous things to be seen is fatty tissue, in large masses.
These often completely surround the alimentary canal, and are held
in place by numerous branches of the tracheae with which they are
supplied. Other and smaller masses of this tissue adhere to the inner
surface of the abdominal wall, in the vicinity of the nervous system,
and at the sides of the body. In adult insects it usually exists in
much less quantity than in larvae.
The chief function of the adipose tissue is the storage of nutriment ;
but it is believed that it also has a urinary function, as concretions of
uric acid accumulate in it during the life of the insect.
VIII. THE NERVOUS SYSTEM
a. THE CENTRAL NERVOUS SYSTEM
The more obvious parts of the central nervous system are the
following: a ganglion in the head above the oesophagus, the brain;
a ganglion in the head below the oesophagus, the subcesophageal
ganglion; a series of ganglia, lying on the floor of the body cavity in
the thorax and in the abdomen, the thoracic and the abdominal
ganglia; two longitudinal cords, the connectives, uniting all of these
ganglia in a series ; and many nerves radiating from the ganglia to the
various parts of the body.
The connectives between the brain and the subcesophageal
ganglion pass one on each side of the oesophagus ; these are termed the
crura cerebri, or the legs of the brain ; in the remainder of their course,
the twro connectives are quite closely parallel (Fig. 124).
124
AN INTRODUCTION TO ENTOMOLOGY
The series of ganglia is really a double one, there being typical!}' a
pair of ganglia in each segment of the body ; but each pair of ganglia
are more or less closely united on the middle line of the body, and
often appear to be a single ganglion.
In some cases the ganglia of adjacent segments coalesce, thus
reducing the number of distinct ganglia in the series. It has been
demonstrated that the brain is composed of the coalesced ganglia of
three of the head segments, and the suboesophageal ganglion of the
coalesced ganglia of the remaining four segments.
B
Fig. 140. — -Successive stages in the coalescence of thoracic and of abdominal
ganglia in Diptera; A, Chironomus; B, Empis; C, Tabanus; D, Sar-
. cophaga (From Henneguy after Brandt).
//'
The three parts of the brain, each of which is composed of the pair
of ganglia of a head segment, are designated as the protocerebrum, the
deutocerebrum, and the tritocerebrum, respectively. The protocere-
brum innervates the compound eyes; the deutocerebrum, the
antennaB; and the tritocerebrum, the labrum.
The suboesophageal ganglion is composed of four pairs of primary
ganglia ; these are the ganglia of the segments of which the mandibles,
the maxillulas, the maxillae, and the labium, respectively, are the
appendages.
The three pairs of thoracic ganglia often coalesce so as to form a
single ganglionic mass; and usually in adult insects the number of
abdominal ganglia is reduced in a similar way.
THE INTERNAL ANATOMY OF INSECTS
125
Successive stages in the coalescence of the thoracic and abdominal
ganglia can be seen by a study of the nervous system of the larva,
pupa, and adult of the same species, a distinct cephalization of the
central nervous system taking place during the development of the
insect. Varying degrees of coalescence of the thoracic and of the
abdominal ganglia can be seen by a comparative study of the nervous
systems of different adult insects (Fig. 140).
The transverse band of fibers that unite the two members of a pair
of ganglia is termed a commissure. In addition to the commissures
that pass directly from one member of a pair of ganglia to the other,
there is in the head a com-
missure that encircles the
oesophagus in its passage
from one side of the brain
to the other, this is the sub-
osophageal commissure (Fig.
141).
The nerves that extend
3 central chain of
to the different
cesophageal commissure; s~g, suboesophageal rfq r f^ hoHv Prp « nflrf-
ganglion; pg, paired ganglion (After Lienard). *
of the central nervous sys-
tem ; the core of each nerve fiber being merely a process of a ganglionic
cell, however long it b
may be.
~-oe
Fig. 141. — Lateral view of the oesophagus of a from
caterpillar, showing the suboesophageal com-
missure; b, brain; oe, oesophagus; sc, sub- ganglia
oes st a\ i'__ r OP
b. THE GESOPHAGEAL
SYMPATHETIC NER-
VOUS SYSTEM
In addition to the
central nervous sys-
tem as defined above
there are three other
nervous complexes
which are commonly
described as separate
systems although
they are connected
to the central nervous
system by nerves.
These are the oeso-
phageal sympathetic
Fig. 142. — Lateral view of the nerves of the head in the
larva of Corydalus; a, antennal nerve; ao, aorta; ar
paired nerves connecting the frontal ganglion with the
brain; b, brain; cl, clypeo-labral nerve; con, connective;
cr, crura cerebri; fg, frontal ganglion; fn, frontal nerve;
i, unpaired nerve connecting the frontal ganglion with
the brain; I, labial nerve; Ig, the paired ganglia; md,
mandibular nerve; m, p, q, s, u, z, nerves of the cesopha-
geal sympathetic system; mx, maxillary nerve; o, optic
nerves; oes, oesophagus; ph, pharynx; pn, pharyngeal
nerve; r, recurrent nerve; sc, subcesophageal commis-
sure; sg, 'suboesophageal ganglion; st, stomagastric
nerve; v, ventricular ganglion (From Hammar).
nervous system, the ventral sympathetic nervous
126
AN INTRODUCTION TO ENTOMOLOGY
system, and the peripheral sensory nervous system. The first of
these* is connected with the brain; the other two, with the thoracic
and abdominal ganglia of the central nervous system.
The oesophageal sympathetic nervous system is intimately
associated with the oesophagus and, as just stated, is connected with
the brain. It is described by different writers under various names ;
among these are visceral, vagus, and stomato gastric. It consists of two,
more or less distinct, divisions, an unpaired median division and a
paired lateral division.
The unpaired division of the oesophageal sympathetic nervous
system is composed of the following parts, which are represented in
Figures 141, 142, 143, and
144: the frontal ganglion
(fg), this is a minute gang-
lion situated above the
oesophagus a short- .distance
in front of the brain; the
unpaired nerve connecting
the frontal ganglion with the
brain (i), this is a small
nerve extending from the
brain to the frontal ganglion ;
the paired nerves connecting
the frontal ganglion with the
brain (ar), these are arching
nerves, one on each side,
extending from the upper
ends of the crura cerebri to
the frontal ganglion; the
frontal nerve (fn), this nerve
arises from the anterior bor-
der of the frontal ganglion
and extends cephalad into
the clypeus, where it bifur-
cates; the pharyngeal
nerves (pn), these extend,
one on each side, from the
frontal ganglion to the
lower portions of the pharynx; the recurrent nerve (r), this is a single
median nerve, which arises from the caudal border of the frontal
ganglion, and extends back, passing under the brain and between the
Fig. 143.— Dorsal view of the nerves of the
head in the larva of Corydalus; e, ocelli;
mnd. mandible; other lettering as in
Figure 142 (From Hammar).
THE INTERNAL ANATOMY OF INSECTS
127
aorta and the oesophagus, to terminate in the ventricular ganglion;
the ventricular ganglion (v), this is a minute ganglion on the middle
line, a short distance caudal of the brain, and between the aorta and
the oesophagus; and the stomogastric nerves (si), these are two nerves
which extend back from the caudal border of the ventricular ganglion,
they are parallel for a short distance,, then they separate and pass, one
on each side, to the sides of the alimentary canal which they follow
to the proventriculus.
The paired division of the cesophageal sympathetic nervous system
varies greatly in form in different insects. In the larva of Corydalus,
there is a single pair of ganglia (Fig. 142 and 143, lg), one on each
side of the oesophagus; each of these ganglia is connected with the
brain by two nerves (m and u) but they are not connected with each
other nor with the unpaired division
of this system. In ~a cockroach
(Fig. 144), there are two pairs of
ganglia (ag and pg); the two ganglia
of each side are connected with each
' other and with the recurrent nerve of
the unpaired division.
As yet comparatively little is
known regarding the function of the
cesophageal sympathetic nervous sys-
tem of insects ; nerves extending from
it have been traced to the clypeus,
the muscles of the pharynx, the oeso-
phagus, the mid-intestine, the salivary
glands, the aorta, and the heart.
Its function is probably analogous to
that of the sympathetic nervous sys-
tem of Vertebrates.
~sn
THE VENTRAL SYMPATHETIC NERV-
OUS SYSTEM
The ventral sympathetic nervous
Fig. 144. — The cesophageal sympa-
thetic nervous system of Peri- c.
planeta orientalis; the outlines of
the brain (b) and the roots of the
antennal nerve which cover a por-
tion of the sympathetic nervous
system are given in dotted lines; system consists of a series of more or
ag, anterior ganglion; pg, posterior , . ... , , , A
ganglion; fgt frontal ganglion; sn, less similar elements, each connected
nerves of the salivary glands; r, with a ganglion of the ventral chain
recurrent nerve (After Hofer). - . m
of the central nervous system. Typi-
cally there is an element of this system arising in each thoracic and
128
AN INTRODUC7ION TO ENTOMOLOGY
ig T . -
the v
abdominal ganglion; and each element consists of a median nerve
extending from the ganglion of its origin caudad between the two
connectives, a pair of lateral branches of this median nerve, and one
or more ganglionic enlargements of each lateral branch. Frequently
the median nerve extends to the ganglion of the following segment.
A simple form of this system exists in the larva of
Cossus ligniperda (Fig. 122); and a more compli-
cated one, in Locusta viridissima (Fig. 145).
From each lateral branch of the median nerve a
slender twig extends to the closing apparatus of the
tracheae.
d. THE PERIPHERAL SENSORY NERVOUS SYSTEM
Immediately beneath the hypodermal layer of the
body-wall, there are many bipolar and multipolar
nerve-cells whose prolongations form a network of
nerves; these constitute the peripheral sensory
part Of nervous system or the subhypodermal nerve -plexus.
ventral chain The fine nerves of this system are banches of
of ganglia of Lo- , n . -, . . ., «
custa viridissima larger nerves which arise in the central nevous sys-
and of the ven- tern; and the terminal prolongations of trjjebipolar
ncrvSSs^ sy s- nerve-cells innervate the sense-hairs of the dbody- wall.
tem; g, ganglion Figure 146 represents a surface view of a small
nervous system- Part °^ tne peripheral sensory nervo us system of the
n, nerve; c, con- tsilkworm, Bombyoo mori, as figuredby Hilton ('02) ;
dfaV nerv^f The ^e bases of several sense hairs arealso shown. The
sympathetic sys- oetails of this figure are as follows: h, h, h, the bases
1km 'of the 8sym- *' sense -hairs; s, s, s, bipolar nerve-cells; m, m, m
pathetic system multipolar cells ; n, n, n, nerves. All of these struct-
ures are united, forming a network. Of especial
interest is the fact that the terminal prolongation of each bipolar
nerve -cell enters the cavity of a sense -hair and that the other pro-
longation is a branch of a larger nerve which comes from the central
nervous system.
The peripheral sensory nervous system is so delicate that it can
not be seen except when it is stained by some dye that differentiates
nervous matter from other tissues. For this purpose the intra vitam
methylen blue method of staining is commonly used.
THE INTERNAL ANATOMY OF INSECTS
129
IX. GENERALIZATIONS REGARDING THE SENSE-
ORGANS OF INSECTS
The sense-organs of insects present a great variety of forms, some
of which are still incompletely understood, in spite of the fact that
they have been investigated by many careful observers. In the
limited space that can be devoted to these organs here only the more
general features of them can be described and some of the disputed
questions regarding them briefly indicated.
A classification of the sense-organs. — The different kinds of sense-
organs are distinguished by the nature of the stimulus that acts on
Fig. 146. — Surface view of subhypodermal nerves and nerve -cells from
the silkworm (From Hilton)
each. This stimulus may be either a mechanical stimulus, a chemical
one, or light. The organs of touch and of hearing respond to mechani-
cal stimuli; the former, to simple contact with other objects; the
latter, to vibratory motion caused by waves of sound. The organs of
taste and of smell are influenced only by soluble substances and it
seems probable that chemical changes are set up in the sense-cells by
these substances ; hence these organs are commonly referred to as the
chemical sense-organs ; no criterion has been discovered by which the
organs of taste and of smell in insects can be distinguished. The
organs of sight are acted upon by light ; it is possible that the action
of light in this case is a chemical one, as it is on a photographic plate,
130
AN INTRODUCTION TO ENTOMOLOGY
but the eyes have not been classed among the chemical sense-organs.
For these reasons the following groups of sense-organs are recognized :
The mechanical sense-organs. — The organs of touch and of hearing.
The chemical sense-organs. — The organs of taste and of smell.
The organs of sight. — The compound eyes and the ocelli.
The cuticular part of the sense-organs. — In most if not all of the
sense-organs of insects there exists one or more parts that are of cuti-
cular formation. The cuticular parts of the organs of sight and of
hearing are described later, in the accounts of these organs; in this
place, a few of the modifications of the cuticula found in other sense-
organs are described.
Each of the cuticular formations described here is found either
within or at the outer end of a pore in the cuticula ; as some of these
formations are obviously setae and others are regarded as modified
setae, this pore is usually termed the trichopore; it has also been
termed the neuropore, as it is penetrated by a nerve-ending.
As the cuticular part of this
group of sense-organs, those other
than the organs of hearing and
of sight, is regarded as a seta,
more or less modified, these
organs are often referred to as
the setiferous sense-organs; they
are termed the Hautsinnesorgane
by German writers.
Special terms have been
applied to the different types of
setiferous sense-organs, based on
the form of the cuticular part of
each; but these types cannot
be sharply differentiated as
intergrades exist between them.
In Figure 147 are represented
the cuticular parts of several of
these different types; these are
designated as follows :
Fig- 147- — Various forms of the cuticular The thick-walled sense-hair,
portion of the setiferous sense-organs. .„ , . , , T ^.u-
The lettering is explained in the text. sensillum tnchodeum—ln this
type the cuticular part is a seta,
the base of which is in an alveolus at the end of a trichopore and is
connected with the wall of the trichopore by a thin articular mem-
brane (Fig. 147, a.)
THE INTERNAL ANATOMY OF INSECTS 131
If the sense-hair is short and stout, it is termed by some writers
a sense-bristle, sensillum ch&ticum; but there is little use for this dis-
tinction.
In the thick-walled sense-hairs, the wall of the seta is fitted to
receive only mechanical stimuli, being relatively thick, and as these
organs lack the characteristic features of the organs of hearing, they
are believed to be organs of touch.
The sense-cones. — The sense-cones vary greatly in form and in their
relation to the cuticula of the body- wall; their distinctive feature is
that they are thin-walled. For this reason, they are believed to be
chemical sense-organs, the thinness of the wall of the cone permitting
osmosis to take place through it. In the sense-cones, too, there is no
joint at the base, as in the sense-hairs, the articular membrane being
of the same thickness as the wall of the cone ; there is, therefore, no
provision for movement in response to mechanical stimuli.
In one type of sense-cone, the sensillum basiconicum, the base of
the cone is at the surface of the body-wall (Fig. 147, 6). In another
type, sensillum cceloconicum, the cone is in a pit in the cuticula of the
body- wall (Fig. 147, c). Two forms of this type are represented in
the figure; in one, the sense-cone is conical; in the other, it is fungi-
form. Intergrades between the* basiconicum and the cceloconicum
types exist (Fig. 147, d).
The flask-like sense-organ, sensillum ampullaceum. — This is a
modification of the sense-cone type, the characteristic feature of
which is that the cone is at the bottom of an invagination of the articu-
lar membrane; in some cases the invagination is very deep so that
the cone is far within the body- wall (Fig. 147, e) ; intergrades between
this form and the more common sensillum cceloconicum exist (Fig.
147, /)•
The pore-plate, sensillum placodeum. — In this type the cuticular
part of the organ is a plate closing the opening of the trichopore; in
some cases, this plate is of considerable thickness with a thin articular
membrane (Fig. 147, g); in others it is thin throughout (Fig. 147, h).
The olfactory pores. — This type of sense-organ is described later.
X. THE ORGANS OF TOUCH
The organs of touch are the simplest of the organs of special sense
of insects. They are widely distributed over the surface of the body
and of its appendages. Each consists of a seta, with all the character-
istics of setae already described, a trichogen cell, which excreted the
132 AN INTRODUCTION TO ENTOMOLOGY
seta,, and a bipolar nerve-cell. These organs are of the type known as
sensillum trichodeum referred to in the preceding section of this
chapter.
According to the observations of Hilton ('02) the terminal pro-
longation of the nerve-cell enters the hair and ends on one side of it at
some distance from its base (Fig. 148). The proximal part of this
nerve-cell is connected with the peripheral sensory nervous system, as
already described (page 128).
The presence of this nervous connection is believed to distinguish
tactile hairs from those termed clothing hairs, and from the scales
that are modified seta?. If this distinction is a good one, it is quite
probable that many hairs and scales that are now regarded as merely
clothing will be found to be sense-organs, when studied by improved
histological methods. In fact Guenther ('01) and others have shown
that some of the scales on the wings of Lepidoptera, especially those
on the veins of the wings, are supplied with nerves ; but the function
of these scales is unknown.
Hilton states that he ' 'found no evidence to indicate nerves ending
in gland cells or trichogen cells by such branches as have been described
and figured by Blanc ('90), but in every case the very fine nerve
termination could be traced up past the hypodermal cell layer with
no branches." Many figures of unbranched nerve fibers ending in
sense-hairs are also given by O. vom Rath ('96).
A very different form of nerve-endings in sense-hairs is given by
Berlese ('09, a). This author represents the nerve extending to a
sense-hair as dividing into many bipolar nerve-endings.
XL THE ORGANS OF TASTE AND OF SMELL
(The chemical sense-organs)
It is necessary to discuss together the organs of taste and of smell,
as no morphological distinction between them has been discovered.
If a chemical sense-organ is so located that it comes in contact with
the food of the insect, it is commonly regarded as an organ of taste, if
not so situated, it is thought to be an organ of smell. In the present
state of our knowledge, this is the only distinction that can be made
between these two kinds of organs.
Many experiments have been made to determine the function of
the various chemical sense-organs but the results are, as yet, far from
conclusive. The problem is made difficult by the fact that these
THE INTERNAL ANATOMY OF INSECTS
133
organs are widely distributed over the body and its appendages, and
in some parts, as on the antennae of many insects, several different
types of sense-organs are closely associated.
Those organs that are characterized by the presence of a thin-
walled sense-cone (Fig. 147, b-f) or by a pore-plate (Fig. 147, g, h) are
believed to be chemical sense-organs. It is maintained by Berlese
('09, a) that an essential feature of these chemical sense-organs is the
presence of a gland -cell, the excretion of which, passing through the thin
wall of the cuticular part, keeps the outer surface of this part, the
sense-cone or pore-plate, moist and thus fitted for the reception of
chemical stimuli. According to this view a chemical sense-organ
consists of a cuticular part, a trichogen cell or cells which produced
3i
Fig. 148. — Sections through the body-wall and sense-hairs of the silk-
worm; c, cuticula; h, hair; hy, hypodermis; n, nerve; 5, bipolar
nerve-cell (From Hilton). The line at the right of the figure indi-
cates one tenth millimeter.
this part, a gland-cell which excretes a fluid which keeps the part
moist, and a nerve-ending.
It is interesting to note that tactile hairs may be regarded as
specialized clothing hairs, specialized by the addition of a nervous
connection, and that sense-cones and pore-plates may be regarded as
specialized glandular hairs with a nervous connection; in the latter
case, the specialization involves a thinning of the wall of the hair so as
to permit of osmosis through it.
In the different accounts of chemical sense-organs there are
marked differences as regards the form of the nerve-endings. In
many of the descriptions and figures of these organs the nerve-ending
is represented as extending unbranched to the chitinous part of the
organ, resembling in this respect those represented in Figure 148.
In other accounts the gland-cell is surrounded by an involucre of
nerve-cells (Fig. 149).
134
AN INTRODUCTION TO ENTOMOLOGY
In the types of chemical sense-organs
action of the chemical stimuli is supposed to
/**
Fig. 149. — Section of the external layers of the wall of
an antenna of Acrida turrita; Ct, cuticula; Ip, hypo-
dermis; JV, nerve; Nv, involucre of nerve-cells sur-
rounding the glandular part of a sense-organ; Sbc,
sensillumbasiconicum; Sec, sensillum coeloconicum.
Three sense-organs are figured; a surface view of the
first is represented, the other two are shown in section.
(From Berlese).
described above the
be dependent upon os-
mosis through a deli-
cate cuticular mem-
brane. It should be
noted, however, that
several writers have de-
scribed sense-cones in
which there is a pore;
but the accuracy of
these observations is
doubted by other
writers.
A very different
type of sense-organs
which has been termed
olfactory pores is de-
scribed in the conclud-
ing section of this
Chapter.
XII. THE ORGANS OF SIGHT
a. THE GENERAL FEATURES
The two types of eyes of insects. — It is shown in the preceding
chapter that insects possess two types of eyes, the ocelli or simple eyes
and the compound or facetted eyes.
Typically both types of eyes are present in the same insect, but
either may be wanting. Thus many adult insects lack ocelli, while
the larvae of insects with a complete metamorphosis (except Corethra)
lack compound eyes.
When all are present there are two compound eyes and, typically,
two pairs of ocelli ; but almost invariably the members of one pair of
ocelli are united and form a single median ocellus. The median ocel-
lus is wanting in many insects that possess the other two ocelli.
The distinction between ocelli and compound eyes. — The most
obvious distinction between ocelli and compound eyes is the fact that
in an ocellus there is a single cornea while in a compound eye there are
many. Other features of compound eyes have been regarded as dis-
tinctively characteristic of them; but in the case of each of these
features it is found that they exist in some ocelli.
THE INTERNAL ANATOMY OF INSECTS 135
Each ommatidium of a compound eye has been considered as a
separate eye because its nerve-endings constituting the retinula are
isolated from the retinulae of other ommatidia by surrounding acces-
sory pigment cells ; but a similar isolation of retinulae exist in some
ocelli.
It has also been held that in compound eyes there is a layer of cells
between the corneal hypodermis and the retina, the crystalline-cone-
cells, which is absent in ocelli ; but in the ocelli of adult Ephemerida
there is a layer of cells between the lens and the retina, which, at least,
is in a position analogous to that of the crystalline-cone-cells; the
two may have had a different origin, but regarding this, we have, as
yet, no conclusive data.
The absence of compound eyes in most of the Apterygota.—
Typically insects possess both ocelli and compound eyes ; when either
kind of eyes is wanting it is evidently due to a loss of these organs and
not to a generalized condition. Although compound eyes are almost
universally absent in the Apterygota in the few cases whtre they
are present in this group they are of a highly developed type and not
rudimentary; the compound eyes of Machilis, for example, are as
perfect as those of winged insects.
The absence of compound eyes in larvae. — The absence of com-
pound eyes in larvae is evidently a secondary adaptation to their
particular mode of life, like the internal development of wings in the
same forms. In the case of the compound eyes of larvae, the develop-
ment of the organs is retarded, taking place in the pupal stage instead
of in an embryonic stage, as is the case with nymphs and naiads.
While the development of the compound eyes as a whole is retarded
in larvae, a few ommatidia may be developed and function as ocelli
during larval life.
b. THE OCELLI
There are two classes of ocelli found in insects : first, the ocelli of
adult insects and of nymphs and naiads, which may be termed the
primary ocelli; and second, the ocelli of most larvae possessing ocelli,
which may be termed adaptive ocelli.
The primary ocelli. — The ocelli of adult insects and of nymphs and
naiads having been originally developed as ocelli are termed the
primary ocelli. Of these there are typically two pairs; but usually
when they are present there are only three of them, and in many cases
only a single pair.
136
AN INTRODUCTION TO ENTOMOLOGY
When there are three ocelli, the double nature of the median ocel-
lus is shown by the fact that the root of the nerve is double, while that
of each of the other two is single.
In certain generalized insects, as some Plecoptera, (Fig. 150) all of
the ocelli are situated in the front; but in most insects, the paired
ocelli have either migrated into the suture between the front and the
vertex (Fig. 151), or have proceeded farther and are situated in the
vertex.
The structure of primary ocelli is described later.
The adaptive ocelli. — Some larvae, as those of the Tenthredinidae,
possess a single pair of ocelli, which in their position and in their
structure agree with the ocelli of the adult insects ; these are doubtless
primary ocelli. But most larvae have lost the primary ocelli; and
if they possess ocelli the position of them and their structure differ
greatly from the positions and structure of primary ocelli.
Except in the few cases where primary ocelli
have been retained by larvae, the ocelli of
larvae are situated in a position corresponding
to the position of the compound eyes of the
adult (Fig. 152); and there are frequently
* several of these ocelli on each side of the head.
This has led to the belief that they represent
a few degenerate ommatidia, which have been
a retained by the larva, while the development of
the greater number of ommatidia has been
retarded. For this reason they are termed
adaptiw ocelli.
The number of adaptive ocelli varies greatly,
and sometimes is not con-
stant in a species; thus
in the larva of Corydalus,
there may be either six or seven ocelli on each
side of the head.
There are also great variations in the struct-
ure of adaptive ocelli. These variations pro-
bably represent different degrees of degeneration
or of retardation of development. The extreme
of simplicity is found in certain dipterous larvae ;
according to Hesse (*oi) an ocellus of Cerato-
pogon consists of only two sense-cells. As examples of com-
plicated adaptive ocelli, those of lepidopterous larvae can be cited.
Fig. 150. — Head of
naiad of Pteronacys;
dtj spots in the cuti-
cula beneath which
the dorsal arms of the
tentorium are at-
tached; the three
ocelli are on the front
(F), between these
two spots.
Fig. 151. — Head of a
cricket.
THE INTERNAL ANATOMY OF INSECTS
137
Fig. 152. — Head of a
larva of Corydalus,
dorsal aspect.
The ocellus of Gastropacha rubi, which is described and figured by
Pankrath ('90), resembles in structure, to a remarkable degree, an
ommatidium, and the same is true of the ocellus
of the larva of Arctia caja figured by Hesse ('01) .
The structure of a visual cell.— The dis-
tinctively characteristic feature of eyes is the
presence of what is termed visual cells. In
insects, and in other arthropods, a visual cell
is a nerve-end-cell, which contains a nucleus
and a greater or less amount of pigment,
and bears a characteristic border, termed the
rhabdomere; this is so called because it forms
a part of a rhab-
dom.
The visual
cells are grouped in such a way that the
rhabdomeres of two or more of them
are united to form what is known as a
rhabdom or optic rod. A group of two
visual cells with the rhabdom formed by
their united rhabdomeres is shown in
Figure 153, A and B.
The form of the rhabdomere varies
greatly in the visual cells of different
insect eyes ; and the number of rhab-
domeres that enter into the composi-
tion of a rhabdom also varies.
Figure 153, C represents in a dia-
grammatic manner the structure of
rhabdomere as described by Hesse ('01). Fig- I53-— Two visual cells from
~, an ocellus of a pupa of Apis
1 he rhabdomere (r) consists of many
minute rodlets each with a minute knob
at its base and connected with a nerve
fibril.
The structure of a primary ocellus.
— The primary ocelli vary greatly in
the details of the form of their parts,
but the essential features of their structure are illustrated by the
accompanying diagram (Fig. 154).
In some ocelli, as for example the lateral ocelli of scorpions,
the visual cells are interpolated among ordinary hypodermal cells,
mellifica. A, longitudinal sec-
tion ; B, transverse section; ?z,
n, nerves; nu, nucleus; r,
rhabdom; p, pigment (After
Redikorzew), C, diagram il-
lustrating the structure of a
rhabdomere; r, rhabdomere;
ct cell-body (From Berlese after
Hesse).
138
AN INTRODUCTION TO ENTOMOLOGY
ret-
the two kinds forming a single layer of cells beneath the
cornea; but in the ocelli of insects, the sense-cells form a distinct
layer beneath the hypo-
dermal cells. In this
type of ocellus the fol-
lowing parts can be dis-
tinguished :
The cornea. — T h e
cornea (Fig. 154, c) is a
transparent portion of
the cuticula of the body-
wall ; this may be lenti-
cular in form or not.
The corneal hypoder-
mis. — The hypodermis
of the body-wall is con-
tinued beneath the
Fig. 154. — A diagram illustrating the structure of cornea (Fig. 1 54, C. hy.) ;
a primary ocellus; c, cornea; c. hy, corneal thi t of the h
nypodermis; ret, retina; n, ocellar nerve; p, . J
accessory pigment cell; r, rhabdom. dermis is termed by
many writers the vitrecus
layer of the ocellus; but the term corneal hypodermis, being a self-
explanatory term, is preferable. Other terms have been applied to it,
as the lentigen layer and the corneagen, both referring to the fact that
this part of the hypodermis produces the cornea.
The retina. — Beneath the corneal hypodermis is a second cellular
layer, which is termed the retina, being composed chiefly or entirely of
visual cells (Fig. 154, ret).
The visual cells of the retina are grouped, as described above (Fig.
I53)f so that the rhabdomeres of several of them, two, three or four,
unite to form a rhabdom; such a group of retinal cells is termed a
retinula.
The visual cells are nerve-end-cells, each constituting the termina-
tion of a fiber of the ocellar nerve, and are thus connected with the
central nervous system.
Accessory pigment cells. — In some ocelli there are densely pig-
mented cells between the retinulas, which serve to isolate them in a
similar way to that in which the retinula of an ommatidium of a com-
pound eye is isolated (Fig. 154, p). Even in cases where accessory
pigment cells are wanting a degree of isolation of the rhabdoms of the
retinulae of an ocellus is secured by pigment within the visual cells
(Fig. 153, P)-
THE INTERNAL ANATOMY OF INSECTS
139
Ocelli of Ephemerida. — It has been found that the ocelli of certain
adult Ephemerida differ remarkably from the more common type of
ocelli described above. These peculiar ocelli have been described and
figured by Hesse ('01) and Seiler ('05). In them the cuticula over the
ocellus, the cornea, is arched but not thickened and the corneal hypo-
dermis is a thin layer of cells immediately beneath it. Under the
hypodermis there is a lens-shaped mass of large polygonal cells ; and
between this lens and the retina there is a layer of closely crowded
columnar cells.
The development of these ocelli has not been studied; hence the
origin of the lens-shaped mass of cells and of the layer of cells between
it and the retina is not known.
C. THE COMPOUND EYES
A compound eye consists of many
quite distinct elements, the ommatidia,
each represented externally by one of
the many facets of which the cuticular
layer of the eye is composed. As the
ommatidia of a given eye are similar,
a description of the structure of one
will serve to illustrate the structure of
the eye as a whole.
The structure of an ommatidium.—
The compound eyes of different insects
vary in the details of their structure;
but these variations are merely modi-
fications of a common plan ; this plan is
well -illustrated by the compound «yes
of MachiliSy the structure of which was
worked out by Seat on ('03). Figure
155 represents a longitudinal section
and a series of transverse sections of an
ommatidium in an eye of this insect,
which consists of the following parts.
The cornea. — The cornea is a hexa-
gonal portion of the cuticular layer of
the eye and is biconvex in form (Fig.
155, c}.
The corneal hypodermis. — Beneath
each facet of the cuticular layer of the eye are two hypodermal cells
Fig- J55- — An ommatidium of
Machilis. The lettering is ex-
plained in the text.
140 AN INTRODUCTION TO ENTOMOLOGY
which constitute the corneal hypodermis of the ommatidium. These
cells are quite distinct in Machilis and their nuclei are prominent
(Fig. 155, hy); but in many insects they are greatly reduced, and
consequently are not represented in many of the published figures
of compound eyes.
The crystalline-cone-ceUs . — Next to the corneal hypodermis there
are four cells, which in one type of compound eyes, the eucone eyes,
form a body known as the crystalline -cone, for this reason these
cells are termed the crystalline-cone-cells (Fig. 155, cc). Two of
these cells are represented in the figure of a longitudinal section
and all four, in that of a transverse section. In each cell there is a
prominent nucleus at its distal end.
The iris -pigment-cells. — Surrounding the crystalline-cone-cells and
the corneal hypodermis, there is a curtain of densely pigment ed cells,
which serves to exclude from the cone light entering other ommatidia ;
for this reason these cells are termed the iris -pigment (Fig. 155, i).
They are also known as the distal retinula cells; but as they are not a
part of the retina this term is misleading.
There are six iris -pigment -cells surrounding each crystalline -cone;
but as each of these cells forms a part of the iris of three adjacent
ommatidia, there are only twice as many of these cells as there are
ommatidia. This is indicated in the diagram of a transverse section
(Fig. 155, *')•
The retinula. — At the base of each ommatidium, there is a group
of visual cells forming a retinula (Fig. 155, r) ; of these there are seven
in Machilis; but they vary in number in the eyes of different insects.
The visual cells are so grouped that their united rhabdomeres form a
rhabdom, which extends along the longitudinal axis of the ommati-
dium (Fig. 155, rh). The distal end of the rhabdom abuts against the
proximal end of the crystalline-cone; and the nerve-fibers of which the
visual cells are the endings pass through the basement membrane
(Fig. 155, b) to the optic nerve.
The visual cells are pigmented and thus aid in the isolation of the
ommatidium.
The accessory pigment -cells. — In addition to the two kinds of pig-
ment-cells described above there is a variable number of accessory
pigment -cells (Fig. 155, ap), which lie outside of and overlap them.
From the above it will be seen that each ommatidium of a eucone
eye is composed of five kinds of cells, three of which, the corneal hypo-
dermis, the crystalline-cone-cells, and the retinular cells produce solid
structures; and three of them are pigmented.
THE INTERNAL ANATOMY OF INSECTS 141
Three types of compound eyes are recognized: first, the eucone
eyes, in these each ommatidium contains a tr^ie crystalline-cone, as
described above, and the nuclei of the cone-cells are in front of the
cone; second, the pseudocone eyes, in these the four cone -cells are
filled with a transparent fluid medium, and the nuclei of these cells are
behind the refracting body; and third, the acone eyes, in which
although the four cone -cells are present they do not form a cone, either
solid or liquid.
d. THE PHYSIOLOGY OF COMPOUND EYES
The compound eyes of insects and of Crustacea are the most com-
plicated organs of vision known to us. It is not strange therefore, that
the manner in which they function has been the subject of much dis-
cussion. It is now, however, comparatively well-understood;
although much remains to be determined.
In studying the physiology of compound eyes, three sets of struc-
tures, found in each ommatidium, are to be considered: first, the
dioptric apparatus, consisting of the cornea and the crystalline -cone;
second, the percipient portion, the retinula, and especially the rhab-
dom; and third, the envelope of pigment, which is found in three
sets of cells, the iris pigment-cells, the retinular cells, and the accessory
or secondary pigment -cells;
The dioptrics of compound eyes is an exceedingly complicated
subject; a discussion of it would require too much space to be intro-
duced here. It has been quite fully treated by Exner ('91). to whose
work those especially interested in this subject are referred. The
important point for our present discussion is that by means of the
cornea and the crystalline -cone, light entering the cornea from within
the limits of a certain angle passes through the cornea and the crystal-
line-cone to the rhabdom, which is formed of the combined rhab-
domeres of the nerve-end-cells, constituting the retinula, the precipient
portion of the ommatidium.
The theory of mosaic vision. — The first two questions suggested by
a study of physiology of compound eyes have reference to the nature
of the vision of such an eye. What kind of an image is thrown upon
the retinula of each ommatidium? And how are these images com-
bined to form the image perceived by the insect? Does an insect
with a thousand ommatidia perceive a thousand images of the object
viewed or only one?
The theory of mosaic vision gives the answers to these questions.
This theory was proposed by J. Muller in 1826; and the most recent
142
AN INTRODUCTION TO ENTOMOLOGY
investigations confirm it. The essential features of it are the follow-
ing: only the rays of light that pass through the cornea and the
crystalline-cones reach the precipient portion of the eye, the others fall
on the pigment of the eye and are absorbed by it ; in each ommatidium
the cornea transmits to the crystalline -cone light from a very limited
field of vision, and when this light reaches the apex of the crystalline-
cone it forms a point of light, not an image; hence the image formed
upon the combined retinulag is a mosaic of points of light, which com-
bined make a single image, and this image is an erect one.
Figure 156 will serve to illustrate the mosaic theory of vision.
In this figure are represented the corneas (c), the crystalline-cones
(cc), and the rhabdoms (r.) of several adja-
cent ommatidia. It can be seen, fiom this
diagram, that each rhabdom receives a
point of light which comes from a limited
portion of the object viewed (O) ; 'and that
the image (I) received by the percipient
portion of the eye is a single erect image,
formed by points of light, each of which
corresponds in density and color to the
corresponding part of the object viewed.
The distinctness of vision of a com-
pound eye depends in part upon the num-
ber and size of the ommatidia. It can be
•0
vson. many small ommatidia will represent the
details of the object better than one formed
by a smaller number of larger ommatidia; the smaller the portion of
the object viewed by each ommatidium the more detailed -will be the
image.
The distinctness of the vision of a compound eye depends also on
the degree of isolation of the light received by each ommatidium,
which is determined by the amount and distribution of the pigment.
Two types of compound eyes, differing in the degree of isolation of the
light received by each ommatidium, are recognized; to one type has
been applied the term day-eyes, and to the other, night-eyes.
Day-eyes. — The type of eyes known as day-eyes are so-called
because they are fitted for use in the day-time, when there is an
abundance of light. In these eyes the envelope of pigment sur-
rounding the transparent parts of each ommatidium is so complete
that only the light that has traversed the cornea and crystalline -cone
THE INTERNAL ANATOMY OF INSECTS
143
of that ommatidium reaches its rhabdom. The image formed in
such an eye is termed by Exner an apposed image; because it is formed
by apposed points of light, falling side by side and not overlapping.
Such an image is a distinct one.
Night-eyes. — In the night-eyes the envelope of pigment surround-
ing the transparent parts of each ommatidium is incomplete ; so that
rays of light entering several adjacent corneas can reach the same
retinula. In such an eye there will be an overlapping of the points of
light; the image thus formed is termed by Exner a superimposed
image. It is obvious that such an image is not as distinct as an ap-
posed image. 4 It is also obvious that a limited amount of light will
produce a greater impression in this type of eye than in one where a
considerable part of the light is absorbed by pigment. Night-eyes are
fitted to perceive objects and the movement of objects in a dim light,
but only the more general features of the object can be perceived by
them.
Eyes with double function. — It is a remarkable fact that with
many insects and Crustacea the compound eyes function in a bright
A B light as day-eyes and in a dim light as night-
eyes. This is brought about by movements in
the pigment. If an insect having eyes of this
kind be kept in a light place for a time and then
killed while still in the light, its eyes will be found
to be day-eyes, that is eyes fitted to.form apposed
images. But if another insect of the same
species be kept in a dark place for a time and
then killed while still in the dark, its eyes will be
found % to be night-eyes, that is eyes fitted to
form superimposed images.
Figure 157 represents two preparations
showing the structure of the compound eyes of
a diving-beetle, studied by Exner. In one
(Fig. 157, A), each rhabdom is surrounded by an
envelope of pigment, so that it can receiv eonly
Fig. 157.— Ommatidia the HSht Passing through the crystalline-cone of
from eyes of Colym- the ommatidium of which this rhabdom is a part,
condition^ Bright- This is the condition found in the individual
eye condition (From killed in the light, and illustrates well the struct-
ure of a day-eye. In the other preparation (Fig.
157, J5), which is from an individual killed in the dark, it can be seen
that the pigment has moved up between the crystalline -cones so that
144 AN INTRODUCTION TO ENTOMOLOGY
the light passing from the tip of a cone may reach several rhabdoms,
making the eye a night-eye. These changes in the position of the
pigment are probably due to amoeboid movements of the cells.
Divided Eyes. — In many insects each compound eye is divided
into two parts; one of which is a day-eye, and the other a night-eye.
The two parts of such an eye can be readily distinguished by a differ-
ence in the size of the facets; the portion of the eye that functions
as a day-eye being composed of much smaller facets than that which
functions as a night-eye.
A study of the internal structure of a divided eye shows that the
distribution of the pigment in the part composed of smaller facets is
that characteristic of day-eyes ; while the part of the eye composed of
larger facets is fitted to produce a superimposed image, which is the
distinctive characteristic of night-eyes.
Great differences exist in the extent to which the two parts of a
divided eye are separated. In many dragon-flies the facets of a part
of each compound eye are small, while those of the remainder of the
eye are much larger ; but the two fields are not sharply separated. In
some Blepharocera the two fields are separated by a narrow band in
which there are no facets, and the difference in the size of the facets of
the two areas is very marked. The extreme condition is reached in
certain May-flies, where the two
parts of the eye are so widely separa-
ted that the insect appears to have
two pairs of compound eyes (Fig 158).
The tapetum. — In the eyes of
many ariimals there is a structure
that reflects back the light that has
entered the eye, causing the well-
known shining of the eyes in the
dark. This is often observed in the
Fig. 158.— Front of head of Cloeon, eyes of cats a d in the eyes of moths
showing divided eyes; a, night-eye; ' J
b, day-eye \c, ocellus (From Sharp), that are attracted to our light at
night. The part of the eye that
causes this reflection is termed a tapetum. The supposed function of a
tapetum is to increase the effect of a faint light, the light being caused
to pass through the retina a second time, when it is reflected from the
. tapetum.
The structure of the tapetum varies greatly in different animals;
in the cat and other carnivores it is a thick layer of wavy fibrous tissue ;
in spiders it consists of a layer of cells behind the retina containing
THE INTERNAL ANATOMY OF INSECTS
145
small crystals that reflect the light ; and in insects it is a mass of fine
tracheae surrounding the retinula of each ommatidium.
XIII. THE ORGANS OF HEARING
a. THE GENERAL FEATURES
The fact that in many insects there are highly specialized organs
for the production of sounds indicates that insects possess also organs
of hearing; but in only a few cases are these organs of such form
that they have been gen-
erally recognized as ears.
The tympana. — In
most of the jumping
Orthoptera there are
thinned portions of the
Fig. 159. — Side view of a locust with the wings CUticula, which are of a
removed; t, tympanum. Structure fitted to.be put
in vibration by waves of
sound. For this reason these have been commonly regarded as organs
of hearing, and have been termed tympana. In the Acridiidse, there
is a tympanum on each side of the first abdominal segment (Fig.
159); and in the Locustidse and in the Gryllidas, there is a pair of
tympana near the proximal
end of each tibia of the first
pair of legs (Fig. 160).
The chordotonal organs. —
An ear to be effective must
consist of something more than
a membrane that will be put
in vibration by means of
sound; the vibrations of such
a tympanum must be trans-
ferred in some way to a nerv-
vous structure that will be
influenced by them if the
sound is to be perceived. Such
structures, closely associated
with the tympana of Orthoptera, were discovered more than a half
century ago by Von Siebold (1844) and have been studied since by
many investigators. The morphological unit of these essential auditory
Fig. 1 60. — Fore leg of a katydid; /, tympa-
num.
146
AN INTRODUCTION TO ENTOMOLOGY
Fig. 161. — Diagrammatic representation of the
auditory organs of a locustid (After Graber) .
structures of insects is a more or less peg-like rod contained in a tubular
nerve-ending (Fig. 161, A and B); this nerve-ending may or may
not be associated with a
specialized tympanum.
To all sense-organs char-
A. //Jl.r JS\ X?m acterized by the presence
of these auditory pegs,
Graber ('82) applied the
term chordotonal organs or
fiddle-string-like organs.
The scolopale and
the scolopophore. — The
peg-like rod
characteris-
tic of a chor-
dotonal organ of an insect was named by Graber the
scolopale; and to the tubular nerve-ending containing
the scolopale, he applied the term scolopophore.
The integumental and the subintegumental scolopo-
phores. — With respect to their position there are two
types of scolopophores ; in one, the nerve-ending is
attached to the body-wall (Fig. 161, A); in the other, it
ends free in the body-cavity (Fig. 161, B). These two
types are designated respectively as integumental scolo-
pophores and subintegumental scolopophores.
The structure of a scolopophore. — In a scolopophore
there can be distinguished an outer sheath (Fig. 161, I),
which appears to be continuous either with the basement
membrane of the hypodermis or with that of the
epithelium of a trachea, and within this sheath the
complicated nerve-ending; this nerve-ending is repre-
sented diagrammatically in Figure 161 from Graber and in
detail in Figure 162 from Hess ('17).
In Figure 162 the following parts are represented: a
bipolar sense-cell (sc) with its nucleus (sen) ; the proximal
pole of this sense -cell is connected with the central nerv-
ous system by a nerve; and its distal pole is connected
with the scolopale (s) by an axis -fiber (of) ; surrounding
the distal prolongation of the sense -cell and the scolopale
there is an enveloping or accessory cell (ec), in which
there is a prominent nucleus , (ecn) ; distad of the enveloping cell is
-sen
Fig. 162.— A
scolopo-
phore of the
i n t e g u -
mental type
(From
Hess).
THE INTERNAL ANATOMY OF INSECTS 147
the cap -cell (cc), in which there is a nucleus (ccn}\ extending from
the end-knob (ek) of the scolopale and surrounded by the cap -cell
there is an attachment fiber or terminal ligament (tl), by which the
scolopophore is attached to the body-wall, the scolopophore repre-
sented being of the integumental type ; at the base of the scolopale
and partly surrounding it, there is a vacuole (v) .
The structure of a scolopale. — The scolopalas or auditory pegs are
exceedingly minute and are quite uniform in size, regardless of the size
of the insect in which they are; but they vary in form in different
insects. They are hollow (Fig. 162, s) ; but the wall of the scolopale
is almost always thickened at its distal end, this forming an end-knob
(Fig. 162, ek). They are traversed by the axis -fiber of the sense-cell.
The vacuole at the base of the scolopale connects with the lumen of
the scolopale; this vacuole is filled with watery
fluid.
In Figure 163 is shown a part of the scolopo-
phore represented in Figure 162, more enlarged
(A), and three cross-sections (B, C, D) of the
scolopale. The wall of the scolopale is composed
at either end of seven ribs (r), each of which is
divided in the central portion, making fourteen
ribs in this part. The entire scolopale, except
possibly the terminal ligament, is bathed in the
watery liquid, and is free to vibrate (Hess '17).
Jt should be remembered that the scolopate of
in Figure 162 more different insects vary greatly in form; the one
enlarged (From figured nere js merely given as an example of
one type.
The simpler forms of chordotonal organs. — In the simplest form
of a chordotonal organ there is a single scolopophore; usually, how-
ever, there are two or more closely parallel scolopophores. In figure
164, which represents a chordotonal organ found in the nex*t to the
last segment of the body of a larva of Chironomus, these two types are
represented, one part of the organ being composed of a single scolopo-
phore, the other of several.
The chordotonal ligament. — In Figure 164 the nerve connecting
the chordotonal organ with the central nervous system is represented
at n; and at li is shown a structure not yet mentioned, the chordo-
tonal ligament, which is found in many chordotonal organs. Figure
165 is a diagrammatic representation of the relations of the chordo-
tonal organs of a larva of Chironomus to the central nervous system
148
AN INTRODUCTION TO ENTOMOLOGY
and to the body-wall. Here each chordotonal organ is approxi-
mately T-shaped; the proximal nerve forming the body of theT;
the scolopophore, one
arm; and the chor-
dotonal ligament, the
other arm.
It will be observed
that in this type of
chordotonal organ
the scolopophore and
the ligament form a
fiddle -string -like
structure between two
points in the wall of
a single segment. It
is believed that in cases
of this kind the integu- Fig. 165.— Diagram
ment acts as a tympa- chordotonaV Organs
Fig. i64.-Chordotonal organ *um or sounding rfa^tTm^C^a-
of a larva of Chironomus board. u T (
(FromGraber).
b. THE CHORDOTONAL ORGANS OF LARVAE
Chordotonal organs have been observed in so many larvag that
we may infer that they are commonly present in larvae. These organs
are very simple compared with those of certain adult insects, described
later. Those figured in the preceding paragraphs will serve to illustrate
the typical form of larval chordotonal organs. Even in the more
complicated ones, there are comparatively few scolopophores ; and, as
a rule, they are not connected with specialized tympana, but extend
between distant parts of the body- wall, which probably acts as a sound-
ing board.
In certain larvae, however, the scolopophores are attached to
specialized areas of the body-wall. Hess ('17) has shown that the
pleural discs of cerambycid larvae, which are situated one on each side
of several of the abdominal segments, serve as points of attachment
of scolopophores.
C. THE CHORDOTONAL ORGANS OF THE ACRIDIID^E
In the Acridiidae there are highly specialized ears situated one on
each side of the first abdominal segment. The external vibrating
THE INTERNAL ANATOMY OF INSECTS
149
Fig. 1 66. — Side view of a locust with the wings
removed; /, tympanum.
part of these organs, the tympanum, is conspicuous, being a thinned
portion of the body- wall (Fig. 166).
Closely applied to the
inner surface of each
tympanum (Fig. 167, T),
there is a ganglion known
as Muller's organ (go),
first described by Muller
(1826). This ganglion
contains many ganglion-
cells and scolopalae and is the termination of a nerve extending
from the central nervous system, the auditory nerve («). Figure
1 68 represents a section of Muller's organ, showing the ganglion -cells
and scolopalse.
Intimately associated with the Muller's organ are two horny
processes (Fig. 167, o and u) and a pear-shaped vesicle (Fig. 167, bi);
and near the margin
of the tympanum,
there is a spiracle
(Fig. 167, si), which
admits air to a space
inside of the tympa-
num, the tympanal
air-chamber.
As the nerve-end-
ings in Muller's organ
are attached to the
tympanum, it is a
chordotonal organ
of the integumental
type; it is attached
to a vibratile mem-
brane, between two
air-spaces.
Fig. 167. — Ear of a locust, Caloptenus if aliens, seen from
inner side; T, typmanum; TR, its border; o, u, two
horn-like processes; bi, pear-shaped vesicle; n, audi-
tory nerve; ga, terminal ganglion or Muller's organ;
st, spiracle ; M, tensor muscle of the tympanum (From
Packard after Graber).
d. THE CHORDOTO-
NAL ORGANS OF THE
LOCUSTID.E AND OF
THE GRYLLID^E
In the long-horned grasshoppers and in the crickets, there is a pai r
tympana near the proximal end of the tibia of each fore leg. In
150
AN INTRODUCTION TO ENTOMOLOGY
S— -
many genera, these tympana are exposed and easily observed (Fig.
169) ; but in some genera each is covered by a fold of the body-wall
and is consequently within a cavity,
which communicates with the out-
side air by an elongated opening
(Fig. 170, a and 6).
Within the legs bearing these
tympana, there are complicated
chordotonal organs. Very de-
--S tailed accounts of these organs
have been published by Graber
('76), Adelung ('92) and Schwabe
('06); in this place, for lack of
space, only their more general
features can be described.
Figure 171 represents a longi-
tudinal section of that part of a
fore tibia of Decticus verrucivorus in
which the chordotonal organs are
situated, and Figure 172 represents
a cross-section of the same tibia,
Fig. 168.— Section of Mullet's organ; g, passing through the tympana and
ganglion-cells; n, nerve; s, s, scolo- the air-chambers formed by the
pabs (After Graber). J
folds of the body- wall. In the fol-
lowing account the references, in most cases, are to both of these figures.
g—
Fig. 169.— Fore leg of a katydid;
num.
tympa-
a
Fig. 1 70. — Tibia of a locustid
with covered tympana; a,
front view; b, side view; o,
opening (After Schwabe).
The trachea of the leg. — The trachea of the leg figured in part here
is remarkable for its great size and for its division into two branches,
THE INTERNAL ANATOMY OF INSECTS
151
the front trachea (Ti) and the hind trachea (Te) ; these two branches
reunite a short distance beyond the end of the chordotonal organs.
It is an interesting
fact that these large
tracheae of the legs
containing the chor-
dotonal organs open
through a pair of
supernumery spir-
acles, differing in this
respect from the tra-
cheae of the other legs.
The spaces of the
leg. — By reference
to Figure 172, it will
be seen that the two
branches of the leg
trachea occupy the
middle space of the
leg between the two
tympana (Tie and
Tii) and separate an
outerspace, theupper
onein the figure, from
an inner space. The
outer space (E) con-
tains a chordotonal
organ, of which the
scolopale is repre-
sented at S ; and the
inner space contains
small tracheae (t) ,
muscles (m) , the
tibial nerve (Ntb),
and a tendon (Tn).
Fig. 171. — Longitudinal section of a fore tibia of
Decticus verrucivorus (From Berlese after Schwabe).
The interstices of the
outer andinner spaces
are filled with blood.
In the outer space some leucocytes and fat-cells (Gr) are represented.
The supra-tympanal or subgenual organ. — In the outer space of
the tibia, a short distance above the tympana, there is a ganglion (Fig.
152
AN INTRODUCTION TO ENTOMOLOGY
171, Os) composed of nerve-endings, which are scolopophores of the
integumenta! type. Two nerves extend to this ganglion, one from
each side of the leg, and
each divides into many
scolopophores. The
attachment fibers of the
scolopophores converge
and are attached to the
wall of the leg. Two
terms have been applied
to this organ, both indicat-
ing its position in the leg;
one refers to the fact that
it is above the tympana,
the other, that it is below
the knee.
The intermediate or-
gan.— Immediately below
Fig. 172 .-Transverse section of the fore tibia of th supra_tvmpanal organ,
Decticus verrucivorus (From Berlese after - A
Schwabe). In comparing this figure with the and between it and the
preceding, note that in that one the external described in the next
parts are at the left, in this one, at the right.
paragraph, is a ganglion
composed of scolopophores of the subintegumental type ; this is
termed the intermediate organ (Fig. 171, Oi) .
Siebold's organ or the crista acustica. — On the outer face of the
front branch of the large trachea of the leg there is a third chordo-
tonal organ, the Siebold's organ or the crista acusitca. A surface view
of the organ is given in Figure 171 and a cross-section is represented in
Figure 172. It consists of a series of scolopophores of the subintegu-
mental type, which diminish in length toward the distal end of the
organ (Fig. 171). The relation of Siebold's organ to the trachea is
shown in Figure 172. It forms a ridge or crest on the trachea, shown
in setion at cr in Figure 172 ; this suggested the name crista acustica,
usedcby some writers.
6. THE JOHNSTON S ORGAN
There has been found in the pedicel of the antenna of many insects,
representing several of the orders, an organ of hearing, which is known
as the Johnston s organy having been pointed out by Christopher
Johnston (1855). This organ varies somewhat in form in different
THE INTERNAL ANATOMY OF INSECTS
153
/
insects and in the two sexes of the same species; but that of a male
mosquito will serve as an example illustrating its essential features.
The following
account is
based on an in-
vestigation by
Professor Ch.
M. Child ('94).
In an an-
tenna of a mos-
quito (Fig. 173)
the scape or
first segment,
which contains
the muscles of
the antenna, is
much smaller
than the pedicel
or second seg-
ment, and is
usually over-
looked, being
concealed b y
the large, glob-
ular pedicel;
the clavola con-
sists of thirteen
slender seg-
ments. Excepting one or two terminal segments, each segment of
the clavola bears a whorl of long, slender setae ; these are more
prominent in the male than in the female.
Figure 174 represents a longitudinal section of the base of an
antenna; in this the following parts are shown: S, scape; P, pedicel,
C, base of the first segment of the clavola; cp, conjunctival plate
connecting the pedicel with the first segment of the clavola; pr,
chitinous processes of the conjunctival plate; m, muscles of the
antenna; N, principal antennal nerve; n, nerve of the clavola;
immediately within the wall of the segments there is a thin layer
of hypodermis; the lumen of the pedicel is largely occupied by a
ganglion composed of scolopophores, the attachment fibers of which
are attached to the chitinous processes of the conjunctival plate.
Fig. 173.— Antennae of mosquitoes, Culex; M, male; F,
female; s, scape; p, pedicel.
154
AN INTRODUCTION TO ENTOMOLOGY
As to the action of the auditory apparatus as a whole, it was shown
experimentally by Mayer ('74) that the different whorls of setas borne
by the segments of the clavola, and which gradually decrease in length
on successive segments, are caused to vibrate by different notes; and
it is believed that the vibrations of the setae are transferred to the
conjunctival plate by the clavola, and thence to the nerve-end-
ings.
It was formerly
believed that the
great specialization
of the Johnston's or-
gan in male mosqui-
toes enabled the
males to hear the
songs of the females
and thus more readily
to find their mates.
But it has been found
that in some species,
at least, of mosquitoes
and of midges in
Fig. 174. — Longitudinal section of the base of an anten- which the males
na of a male mosquito, Corethra culiciformis (After -, , * •
Child)< have this organ
highly specialized the
females seek the males. This has led some writers to doubt that the
Johnston's organ is auditory in function. But the fact remains that
its distinctive feature is the presence of scolopalae, which is the dis-
tinctive characteristic of the auditory organs of other insects.
N
XIV. SENSE-ORGANS OF UNKNOWN FUNCTIONS
In addition to the sense-organs discussed in the foregoing account
there have been described several types of supposed sense-organs
which are as yet very imperfectly understood. Among these there is
one that merits a brief discussion here on account of the frequent
references to it in entomological literature. Many different names
have been applied to the organs of this type; of these that of sense-
domes is as appropriate as any, unless the conclusions of Mclndoo,
referred to below, are confirmed, in which case his term olfactory pores
will be more descriptive.
THE INTERNAL ANATOMY OF INSECTS
155
a b
Fig. 175. — Sense-domes (From Berlese).
The sense-domes are found in various situations, but they occur
chiefly on the bases of the wings and on the legs. Each sense-dome
consists of a thin, hemispherical or more nearly spherical membrane,
which either projects from the
outer end of a pore in the
cuticula (Fig. 175, a) or is
more or less deeply enclosed
in such a pore (Fig. 175, 6);
intergrades between the two
types represented in the accom-
panying figures occur.
When a sense-dome is
viewed in section a nerve-
ending is seen to be connected
with the dome-shaped or bell-
like membrane. A striking
feature of these organs is the
absence of any gland -cells connected with them, such as are found
in the chemical sense-organs described on an earlier page,
In one very important respect there is a marked difference in the
accounts of these organs that have been published. The organs were
first discovered long ago by Hicks ('57); but they have been more
carefully studied in recent years by several writers, who have been
able to make use of a greatly improved histologicai technic; among
these writers are Berlese ('09 a), Vogel ('n), Hochreuter (12'), Lehr
('14), and Mclndoo ('14).
All of the writers mentioned above except the last named maintain
that the sense-cell ends in a structure, in the middle of the sense-dome,
which differs in appearance from both the membrane of the sense-
dome and the body of the sense-cell. This
structure varies in form in different sense-
domes; in some it is cylindrical, and is
consequently described as a peg; in others,
it is greatly flattened so that it is semilunar
in form when seen in section. In Figure
J75> b, which represents a section made
transversely to the long axis of this part it
appears peglike ; but in Figure 175,0, which
represents a longitudinal view of it, it is
semilunar in form.
According to Mclndoo (Fig. 176) no structure of this kind is
Fig. 176 — Olfactory pore
of Mclndoo '(From
Mclndoo)
156 AN INTRODUCTION TO ENTOMOLOGY
present, but the sense- fiber of the sense-cell pierces the bottom of the
cone and enters the round, oblong, or slitlike pore-aperture. "It is
thus seen that the cytoplasm in the peripheral end of the sense-
fiber comes in direct contact with the air containing odorous par-
ticles and that odors do not have to pass through a hard membrane
in order to stimulate the sense-cell as is claimed for the antennal
organs".
XV. THE REPRODUCTIVE ORGANS
a. THE GENERAL FEATURES
In insects the sexes are normally distinct except in a single genus
of wingless, very aberrant Diptera, Termitoxinia, the members of
which live in the nests of Termites ; these have been found to be her-
maphroditic.
Individuals in which one side has the external characters of the
male and the other those of the female are not rare ; such an individual
is termed a gynandromorph; in some gynandromorphs, both testes
and ovaries are present but in no case are both functional ; these there-
fore are not true hermaphrodites.
In females the essential reproductive organs consist of a 2air__of.
ovar^es^ the organs in which the ova or eggs are developed, and a tube
leading from each ovary to an external opening, the oviduct. In the
male, the essential reproductive organs are a pair of testes > in which
the spermatozoa are developed and a tube leading from each testis to
an external opening, the my deferens. In addition to these essential
organs, there are in most insects accessory organs, these consist of
glands and of reservoirs for the reproductive elements.
The.formof the essential reproductive organs and the number and
form of the accessory organs vary greatly in different insects. It is
impossible to indicate the extent of these variations in the limited
space that can be devoted to this subject in this work. Instead of
attempting this it seems more profitable to indicate by diagrams, one
for each sex, the relations of the accessory organs that may exist to
the essential organs.
In adult insects the external opening of the reproductive organs is
on the ventral side of the abdomen near the caudal end of the body.
The position of the opening appears to differ in different insects and in
some cases in the two sexes of the same species. The lack of uni-
formity in the published accounts bearing on this point is partly due
to differences in numbering the abdominal segments; some authors
describing the last segment of the abdomen as the tenth while others
THE INTERNAL ANATOMY OF INSECTS
157
believe it to be the eleventh; embryological evidence supports the
latter view.
In most insects there is a single external opening of the reproduc-
tive organs ; but in the Ephemerida and in a few other insects the two
efferent ducts open separately.
Secondary sexual characters. — In addition to differences in the
essential reproductive organs and in the genital appendages of the
two sexes, many insects exhibit what are termed secondary sexual
characters. Among the more striking of these are differences in size,
coloring, and in the form of certain organs. Female insects are
usually larger than the males of the same species; this is due to the
fact that the females carry the eggs ; but in those cases where the males
fight for their mates, as stag-beetles, the males are the larger. Strik-
ing differences in the color-
ing of the two sexes are
common, especially in the
Lepidoptera. In many
insects the antennae of the
male are more highly
specialized than those of
the female; and this is
true also of the eyes of
certain insects. These are
merely a few of the many
known secondary sexual
characters found in insects.
Fig. 178-
Repro-
ductive
organs of
Japyx,
female
(After
Grassi) .
Fig. 177. — Diagram of thereproduc-
tive organs of a female insect; o,
ovary; od, oviduct; c, egg-calyx; v,
vagina; s,spermatheca; &c,bursa
copulatrix; sg, spermathecal
gland; eg, colleterial glands.
there is a single ovarian tube
b. THE REPRODUCTIVE
ORGANS OF THE
FEMALE
The general features of
the ovary. — In the more
usual form of the ovaries
of insects, each ovary is
a compact, more or less spindle-
shaped body composed of many paral-
lel ovarian tubes (Fig. 177, o), which
open into a common efferent tube,
the oviduct. In Campodea, however,
and in certain other Thysanura the
ovarian tubes have a metameric arrangement (Fig. 178). The num-
158
AN INTRODUCTION TO ENTOMOLOGY
her of ovarian tubes differs greatly in different insects ; in many
Lepidoptera there are only four in each ovary; in the honeybee,
about 150; and in some Termites, 3000 or more.
The wall of an ovarian tube.. — The ovarian tubes are lined with
an epithelial layer, which is supported by a basement membrane; out-
side of this there is a peritoneal envelope, composed of connective tis-
sue; and sometimes there are muscles in the peritoneal envelope.
The zones cf an ovarian tube. — Three different sections or zones are
recognized in an ovarian tube; first,
the terminal filament, which is the
slender portion which is farthest from
the oviduct (Fig. 179, i)\ second, the
germarium, this is a comparatively short
chamber, between the other two zones
(Fig. 179, g); and third, the vitellarium,
which constitutes the greater portion of
the ovarian tube.
The contents of an ovarian tube. — In
the germarium are found the primordial
germ-cells from which the eggs are devel-
oped; and in the vitellarium are found
the developing eggs. In addition to the
cells that develop into eggs there are
found, in the ovarian tubes of many
insects, cells whose function is to furnish
nutriment to the developing eggs; these
are termed nurse-cells.
Depending upon the presence or ab-
sence of nurse-cells and on the location of
the nurse-cells when present, three types
of ovarian tubes are recognized: first,
those without distinct nurse-cells (Fig.
179, A) ; second, those in which the eggs
and masses of nurse-cells alternate in the
ovarian tube (Fig. 179, B); and third,
those in which the nurse-cells are
restricted to the germarium (Fig. 179, C), which thus becomes a nutri-
tive chamber. In the latter type the developing eggs are each con-
nected by a thread with the nutritive chamber.
The egg-follicles. — The epithelium lining of the ovarian tube
becomes invaginated between the eggs in such a way that each egg is
A
Fig. 179. — Three types
ovarian tubes; e, e, e,
eggs; n, n, n, nurse- cells
(After Berlese).
THE INTERNAL ANATOMY OF INSECTS 159
enclosed in an epithelial sac or egg-follicle, which passes down the tube
with the egg (Fig. 179). There is thus a tendency to strip the tube of
its epithelium, but a new one is constantly formed.
The functions of the follicular epithelium. — It is believed that in
some cases, and especially where the nurse-cells are wanting, the
follicular epithelium serves a nutritive function. But the most
obvious function of this epithelium is the formation of the chorion or
egg-shell, which is secreted on its inner surface. The pit-like mark-
ings so common on the shells of insect eggs indicate the outlines of the
cells of the follicular epithelium.
The ligament of the ovary. — In many insects, the terminal fila-
ments of the several ovarian tubes of an ovary unite and form a
slender cord, the ligament of the ovary, which is attached to the dorsal
diaphragm ; but in other insects this ligament is wanting, the terminal
filaments ending free in the body cavity.
The oviduct. — The common outlet of the ovarian tubes is the ovi-
duct (Fig. 177, od). In most insects the oviducts of the two ovaries
unite and join a common outlet, the vagina; but in the Ephemerida
and in some Dermaptera each oviduct has a separate opening.
The egg-calyx. — In some insects each oviduct is enlarged so as to
form a pouch for storing the eggs, these pouches are termed the egg-
calyces (Fig. 177, c).
The vagina. — The tube into which the oviducts open is the vagina
(Fig. 177, v). The vagina differs in structure fronrihe oviducts, due
to the fact that it is an invagination of the body-wall, and, like other
imaginations of the body-wall, is lined with a cuticular layer.
The spermatheca. — The spermathecaisa sac for the storage of the
seminal fluid (Fig. 1 7 7 , s) . As the pairing of the sexes takes place only
once in insects and as the egg-laying period may extend over a long
time, it is essential that provision be made for the fertilization of the
eggs developed after the union of the sexes. The eggs become full-
grown and each is provided with a shell before leaving the ovarian
tubes. At the time an egg is laid a spermatozoan may pass from the
spermatheca, where thousands of them are stored, into the egg through
an opening in the shell, the micropyle, which is described in the next
chapter (Fig. 184 and 185).
In some social insects, eggs that are developed years after the'
pairing took place are fertilized by spermatozoa that have been stored
in the spermatheca.
The bursa copulatrix. — In many insects there is a pouch for the
reception of the seminal fluid before it passes to the spermatheca.
160 AN INTRODUCTION TO -ENTOMOLOGY
This pouch is known as the bursa copulatrix or copulatory pouch. In
some insects this pouch is a diverticulum of the vagina (Fig. 177,6*;);
in others it has a distinct external opening, there being two external
openings of the reproductive organs, the opening of the vagina and the
opening of the bursa copulatrix.
When the bursa copulatrix has a distinct external opening there
may or may not be a passage from it to the vagina. In at least some
Orthoptera (Melanoplus) there is no connection between the two;
when the eggs are laid they are
pushed past the opening of the
bursa copulatrix where they are
fertilized.
In the Lepidoptera (Fig. 180),
there is a passage from the bursa
copulatrix to the vagina. In
this case the seminal fluid is
Fig. i8o.-Reproductive organs of the received by the bursa copulatrix
female of the milkweed butterfly; a, at the time of pairing, later it
' &&&*££u3ti P^ to the spermatheca, and
filaments of the ovary; v, opening from here it passes to the vagina.
A bursa copulatrix is said to
be wanting in Hymenoptera, Diptera, Heteroptera and Homoptera
except the Cicadas.
The colleterial glands. — There are one or two pairs of glands that
open into the vagina near its outlet (Fig. 177, eg) ; to these has been
applied the general term colleterial glands. Their function differs in
different insects; in some insects they secrete a cement for gluing the
eggs together, in others they produce a capsule or other covering
which protects the eggs.
The spermathecal gland.— In many insects there is a gland that
opens either into the spermatheca or near the opening of the sperma-
theca, this is the spermathecal gland (Fig. 177, sg).
C. THE REPRODUCTIVE ORGANS OF THE MALE
The reproductive organs of the male are quite similar in their more
general features to those of the female; but there are striking differ-
ences in details of form.
The general features of the testes. — As the reproductive elements
developed in the testes, the spermatazoa, always remain small, the
testes of a male are usually much smaller than the ovaries of the female
of the same species.
THE INTERNAL ANATOMY OF INSECTS
161
In the more common form, each testis is a compact body (Fig.
181, t) composed of a variable number of tubes corresponding with
the ovarian tubes, these are commonly called
the testicular follicles; but it would have been
better to have termed them the testicular tubes,
reserving the term follicle for their divisions.
The testicular follicles vary in number,
form, and in their arrangement. In many
insects as the Neuroptera, the Hemiptera, the
Diptera, and in Campodea and Japyx, each
testis is composed of a single follicle. In some
beetles, Carabidae and Elateridae, the follicle
is long and rolled into a ball. In some Thy-
sanura the testicular follicles have a metameric
arrangement.
In some Coleoptera, each testis is separated
into several masses, each having its own outlet
leading to the vas deferens; while in some
other insects the two testes approach each other
during the pupal stage and constitute in the
adult a single mass.
The structure of a testicular follicle. — Like
the ovarian tubes, the testicular follicles are
lined with an epithelial layer, which is sup-
ported by a basement membrane, outside of
which there is a peritoneal envelope composed
And in these follicles a series of zones are
distinguished in which the genital cells are found in different stages
of development, corresponding to the successive generations of these
cells. In addition to the terminal filament four zones are recog-
nized as follows:
The germarium. — This includes the primordial germ-cells and the
spermatogonia.
The zone of growth. — Here are produced the spermatocytes of the
first order and the spermatocytes of the second order.
The zone of division and reduction. — In this zone are produced the
Spermatids or immature spermatozoa.
The zone of transformation. — Here the spermatids become sper-
matozoa.
A discussion of the details of the development of the successive
generations of the genital cells of the male, or spermatogenesis, does
not fall within the scope of this volume.
Fig. 181. — Diagram of
the reproductive or-
gans of a male insect ;
the right testis is shown
in section; ag, acces-
sory glands; ed, eja-
culatory duct; sz;,semi-
of connective tissue.
162 AN INTRODUCTION TO ENTOMOLOGY
The spermatophores. — In some insects the spermatozoa become
enveloped in a sac in which they are transferred to the female; this
sac is the spermatophore. Spermatophores have been observed in
Gryllidae, Locustidse, and certain Lepidoptera.
Other structures. — A ligament of the testis, corresponding to the
ligament of the ovary, is often present ; the common outlet of the testi-
cular follicles, corresponding to the oviduct is termed the vas defer ens
(Fig. 181, vd)-, an enlarged portion of the vas deferens serving as a
reservoir for the products of the testis is known as a seminal vesicle
(Fig. 181, sv); the invaginated portion of the body- wall, correspond-
ing with the vagina of the female, is the ejaculatory duct (Fig. 181, ed);
accessory glands, corresponding to the colleterial glands of the female,
are present (Fig. 181, ag)\ the function of these glands has not been
determined, they may secrete the fluid part of the semen, and they
probably secrete the spermatcphore when one is formed; the penis,
this is merely the chitinized terminal portion of the ejaculatory duct,
which can be evaginated with a part of the invaginated portion of the
body-wall. It is furnished with powerful muscles for its protrusion
and retraction.
XVI. THE SUSPENSORIA OF THE VISCERA
The organs discussed here do not constitute a well-defined system,
but are isolated structures connected with
different viscera. As in most cases they
appear to serve a suspensory function, they
are grouped together provisionally as the sus-
pensoria of the viscera.
The dorsal diaphragm. — This is a mem-
branous structure which extends across the
abdominal cavity immediately below the
P... -p.. heart, to which it is attached along its median
Fig. 182.— Diagram show- ,. ' .
ing the relation of the nne. The lateral margins of this diaphragm
dorsal diaphragm and are attached to the sides of the body by a
the ventral diaphragm . . • i
to other viscera; a, series or triangular prolongations, which have
daS^lS hfa™1' h been commonly known as the wings of the
heart; n, ventral nerv- heart (Fig. 139, c). The dorsal diaphragm is
V> Ventral comP°sed largely of very delicate muscles.
Its relation to the heart is illustrated by the
accompanying diagram (Fig. 182, d).
There are differences of opinion as to the function of the dorsal
diaphragm. ^ An important function is probably to protect the heart
THE INTERNAL ANATOMY OF INSECTS 163
from the peristaltic movements of the alimentary canal. It also
supports the heart; and it may play a part in its expansion.
The dorsal diaphragm is also known as the pericardial diaphragm.
The ventral diaphragm. — The ventral diaphragm is a very delicate
membrane which extends across the abdominal cavity immediately
above the ganglia of the central nervous system. It is quite similar
in form to the dorsal diaphragm; it is attached along each side of
the body, just lateral of the great ventral muscles, by a series of pro-
longations resembling in form the wings of the heart. The .position of
the ventral diaphragm is illustrated in Figure 182, v.
This diaphragm has been described as a ventral heart; but I
believe that its function is to protect the abdominal ganglia of the
central nervbus system from the peristaltic movements of the alimen-
tary canal.
The thread-like suspensoria of the viscera. — Under this head may
be classed the ligament of the ovary and the ligament of the testis,
already described. In addition to these, there is, in some insects at
least, a thread-like ligament that is attached to the intestine.
XVII. SUPPLEMENTARY DEFINITIONS
There are found in the bodies of insects certain organs not referred
to in the foregoing general account of the internal anatomy of insects.
These organs, though doubtless very important to the insects in which
they occur, are not likely to be studied in an elementary course in
entomology and, therefore, a detailed account of them may well be
omitted from an introductory text-book. This is especially true as
our knowledge of the structure and functions of these organs is so
incomplete that an adequate discussion of the conflicting views now
held would require more space than can be devoted to it here. The
organs in question are the following:
The oenocytes. — The term cenocytes is applied to certain very large
cells, that are found in clusters, often metamerically arranged, and
connected with the tracheae and the fat body of insects. The name
was suggested by the light yellow color which often characterizes
these cells, the color of certain wines ; but the name is not a good one,
as oenocytes vary greatly in color. Several other names have been
applied to them but they are generally known by the name used here.
Two types of oenocytes are recognized: first, the larval oenocytes;
and second, the imaginal oenocytes.
164 AN INTRODUCTION TO ENTOMOLOGY
The larval oenocytes are believed by Verson and Bisson ('91) to be
ductless glands which take up, elaborate, and return to the blood
definite substances, which may then be taken up by other cells of the
body. Other views are held by other writers, but the view given
above seems, as this time to be the one best supported by the evidence
at hand.
As to the function of the imaginal oenocytes, there are some obser-
vations that seem to show that they are excretory organs without
ducts, cells that serve as storehouses for excretory products, becoming
more filled with these products with the advancing age of the insect.
The pericardial cells. — The term pericardia] cells is applied to a
distinct type of cells that are found on either side of the heart in the
pericardial sinus or crowded between the fibers of the pericardial
diaphragm.
These cells can be rendered very conspicuous by injecting ammonia
carmine into the living insect some time before killing and dissecting
it ; by this method the pericardial cells are stained deeply while the
other cells of the body remain uncolored.
It is believed that the pericardial cells absorb albuminoids origina-
ting from the food and transform them into assimilable substances.
The phagocytic organs. — The term phagocyte is commonly applied
to any leucocyte or white blood corpuscle that shows special activity
in ingesting and digesting waste and harmful materials, as disinte-
grating tissue, bacteria, etc. The action of phagocytes is termed
phagocytosis; an excellent example of phagocytosis is the part played
by the leucocytes in the breaking down and rebuilding of tissues in the
course of the metamorphosis of insects; this is discussed in the next
chapter.
Phagocytosis may take place in any part of the body bathed by the
blood and thus reached by leucocytes. In addition to this widely
distributed phagocytosis, it is believed that in certain insects there are
localized masses of cells which perform a similar function; these
masses of cells are known as the phagocytic organs.
Phagocytic organs have been found in many Orthoptera and in
earwigs; they are situated in the pericardial region; and can be made
conspicuous by injecting a mixture of ammonia carmine and India ink
into the body cavity; by this method the pericardial cells are stained
red and the phagocytic organs black.
The light-organs. — The presence of organs for producing light is
widely distributed among living forms both animal and vegetable.
THE INTERNAL ANATOMY OF INSECTS 165
The most commonly observed examples of light-producing insects are
certain members of the Lampyridse, the fireflies and the glow-worms,
and a member of the Elateridae, the "cucujo" of the tropics. With
these insects the production of light is a normal function of highly
specialized organs, the light-organs.
Examples of insects in which the production of light is occasionally
observed are larvae of mosquitoes, and certain lepidopterous larvae.
In these cases the production of light is abnormal, being due either to
the presence in the body of light -producing bacteria or to the ingestion
of luminescent food.
The position of the specialized light-organs of insects varies
greatly; in the fireflies, they are situated on the ventral side of the
abdomen ; in the glow-worms, along the sides of the abdomen; and in
the cucujo, the principal organs are in a pair of tubercles on the dorsal
side of the prothorax and in a patch in the ventral region of the
metathorax.
The structure of the light-organs of insects varies in different
insects, as is shown by the investigations of several authors. A good
example of highly specialized light-organs are those of Photinus
marginellus, one of our common fireflies. An excellent account of
these is that of Miss Townsend ('.04), to which the reader is referred.
CHAPTER IV.
THE METAMORPHOSIS OF INSECTS
MANY insects in the course of their lives undergo remarkable
changes in form ; a butterfly was once a caterpillar, a bee lived first the
life of a clumsy footless grub, and flies, which are so graceful and active,
are developed from maggots.
In the following chapters considerable attention is given to
descriptions of the changes through which various insects pass ; the
object of this chapter is merely to discuss the more general features of
the metamorphosis of insects, and to define the terms commonly used
in descriptions of insect transformations.
I. THE EXTERNAL CHARACTERISTICS OF THE META-
MORPHOSIS OF INSECTS
The more obvious characteristics of the metamorphosis of insect8
are those changes in the external form of the body that occur during
postembryonic development. In some cases there appears to be but
little in common between the successive forms presented by the same
insect, as the caterpillar, chrysalis, and adult stages of a butterfly.
On the other hand, in certain insects, the change in the form of the
body during the postembryonic life is comparatively little. Based
on these differences, several distinct types of metamorphosis have
been recognized; and in those cases where the insect in its successive
stages assumes different forms, distinctive terms are applied to the
different stages.
a. THE EGG
Strictly speaking, all insects are developed from eggs, which are
formed from the primordial germ-cells in the ovary of the female.
As a rule, each egg is surrounded by a shell, formed by the follicular
epithelium of the ovarian tube in which the egg is developed; and
this egg, enclosed in its shell, is deposited by the female insect, usually
on or near the food upon which the young insect is to feed. In some
cases, however, the egg is retained by the female until it is hatched;
thus flesh-flies frequently deposit active larvae upon meat, especially
when they have had difficulty in finding it ; and tViere are other vivi-
parous insects, which are discussed later. In th:"s place is discussed
(166)
THE METAMORPHOSIS OF INSECTS
167
the more common type of insect eggs, those that are laid while still
enclosed in their shell.
The shape of the egg. — The terms ovoid and ovate have a definite
meaning which has been derived from the shape of the eggs of birds ;
but while many eggs of
insects are ovate in form,
many others are not.
The more common
form of insect eggs is
an elongate oval, some-
what curved; this type is
illustrated by the eggs
of crickets (Fig. 183, i);
many eggs; are approx-
imately spherical, as those
of some butterflies (Fig.
183, 2) ; while some are of
remarkable shape, two of
these are represented in
Figure 183,3, 4.
The sculpture of the
shell. — Almost always the
external ^surf ace of the shell
of an insect egg is marked with small, hexagonal areas ; these are the
imprints of the cells of the follicular epi-
thelium, which formed the shell. In
many cases the ornamentation of the
shell is very conspicuous, consisting of
prominent ridges or series of tubercles ;
this is well -shown in the eggs of many
Lepidoptera (Fig. 184).
The micropyle. — It has been shown,
in the course of the discussion of the
reproductive organs of the female, that
the egg becomes full-grown, and the
protecting chorion or egg-shell is formed
about it before it is fertilized. This
renders necessary some provision for the
entrance of the male germ -cell into the
egg; this provision consists of one or
more openings in the shell through which a spermatozoan may enter
This opening or group of openings is termed the micropyle.
Fig. 183. — -Eggs of insects; I, (Ecanthus nigri-
cornis; 2, (Enis semidea; 3, Piezosterum
subulatum; 4, Hydrometra martini.
Fig. 184. — Egg of the cotton-
worm moth; the micropyle is
shown in the center of the lower
figure.
168
AN INTRODUCTION TO ENTOMOLOGY
of
The number and position of the micropylar openings varies greatly
in the eggs of different insects. Frequently they present an elaborate
pattern at one pole of the egg (Fig. 184); and sometimes they open
through more or less elongated papillae (Fig. 185).
While in most cases it is necessary that an egg be fertilized in order
that development may continue; there are many instances of par-
thenogenesis among insects.
The number of eggs produced by insects.—
A very wide variation exists in the number of
eggs produced by insects. In the sheep-tick, for
example, a single large egg is produced at a time,
and but few are produced during the life of the
insect; on the other hand, in social insects, as
ants, bees, and termites, a single queen may
produce hundreds of thousands of eggs during her
lifetime.
These, however, are extreme examples; the
peculiar mode of development of the larva of the
Drosophila ampelo- sheep-tick within the body of the female makes
ptnia; m, micropyle.
possible the production of but few eggs; while
the division of labor in the colonies of social insects, by which the func-
tion of the queen is merely the production of eggs, makes it possible
for her to produce an immense number ; this is especially true where
the egg-laying period of the queen extends over several years.
The following may be taken as less extreme examples. In the
solitary nest-building insects, as the fossores, the solitary wasps, and
the solitary bees, the great labor involved in making and provisioning
the nest results in the reduction of the number of eggs produced to a
comparatively small number; while many insects that make no pro-
vision for their young, as moths, for example, may lay several hundred
eggs.
With certain chalcis-flies the number of young produced is not
dependent upon the number of eggs laid; for with these insects many
embryos are developed from a single egg. This type of development
is termed polyembryony.
Modes of laying eggs. — Perhaps in no respect are the wonderful
instincts of insects exhibited in a more remarkable way than in the
manner of lajdng their eggs. If insects were reasoning beings, and if
each female knew the needs of her young to be, she could not more
accurately make provision for them than is now done by the great
majority of insects.
THE METAMORPHOSIS OF INSECTS 169
This is especially striking where the life of the young is entirely
different from that of the adult. The butterfly or moth may sip
nectar from any flower; but when the female lays her eggs, she selects
with unerring accuracy the particular kind of plant upon which her
larvae feed. The dragonfly which hunts its prey over the field, returns
to water and lays her eggs in such a position that the young when it
leaves the egg is either in or can readily find the element in which alone
it is fitted to live.
The ichneumon-flies frequent flowers; but when the time comes
for a female to lay her eggs, she seeks the particular kind of larva
upon which the species is parasitic, and will lay her eggs in no other.
It is a remarkable fact that no larva leads so secluded a life that it
cannot be found by its parasites. Thus the larvae of Tremex Columba
bore in solid wood, where they are out of sight and protected by a
layer of wood and the bark of the tree in which they are boring;
nevertheless the ichneumon-fly
Thalessa lunator, which is para-
sitic upon it, places her eggs in
the burrows of the Tremex by
means of her long drill-like
ovipositor (Fig. 186).
In contrast with the exam-
ples just cited, some insects
exhibit no remarkable instinct
in their egg-laying. Our com-
mon northern walking-stick,
Diapheromera, drops its eggs-on
the ground under the shrubs
and trees upon which it feeds.
This, however, is sufficient pro-
Fig. 1 86. — Thalessa lunator.
throughout the winter by the
fallen leaves, and the young when hatched, readily find their food.
Many species, the young: of which feed upon foliage lay their eggs
singly upon leaves; but many others, and this is especially true of
those, the young of which are gregarious, lay their eggs in clusters.
In some cases, as in the squash bug, the mass of eggs is not protected
(Fig. 187) ; in others, where the duration of the egg-state is long, the
eggs are protected by some covering. The females of our tent-
caterpillars cover their eggs with a water-proof coating; and the
tussock moths of the genus Hemerocampa cover their egg-clusters with
a frothy mass.
170
AN INTRODUCTION TO ENTOMOLOGY
The laying of eggs in compact masses, however, is not correlated,
in most cases, with gregarious habits of the larvae. The water-
scavenger beetles, Hydrophilidae, make egg-sacks out of a hardened
silk-like secretion (Fig. 188) ; the locusts, Acridiidse, lay their eggs in
oval masses and cover them with a
..-—• "/.v^. .-x-!.:.;^ tough substance; the scale-insects
t?^-.': }: /•:' .. .^::ri|y"^-.K of the genus Pulvinaria excrete a
£:--;;:'.\ large cottony egg-sac (Fig. 189);
Fig. 187 — Egg-mass of the
squash-bug.
F ig . 1 88. — Egg-sac of Hydrophilus
(After Miall).
the eggs of the praying mantis are laid in masses and overlaid with
a hard covering of silk (Fig. 190) ; and cockroaches produce pod-like
egg-cases, termed
ootheca, each
containing many
eggs (Fig. 191).
Among the
more remarkable
of the methods of Fi&- 1 89- — Pulvinaria innumerabilis, females on
grape with egg -sacs
caring for eggs is
that of the lace-winged flies, Chrysopa. These insects place
each of their eggs on the summit of a stiff stalk of hard silk
(Fig. 192).
Duration of the egg-state. — In the life-cycle of most insects,
a few days, and only a few, intervene between the laying of
p. r*^0 an egg and the emergence of the nymph, naiad, or larva from
— E g g- it. In some the duration of the egg-state is even shorter, the
m ^ s | hatching of the egg taking place very soon after it is laid, or
pray- even, as sometimes in flesh-flies, before it is laid. On the
man? otner nan(i, in certain species, the greater part of the life of an
tis. individual is passed within the egg-shell. The common
apple-tree tent-caterpillars, Clisiocampa americana, lays
its' eggs in early summer; but these eggs do not hatch till the fol-
lowing spring; while the remainder of the life-cycle occupies only a
THE METAMORPHOSIS OF INSECTS
171
few weeks. The eggs of Bittacus are said to remain unhatched for
two years; and a similar statement is made regarding the eggs of
our common walking-stick.
b. THE HATCHING OP YOUNG INSECTS
Only a few accounts have been published
regarding the manner in which a young insect
frees itself from the embryonic envelopes. In FigcI09ckr7adi°theCa °f *
some cases it is evident that the larva cuts its
way out from the egg-shell by means of its mandibles ; but in otners, a
specialized organ has been developed for this purpose.
The hatching spines. —
An organ for rupturing
the embryonic envelopes
is probably commonly pre-
sent. It has been des-
cribed under several
names. It was termed an
egg-burster by Hagen, the
ruptor ovi by C. V. Riley
an egg-tooth by Heymons,
and the hatching spines
by Wheeler.
Fig. 192. — Eggs, larva, cocoon, and adult of
Chrysopa.
C. THE MOLTING OF INSECTS
The young of insects
cast periodically the outer
parts of the cuticula ; this process is termed molting or ecdysis.
General features of the molting of insects. — The chitinization of
the epidermis or primary cuticula adds to its efficiency as an armor, but
it prevents the expansion of the body-wall rendered necessary by the
growth of the insect; consequently as the body grows, its cuticula
becomes too small for it. When this occurs a second epidermis is
formed by the hypodermis; after which the old epidermis splits open,
usually along the back of the head and thorax, and the insect works
itself out from it. The new epidermis being elastic, accommodates
itself to the increased size of the body; but in a short time it becomes
chitinized; and as the insect grows it in turn is cast off. The cast
skin of an insect is termed the exwuice, the plural noun being used as in
English is the word clothes.
172 AN INTRODUCTION TO ENTOMOLOGY
Coincident with the formation of the new epidermis, new setae
are formed beneath the old epidermis ; these lie closely oppressed to
the outer surface of the new epidermis until released by the molting
of the old epidermis.
In the above account on1y the more general features of the process of molting
are indicated, the details, according to the observations of Tower ('06) are as
follows. (See Figure 1 1 3, p. 99) . In the formation of the new epidermis it appears
as a thin, delicate lamella, spread evenly over the entire outer surface of the
hypodermis; it grows rapidly in thickness until finally, just before ecdysis takes
place, it reaches its final thickness. After ecdysis the epidermis hardens rapidly
and its coloration is developed. As soon as ecdysis is over the deposition of the
dermis or secondary cuticula begins. This layer is a carbohydrate related
to cellulose, and is deposited in layers of alternating composition, through the
period of reconstruction and growth, during which it reaches its maximum thick-
ness. Preliminary to ecdysis a thin layer of molting fluid is formed, and through
its action the old dermis is corroded and often almost entirely destroyed, thus
facilitating ecdysis. This dissolving of the dermis, is, according to Tower, a most
constant phenomenon in ecydsis' and has been found in all insects examined by
him in varying degrees.
It is said that the Ccllembola molt after reacmng sexual maturity,
in this respect agreeing with the Crustacea and the "Myriapoda," and
differing from the Arachnida and from all other insects (Brindley '98).
The molting fluid. — As indicated above, the process of molting is
facilitated by the excretion of a fluid known as the molting fluid. This
is produced by unicellular glands (Fig. 113, p. 99) which are modified
hypodermal cells. These glands are found all through the life of the
insect and upon all parts of the body; but are most abundant upon
the pronotum, and are more abundant at pupation than at any other
period.
The number of postembryonic molts.— A very wide range of vari-
ation exists as to number of molts undergone by insects after they leave
the egg-shell. According to Grassi ('98, p. 292), there is only a single
partial molt with Campodea and Japyx, while the May-fly Chloeon
molts twenty times. Between these extremes every condition exists .
Probably the majority of insects molt from four to six times; but
there are many records of insects that molt many more times than this.
Stadia. — The intervals between the ecdyses are called stadia. In
numbering the stadia, the first stadium is the period between hatching
and the first postembryonic ecdysis.
Instars. — The term instar is applied to the form of an insect during
a stadium; in numbering the instars, the form assumed by the insect
between hatching and the first postembryonic molt is termed the first
instar.
THE METAMORPHOSIS OF INSECTS
173
Head measurements of larvse. — It was demonstrated by Dyar ('90)
that the widths of the head of a larva in its successive instars follow
a regular geometric progression in their increase. The head was
selected as a part not subject to growth during a stadium; and the
width as the most convenient measurement to take. By means of
this criterion, it is possible to determine, when studying the transfor-
mations of an insect, whether an ecdysis has been overlooked or not.
Experience has shown that slight variations between the computed
and the actual widths may occur; but these differences are so slight
that the overlooking of an ecdysis can be readily discovered. The
following example will serve to illustrate the method employed.
A larva of Papilio thoas was reared from the egg; and the widths
of the head in the successive instars was found to be, expressed in
millimeters, as follows: .6; i.i; 1.6; 2.2; 3.4.
By dividing 2.2. by 3.4 (two successive members of this series), the
ratio of increase was found to be .676+ ; the number, .68 was taken,
therefore, as sufficiently near the ratio for practical purposes. By
using this ratio as a factor the following results were obtained :
Width found in fifth instar = 3.4
Calculated width in fourth ins tar (3.4 X .68) = 2.312
" "third " (2.3 12 X. 68) =.... 1.57
" " second " (1.57 X .68) = 1.067
" "first (1.067 X .68)j= 725
By comparing the two series, as is done below, so close a correspond-
ence is found that it is evident that no ecdysis was overlooked.
Widths found: — .6; i.i; 1.6; 2.2; 3.4
" calculated: — .7; i.i-; 1.6-; 2.3.
* The reproduction of lost limbs. — The repro-
duction of lost limbs has been observed in many
insects ; but such reproduction occurs here much
less frequently than in the other classes of the
Arthropoda. The reproduction takes place dur-
ing the period of ecdysis, the reproduced part
becoming larger and larger with each molt;
hence with insects, and with Arachnida as well,
the power of reproducing lost limbs ceases with
the attainment of sexual maturity; but not so
with the Crustacea and the "Myriapoda" which
molt after becoming sexually mature. In none
Fig- 93- — A spider in of the observed examples of the reproduction
which lost legs weie v- * 1 11
being reproduced. of appendages has an entire leg been reproduced.
174 AN INTRODUCTION TO ENTOMOLOGY
It appears to be necessary that the original coxa be not removed in
order that the reproduction may take place. Figure 193 represents
a spider in our collection in which two legs, the left fore leg and the
right hind leg, were being reproduced when the specimen was captured.
d. DEVELOPMENT WITHOUT METAMORPHOSIS
(Ametabolous* Development)
While most insects undergo remarkable changes in form during
their postembryonic development, there are some in which this is
not the case. In these the young insect just hatched from the egg is
of practically the same form as the adult insect. These insects grow
larger and may undergo slight changes in form of the body and its
appendages ; but these changes are not sufficiently marked to merit
being termed a metamorphosis. This type of development is known
technically as ametabolous development.
Development without metamorphosis is characteristic of- .the two
orders Thysanura and Collembola, which in other respects, also, are
the most generalized of insects.
The nature of the changes in form undergone by an insect with an ametabolous
development is illustrated by the development of Machilis alternate, one of the
Thysanura. The first instar of this insect, according to Heymons ('07), lacks
the clothing of scales, the styli on the thoracic legs, and the lateral rows of eversi-
ble sacs on the abdominal segments; and the antennae and cerci are relatively
shorter and consist of a much smaller number of segments than those of the adult.
These changes, however, are comparable with those undergone by many animals
in the course of their development that are not regarded as having a metamorpho-
sis. In common usage in works on Entomology the term metamorphosis is used
to indicate those marked changes that take place in the appearance of an insect
that are correlated with the development of wings.
In addition to the Thysanura and the Collembola there are certain
insects that develop without metmorphosis, as the Mallophaga
and the Pediculidae. But their ametabolous condition is believed to be
an acquired one. In other words, it is believed that the bird-lice and
the true lice are descendants of winged insects whose form of body and
mode of development have been modified as a result of parasitic life.
The Ametabola. — Those insects that develop without meta-
morphosis are sometimes referred to as the Ametabola. This term was .
first proposed by Leach (1815), who included under it the lice as well
as the Thysanura and Collembola. But with our present knowledge, if
it is used it should be restricted to the Thysanura and Collembola
those insects in which a development without metamorphosis is a
primitive not an acquired condition.
*Ametabolous: Greek a, without; metabole Cfcera/SoXiJ), change.
7 HE METAMORPHOSIS OF INSECTS
6. GRADUAL METAMORPH3-IS
175
(Paurometabolous* Development)
In several orders of insects there exists a type of development that
is characterized by the fact that the young resemble the adult in the
general form of the body and in manner of life. There is a gradual
growth of the body and of the wing rudiments and genital appendages.
Fig. 194.' — Nymph of Mela-
no plus, first instar (After
Emerton).
Fig. 195. — Nymph of Mela-
noplus, second instar
(After Emarton).
Fig. 196.— Nymph of Melano-
plus, third instar (After Emer-
ton)
Fig. 197.— Nymph of Melano-
plus, fourth instar (After
Emerton).
Fig. 198. — Nymph of Melano-
plus, fifth instar (After Emer-
ton).
Fig. 199. — Melanoplus,
adult.
But the changes in form take place gradually and are not very great
between any two successive instars except that at the last ecdysis
there takes place a greater change, especially in the wings, than at
any of the preceding ecdyses. This type of metamorphosis is desig-
nated as gradual metamorphosis or paurometabolous development.
The characteristic features oi paurometabolous development are
correlated with the fact that the mode of life of the young and of the
*Paurometabolous: pauros (ira&pos), little; metabole
, change.
176 AN INTRODUCTION TO ENTOMOLOGY
adult are essentially the same; the two living in the same situation,
and feeding on the same food. The adult has increased power of loco-
motion, due to the completion of the development of the wings ; this
enables it to more readily perform the functions of the adult, the spread
of the species, and the making of provision for its continuance; but
otherwise the life of the adult is very similar to that of the young.
The development of a locust or short-horned grasshopper will
serve as an example of gradual metamorphosis. Each of the instars
of our common red-legged locust, Melanoplus femur-rubntm, is repre-
sented in the accompanying series of figures. The adult (Fig. 199)
is represented natural size; each of the other instars, somewhat
enlarged; the hair line above the figure in each case indicates the
length of the insect.
The young locust just out from the egg-shell can be easily recog-
nized as a locust (Fig. 194). It is of course much smaller than the
adult; the proportion of the different regions of the body are some-
what different ; and it is not furnished with wings ; still the form of the
body is essentially the same as that of the adult. In the second and
third instars (Fig. 195 and 196) there are slight indications of the
development of wing-rudiments; and these rudimentary wings are
quite conspicuous in the fourth and fifth instars (Fig. 197 and 198).
The change at the last ecdysis, that from the fifth instar to the adult,
is more striking than that at any preceding ecdysis; this is due to the
complete expansion of the wings, which takes place at this time.
The Paurometabola. — Those orders of insects that are characterized
by a gradual metamorphosis are grouped together as the Paurometa-
bola. This is not a natural division of the class Hexapoda but merely
indicates a similarity in the nature of the metamorphosis in the orders
included. This group includes the Isoptera, Dermaptera, Orthop-
tera, Corrodentia, Thysanoptera, Homoptera, and Efeteroptera.
The term nymph. — An immature instar of an insect that undergoes
a gradual metamorphosis is termed a nymph.
In old entomological works, and especially in those written in the
early part of the last century, the term nymph was used as a synonym
of pupa ; but in more recent works it is applied to the immature instar
of insects that undergo either a gradual or incomplete metamorphosis.
/ In this book I restrict the use of this term to designate an immature
instar of an insect that undergoes a gradual metamorphosis.
Deviation from the usual type. — It is to be expected that within so
large a group of organisms as the Paurometabola there should have
THE METAMORPHOSIS OF INSECTS 177
been evolved forms that exhibit deviations from the usual type of
development. The more familiar examples of these are the following :
The Saltitorial Orthoptera. — In the crickets, locusts, and long-
horned grasshoppers, the wings of the nymphs are developed in an
inverted position; that surface of the wing which is on the outside in
the adult is next to the body in the nymphal instars; and the rudi-
mentary hind wings are outside of the fore wings, instead of beneath
them, as in the adult. At the last ecdysis the wings assume the normal
position.
The Cicadas. — In the Cicadas there exists a greater difference
between the nymphal instars and the adult than is usual with insects
in which the metamorphosis is gradual. The nymphs live below the
surface of the ground, feeding upon the roots of plants ; the adults
live in the open air, chiefly among the branches of trees. The forelegs
of the nymphs are fossorial (Fig. 200); this is an
adaptation for subterranean life, which is not needed
and not possessed by the adults. And it is said that
the last nymphal instar is quiescent for a period.
The Coccida. — In the Coccidae the mode of develop-
ment of the two sexes differ greatly. The female
never acquires wings, and in so far as external form is
concerned the adult is degenerate. The male, on
the other hand, exhibits a striking approach to com-
plete metamorphosis, the last nymphal instar being'
enclosed in a cocoon, and the legs of the adult are not
those of the nymph, being developed from imaginal
disks. But the wings are developed externally.
- The Aleyrodida.—In this family the type of meta-
morphosis corresponds quite closely with that described
later as complete metamorphosis; consequently the
term larva is applied to the immature instars except the last, which is
designated the pupa.
The wings arise as histoblasts in the late embryo, and the growth
of the wing-buds during the larval stadia takes place inside the body-
wall. The change to the pupal instar, in which the wing-buds are
external, takes place beneath the last larval skin, which is known as
the pupa case or puparium. The adult emerges through a T-shaped
opening on the dorsum of the puparium. Both sexes are winged.
The Aphidida. — In the Aphididae there exists a remarkable type
of development known as heterogamy or cyclic reproduction. This is
characterized by an alternation of several parthenogenetic generations
178
AN INTRODUCTION TO ENTOMOLOGY
with a sexual generation. And within the series of parthenogenetic
forms there may be an alternation of winged and wingless forms. In
some cases the reproductive cycle is an exceedingly complicated one;
and different parts of it occur on different food plants.
The Thysanoptera. — In the Thysanoptera, as in most other insects
with a gradual metamorphosis, the nymphs resemble the adults in the
form of the body, and the wings are developed externally; but the last
nymphal instar is quiescent or nearly so and takes no nourishment.
This instar is commonly described as the pupa.
/. INCOMPLETE METAMORPHOSIS
(Hemimetabolous* Development)
In three of the orders of insects, the Plecoptera, Ephemerida, and
Odonata, there exists a type of metamorphosis in which the changes
Fig. 201. — Transformation of a May-fly, Ephemera varia; A,
adult; B, naiad (After Needham).
that take place in the form of the body are greater than in gradual
metamorphosis but much less marked than in complete metamorpho-
sis. For this reason the terms incomplete metamorphosis and hemi-
metabolous development have been applied to it.
Both incomplete metamorphosis and complete metamorphosis are
characterized by the fact that the immature instars exhibit adaptive
modifications of form and structure, fitting them for a very different
mode of life than that followed by the adult. This is often expressed
by the statement that the immature instars are "sidewise developed" ;
fpr it is believed that in these cases the development of the individual
does not repeat the history of the race to which the individual belongs.
*Hemimetabolous : hemi (-fjfj-i), half; metabole (/4rra/3oXiJ), change.
THE METAMORPHOSIS OF INSECTS 179
This mode of development is termed cenogenisis* It contrasts
strongly with gradual metamorphosis, where there is a direct develop-
ment from the egg to the adult.
In each of the orders that are characterized by an incomplete
metamorphosis, the adaptive characteristics of the young insects fit
them for aquatic life; while the adults lead an aerial existence. The
transformations of a May-fly (Fig. 201) will serve to illustrate this
type of metamorphosis.
The primitive insects were doubtless terrestrial ; this is shown by
the nature of the respiratory system, which is aerial in all insects. In
the course of the evolution of the different orders of insects, the
immature forms of some of them invaded the water in search of food.
This resulted in a sidewise development of these immature forms to
better fit them to live in this medium ; while the adult continued their
development in, what may be termed by contrast, a direct line. In
some of the Plecoptera, as Capnia and others, the results of the ceno-
genetic development are not marked except that the immature forms
are aquatic.
In the three orders in which the metamorphosis is incomplete, the
cenogenetic development of the immature instars involved neither a
change in the manner of development of the wings nor a retarding of
the development of the compound eyes ; consequently these immature
forms, although sidewise developed, constitute a class quite distinct
from larvae.
The Hemimetabola. — The three orders in which the development is
a hemimetabolous one are grouped together as the Hemimetabola;
these are the Plecoptera, Ephemerida, and Odonata. This grouping
together of these three orders is merely for convenience in discussions
of types of metamorphosis and does not indicate a natural division of
the class Hexapoda. The radical differences in the three types of
aquatic respiratory organs characteristic 'of the three orders indicate
that they were evolved independently.
The term naiad. — The immature instars of insects with an incom-
plete metamorphosis have been termed nymphs; but as a result of
their sidewise development they do not properly belong in the same
class as the immature instars of insects with a gradual metamorphosis.
I, therefore, proposed to designate them as naiads (Comstock '18, b).
The adoption of the term naiad in this sense affords a distinctive
term for each of the three classes of immature insects corresponding to
the three types of metamorphosis, i. e., nvnjphs, naiads, and larvae.
*Cenog£nisis: kainos (KO/POJ), new; genesis.
180 AN INTRODUCTION TO ENTOMOLOGY
Deviation from the usual type of incomplete metamorphosis. — The
more striking deviations from the usual type of hemimetabolous devel-
opment are the following: f
The Odonata. — In the Odonata the wings of the naiads are inverted ;
these insects resembling in this respect the Saltitorial Orthoptera.
What is the upper surface of the wings with naiads becomes the lower
surface in the adults, the change taking place at the last ecdysis.
The Ephemerida. — In the Ephemerida, there exists the remarkable
phenomenon of an ecdysis taking place after the insect has left the
water and acquired functional wings. The winged instar that is
interpolated between the last aquatic one and the adult is termed the
sub-imago.
g. COMPLETE METAMORPHOSIS
(Holometabolus* Development)
The representatives of several orders of insects leave the "egg-shell
in an entirely different form from that they assume when they reach
maturity; familiar examples of these are caterpillars which develop
into butterflies, maggots which develop mto flies, and grubs which
develop into beetles. These insects and others that when they
emerge from the egg-shell bear almost no resemblance in form to the
adult are said to undergo a complete metamorphosis or a holometdbolous
development. t ~
The Holometabola. — Those orders that are characterized by a
holometabolous development are grouped together as the Holometab-
ola. This group includes the Neuroptera, Mecoptera, Trichoptera,
Lepidoptera, Diptera, Siphonaptera, Coleoptera, and Hymenoptera.
This grouping together of these orders, while convenient for dis-
cussions of metamorphosis, is doubtless artificial. It is not at all
probable that the Holometabola is a monophylitic group. In other
words complete metamorphosis doubtless arose several times inde-
pendently in the evolution of insects.
The term larva. — The form in which a holometabolous insect
leaves the egg is called larva. The term was suggested by a belief of
the ancients that the form of the perfect insect was masked, the Latin
word larva meaning a mask.
Formerly the term larva was applied to the immature stages of all
insects; but more recent writers restrict its use to the immature in-
*Holometabolous : holos (^os)i complete; metabole (/texa/SoX^), change.
THE METAMORPHOSIS OF INSECTS 181
stars of insects with a complete metamorphosis; and in this sense
only is it used in this book.
The adaptive characteristics of larvae. — The larvae of insects with
complete metamorphosis, like the naiads of thos*e with incomplete
metamorphosis, exhibit an acquired form of body adapting them to
special modes of life; and in this case the cenogenetic or "sidewise
development" is much more marked than it is in insects with an
incomplete metamorphosis. Here the form of the body bears but little
relation to the form to be assumed by the adult, the nature of the
larval life being the controlling factor.
The differences in form between larvae and adults are augmented
by the fact that* not only have larvae been modified for special modes
of life, but in most cases the adults have been highly specialized for a
different mode of life; and so great are these differences that a
quiescent pupa stage, during which certain parts of the body can be
made over, is necessary.
Here, as in the case of insects with an incomplete metamorphosis, we have an
illustration of the fact that natural selection can act on any stage in the develop-
ment of animal to better adapt that particular stage to the conditions under which
it exists. Darwin pointed out in his "Origin of Species" that at whatever age
a variation first appears in the parent it tends to reappear at a corresponding age
in the offspring. This tendency is termed homochronous heredity*.
It is obvious that the greater the adaptive characteristics of the immature
forms, the less does the ontogeny of a species represent - the phylogeny of the
race to which it belongs. This fact led Fritz Muller, in his "Facts for Darwin",
to make the aphorism "There were perfect insects before larvae and pupae." The
overlooking of this principle frequently results in the drawing of unwarranted con-
clusions, by those writers on insects who cite adaptive larval characteristics as
being more generalized than the corresponding features of the adult.
The more obvious of the adaptive characteristics of larvae are the
following -.
The form of the body. — As indicated above the form of the body of a
larva bears but little relation to the form to be assumed by the adult,
the nature of the larval life being the controlling factor in determining
the form of the body. As different larvae live under widely differing
situations, various types of larvae have been developed; the more
important of these types are described later.
The greater or less reduction of the thoracic legs. — In the evolution
of most larvae there has taken place a greater or less reduction of the
thoracic legs; but the extent of this reduction varies greatly. The
larvae of certain Neuroptera, as Corydalus for example, have as perfect
*HomSchronous: homos (OACO'I), one and the same; chronos (x/x^oj), time.
182 AN INTRODUCTION TO ENTOMOLOGY
legs as do naiads of insects with an incomplete matamorphosis. The
larvae of Lepidoptera have short legs which correspond to only a part
of the legs of the adult. While the larvae _of Diptera have no external
indications of legs.
The development of prolegs in some larva. — A striking feature of
many larvae is the presence of abdominal organs of locomotion ; these
have been termed prolegs; the prolegs of caterpillars are the most
familiar examples of these organs.
The prolegs were so named because they were believed to be merely adaptive
cuticular formations and not true legs ; this belief arose from the fact that they are
shed with the last larval skin. Some recent writers, howeve», regard the prolegs
as true legs. It is now known that abdominal appendages are common in the
embryos of insects; and these writers believe that the prolegs are developed
from these embryonic appendages, and that, therefore, they must be regarded as
true legs.
If this is true, there has taken place a remarkable reversal in the course of
development. The abdominal legs, except those that were modified into append-
ages of the reproductive organs, the gonapophyses, were lost early in the phylogeny
of the Hexapoda. The origin of complete metamorphosis must have taken place
at a much later period; when, according to this belief, the abdominal appendages,
which had been latent for a long time, were redeveloped into functional organs.
The development of tracheal gills. — A striking feature of many larvae
is the possession of tracheal gills. This is obviously an adaptive
characteristic the development of which was correlated with the
assumption of aquatic life by forms that were primarily aerial; and
it is also obvious that the development of tracheal gills has arisen
independently many times ; for they exist in widely separated families
belonging to different orders of insects that are chiefly aerial. They
are possessed by a few lepidopterous larvae, and by the representatives
of several families of Neuroptera, Coleoptera and Diptera. On the
other hand, in the Trichoptera the possession of tracheal gills by the
larvae is characteristic of nearly all members of the order.
The internal development of wings. — This is perhaps the most re-
markable of the sidewise developments of larvae. Although larvae
exhibit no external indications of wings, it has been found that the
rudiments of these organs arise at as early a period in insects with a
complete metamorphosis as they do in those with an incomplete
metamorphosis ; and that during larval life the wing rudiments attain
an advanced stage in their development. But as these rudiments are
invaginated there are no external indications of their presence during
larval 1ife. The details of the internal development of wings are dis-
cussed later.
THE METAMORPHOSIS OF INSECTS 183
Occasionally atavistic individual larvae are found which have
external wing-buds.
As to the causes that brought about the internal development of wings we
can only make conjectures. It has occurred to the writer that this type of wing-
development may have arisen as a result of boring habits, or habits of an analogous
nature, of the stem forms from which the orders of the Holometabola sprang.
Projecting wing-buds would interfere with the progress of a boring insect; and,
therefore, an embedding of them in the body, thus leaving a smooth contour,
would be advantageous.
In support of this theory attention may be called to the fact that the larvae
of the most generalized Lepidoptera, the Hepialidas, are borers; the larvae of the
Siricidac, which are among the more generalized of the Hymenoptera are borers;
so too are many Coleoptera; most larvae of Diptera are burro wers; and the larvae
of Trichoptera live in cases.
The retarding of the development of the compound eyes. — One of the
most distinctively characteristic features of larvae is the absence of
compound eyes. The life of most larvae is such that only limited
vision is necessary for them ; and correlated with this fact is a retard-
ing of the development of the greater portion of the compound eyes ;
only a few separate ommatidia being functional during larval life.
In striking contrast with this condition are the well -developed eyes
of nymphs and naiads.
The larvae of Corethra are the only larvae known to me that
possess compound eyes.
The invaginated conditions of the head in the larva of the more
specialized Diptera. — The extreme of sidewise development is exhibited
by the larvae of the more specialized Diptera. Here not only are the
legs and wings developed internally but also the head. This phe-
nomenon is discussed later.
The different types of larvae. — As a rule, the larvae of the insects of
any order resemble each other in their more general characteristics,
although they bear but little resemblance to the adult forms. Thus
the grubs of Coleoptera, the caterpillars of Lepidoptera, or the mag-
gots of Diptera, in most cases, can be recognized as such. Still in
each of these orders there are larvae that bear almost no resemblance
to the usual type. As examples of these may be cited the water-
pennies (Parnidas, Coleoptera), the slug-caterpillars (Cochlidiidae,
Lepidoptera), and the larvae of Microdon (Diptera).
To understand the variations in form of larvae it should be borne
in mind that the form of the body in all larvae is the result of secondary
adaptations to peculiar modes of life; and that this modification of
form has proceeded in different directions and in varying degrees in
different insects.
184
AN INTRODUCTION TO ENTOMOLOGY
Among the many types of larvae, there are a few that are of such
common occurrence as to merit distinctive names; the more im-
portant of these are the following:
Campodeiform. — In many paurometabolous
insects and in some holometabolous ones, the
early instars resemble Campodea more or less in
the form of the body (Fig. 202) ; such naiads
and larvae are described as campodeiform.
In this type, the body is long, more or less
flattened, and with or without caudal setae ; the
mandibles are well developed; and the legs are
not greatly reduced. Among the examples of
this type are the larvae of most Neuroptera, and
the active larvae of many Coleoptera (Cara-
bidae, Dysticidae, and the first instar of Me-
loidae) .
Eruciform. — The cruciform type of larvae is
well-illustrated by most larvae of Lepidoptera
an.d of Mecoptera; it is the caterpillar form
(Fig. 203). In this type the body is cylindrical ;
the thoracic legs are short, having only the
terminal portions of them developed; and the
abdomen is furnished with prolegs or with
proleg-like cuticular folds. Although these
larvae move freely, their powers of locomo-
tion are much less than in the campodeiform
type.
Fig. 202. — Campodea
staphylinus (After
Lubbock).
Scarabeiform. — The common white grub, the larva of the May-
beetle (Fig. 204) is the most familiar example of a scarabeiform larva .
Fig. 203. — The silk- worm, an eruciform larva (After Verson).
In this type the body is nearly cylindrical, but usually, especially
when at rest, its longitudinal axis is curved; the legs are short; and
THE METAMORPHOSIS OF INSECTS.
185
prolegs are wanting. This type is quite characteristic of the larvae
of the Scarabaeidae, hence the name; but it occurs in other groups
of insects.
The movements of these larvae are
slow; most of them live in the ground,
or in wood, or in decaying animal or
vegetable matter.
Vermiform. — Those larvae that are
more or less worm-like in form are
termed vermiform. The most striking
features of this type are the elongated
Fig. 204. — Larva of Melolontha form of the body and an absence of
vulgaris (After Schiodte). , . j /T7.
locomotive appendages (Fig. 205).
Naupliiform. — The term naupliiform is applied to the first instar
of the larva of Platygaster (Fig. 206), on account of its
resemblance to the nauplius of certain Crustacea.
The prepupa. — Usually the existence of an instar
between the last larval one and the pupal instar is not
recognized. But such a form exists; and the recogni-
tion of it becomes important when a careful study is
made of the development of holometabolous insects.
As is shown later, during larval life the develop-
ment of the wings is going on within the body. As
the larva approaches maturity, the wings reach an
advanced stage of development within sac-like invagi-
nations of the body-wall. Near the close of the last
larval stadium the insect makes preparation for the
change to the pupa state. Some form a cell within
which the pupa state is passed, the larvae of butter-
flies suspend themselves, and most larvae of moths spin
a cocoon. Then follows a period of apparent rest before
the last larval skin is shed and the pupal state assumed.
But this period is far from being a quiet one ; within ;
the apparently motionless body important changes ^
take place. The most easily observed of these Larva of a
changes is a change in the position of the wings. crane-fly-
Each of these passes out through the mouth of the sac in which it has
been developed, and lies outside of the newly developed pupal cuti-
cula, but beneath the last larval cuticula. Then follows a period of
variable duration in different insects, in which the wings 'are really
186
AN INTRODUCTION TO ENTOMOLOGY
Fig. 206 —
Larva of
Platygaster
(After Ganin.)
outside of the body although still covered by the last larval cuticula ;
this period is the prepupal stadium. The prepupal instar differs
markedly from both the last larval one and from the
pupa ; for after the shedding of the last larval cuticula
important changes in the form of the body take place
before the pupal instar is assumed.
The pupa. — The most obvious characteristics of the
pupa state are, except in a few cases, inactivity and help-
lessness. The organs of locomotion are functionless,
and may even be soldered to the body throughout their
entire length, as is usual with the pupae of Lepidoptera
(Fig. 207). In other cases, as in the Coleoptera (Fig.
208) and in the Hymenoptera, the wings and legs are
free, but enclosed in more or less sac-like cuticular
sheaths, which put them in the condition of the pro-
verbial cat in gloves. More than this, in most cases, the legs of the
adult are not fully formed till near the end of the pupal stadium.
The term pupa, meaning girl, was applied to this instar by Linnaaus
on account of its resemblance to a baby that has been swathed or
bound up, as is the custom with
many peoples.
Although the insect during the pupal
stadium is apparently at rest, this, from a
physiological point of view, is the most
active period of its postembryonic exist-
ence; for wonderful changes in the struc- pig 2O7._pupa of a moth.
cure of the body take pla^e at this time.
In the development of a larva the primitive form of the body has been greatly
modified to adapt it to its peculiar mode of life; this sidewise development results
in the production of a type of body that is not at all fitted for the
duties of adult life. In the case of an insect with-incomplete meta-
morphosis, the full grown naiad needs to be modified comparatively
little to fit it for adult life; but the change from a maggot to a fly,
or from a caterpiller to a butterfly, involves not merely a change
in external form but a greater or less remodeling of its entire
structure. These changes take place during the period of apparent
rest, the prepupal and pupal stadia.
The chrysalis. — The term chrysalis is often applied to
the pupse of butterflies. It was suggested by the golden
spots with which the pupae of certain butterflies are
ornamented.
Two forms of this word are in use: first, chrysalis, the plural of
which is chrysalides; and second, chrysalid, the plural of which is
THE METAMORPHOSIS OF INSECTS
chrysalids. The singular of the first form and the plural of the second
are those most often used.
Active pupa. — The pupae of mosquitoes and of certain midges are
remarkable for being active. Although the wings and legs are func-
tionless, as with other pupae, these creatures are able to swim by
means of movements of the caudal end of the body.
In several genera of the Neuroptera (Chrysopa, Hemerobius, and
Raphidia) the pupa becomes active and crawls about just before
transforming to the adult state.
Movements of a less striking character are made by many pupae,
which work their way out of the ground, or from burrows in wood,
before transforming. In some cases, as in the pupae of the carpenter-
moths (Cossidae) the pupa is armed with rows of backward projecting
teeth on the abdominal segments, which facilitate the movements
within the burrow.
^he cremaster. — Many pupa?, and especially those of most Lepidop-
tera, are provided with a variously shaped process of the posterior
end of the body, to which the term cremaster is applied. This process
is often provided with hooks which serve to suspend the pupa, as in
butterflies, or to hold it in place, after it has partly emerged from the
cocoon, and while the adult is emerging from the pupal skin, as in
cocoon-making moths. In its more simple form, where hooks are
lacking, it aids the pupa in working its way out of the earth, or from
other closed situations.
The method of fixing the cremaster in the disk of silk from which
the pupa of a butterfly is suspended was well-illustrated by C. V. Riley
('79). The full grown larva spins this disk and hangs from it during
•the prepupal stadium
by means of its anal
prolegs (Fig. 209, a).
When the last larval
skin is shed, 'it is
worked back to the
caudal end of the body
(Fig. 209, 6); and is
then grasped between
two of the abdominal
segments (Fig. 209, c,)
while the caudal end of the body is removed from it; and thus the
cremaster is freed, and is in a position from which it can be inserted
in the disk of silk.
Fig. 209. — Transformations of the milkweed button
fly (From Riley).
188 AN INTRODUCTION TO ENTOMOLOGY
The cocoon,— The pupal instar is an especially vulnerable one.
During the pupal life the insect has no means of offence, and having
exceedingly limited powers of motion, it has almost no means of
defense unless an armor has been provided.
Many Iarva3 merely retreat to some secluded place in which the
pupal stadium is passed ; others bury themselves in the ground ; and
still others make provision for this helpless period by spinning a silken
armor about their bodies. Such an armor is termed a cocoon.
The cocoon is made by the full-grown larva; and this usually
takes place only a short time before the beginning of the pupal stadium.
But in some cases several months elapse between the spinning of the
cocoon and the change to pupa, the cocoon being made in the autumn
and the change to pupa taking place in the spring. Of course a
greater or less portion of this period is occupied by the prepupal
stadium.
Cocoons are usually made of silk, which is spun from glands
already described. In some cases, as in the cocoons of Bombyx, the
silk can be unwound and utilized by man.
While silk is the chief material used in the making of cocoons, it is
by no means the only material. Many wood-boring
larvae make cocoons largely of chips. Many insects that
undergo their transformation in the ground incorporate
earth in the walls of their cocoons. And hairy cater-
pillars use silk merely as a warp to hold together a
woof of hair, the hairs of the larva being the most con-
spicuous element in the cocoon.
In those cases in which silk alone is used there is a
great variation in the nature of the silk, and in the den-
sity of the cocoon. The well-known cocoons of the
saturniids illustrate one extreme in density, the cocoons
of certain Hymenoptera, the other.
The fiberous nature of the cocoon is usually obvious ;
but the cocoons of saw-flies appear parchment-like, and
Fig. 210. — the cocoons of the sphecids appear like a delicate foil.
cocoon of While in the more common type of cocoons the
Trichostibas wall is a closely woven sheet, there are cocoons that
from which are lace-like in texture (Fig. 210).
theadulthas Modes of escape from the cocoon. — The insect, having
walled itself in with a firm layer of silk, is forced to meet
the problem of a means of escape from this inclosure; a problem
which is solved in greatly varied ways.
THE METAMORPHOSIS OF INSECTS
189
In many insects in which the adult has biting mouth parts, the
adult merely gnaws its way out by means of its mandibles In some
cases, as the Cynipidae, it is said that this is the only use made of
its mandibles by the adult.
In some cases the mandibles with which the cocoon is pierced per-
tain to the pupal instar, this is true of Chrysopa and Hemerobius;
and the Trichoptera break out from their cases, by means of their
mandibles, while yet in the pupal state.
For those insects in which the adult has sucking mouth parts, the
problem is even more difficult. Here it has been met in several quite
distinct ways. The pupae of many Lepidop-
tera possess a specialized organ for breaking
through the cocoon; in some the anterior
end of the pupa is furnished with a toothed
crest (Lithocolletes hamadryelld); in certain satur-
niids there is a pair of large, stout, black spines,
one on each side
of the thorax, at
the base of the
fore wings with
which the pupa
cuts a slit in the Fig. 212. — Cocoon of Megalopyge oper-
cocoon through cularis'
which the adult emerges, this was observed by
Packard in Tropaa luna; but as these spines are
present in other saturniids, where the cocoon is too
dense to be cut by them, and where an opening is
made in some other way,.
it is probable that, as a
rule, their function is loco-
motive, aiding the pupa to
work its way out from the
cocoon, by a wriggling
motion.
One of the ways in
which saturniids pierce
their cocoons is that practiced by Bombyx and Telea.
These insects soften one end of the cocoon by a
liquid, which issues from the mouth; and then, by
forcing the threads apart or by breaking them, make an opening.
Fig. 211. — Longi-
tudinal section
of a cocoon of
Callosamia pro-
methca;v, valve-
like arrange-
ment for the
escape of the
adult.
Fig. 213. — Old cocoon of
Megalopyge opercularis.
190 AN INTRODUCTION TO ENTOMOLOGY
Far more wonderful than any of the methods of emergence from
the cocoon described above are those in which the larva makes pro-
vision for the escape of the adult. The most familiar of these is that
practiced by the larvae of Samia cecropia and Callosamia promethea.
These larvae when they spin their cocoons construct at one end a coni-
cal valve-like arrangement, which allows the adult to emerge without
the necessity of making a hole through the cocoon (Fig. 211, v). A
less familiar example, but one that is fully as wonderful, is that of
a Megalopyge. The larva of this species makes a cocoon of the
form shown in Figure 212. After an outer layer of the cocoon has
been made, the larva constructs, near one end of it, a hinged partition ;
this serves as a trap door, through which the moth emerges. That
part of the cocoon that is outside of the partition is quite delicate and
is easily' destroyed. Hence most specimens of the cocoons in col-
lections present the appearance represented in Figure 213.
The puparium. — The pupal stadium of most Diptera is" 'passed
within the last larval skin, which is not broken till the adult fly is
ready to emerge. In this case the larval skin, which becomes hard
and brown, and which serves as a cocoon, is termed a
puparium. In some families the puparium retains the
form of the larva; in others the body of the larva
shortens, assuming a more or less barrel-shaped form,
before the change to a pupa takes place (Fig. 214).
Modes of escape from the puparium. — The pupae of
the more generalized Diptera escape from the pupa-
rium through a T-shaped opening, which is formed by
a lengthwise split on the back near the head end and a
crosswise split at the front end of this (Fig. 215), or
rarely, through a cross-wise split between the seventh
Fig. 214. — Pupa- and eighth abdominal segments. In the more special-
™™ °f Try~ ized Diptera there is developed a large bladder-like
organ, which is pushed out from the front of the head,
through what is known as the frontal suture, and by which the head
end of the puparium is forced off. This organ is known as the ptilinum.
After the adult escapes, the ptilinum is withdrawn into the head.
The Different types of pupae . — Three types 3
of pupae are commonly recognized; these
are the following : Fig. 215. — Puparium of a
Exarate pupa.—Pwpaz which, like those s^110111^-
of the Coleoptera and Hymenoptera, have the legs and wings free,
are termed exarate pupae.
THE METAMORPHOSIS OF INSECTS 191
Obtected pupa. — Pupag which like the pupae of Lepidoptera, have
the limbs glued to the surface of the body, are termed obtected pupae.
C oar date Pupa. — Pupae that are enclosed within the hardened
larval skin, as is the case with the pupae- of most of the Diptera, are
termed coarctate pupae.
The imago — The fully developed or adult insect is termed the
imago.
The life of the imago is devoted to making provision for the
perpetuation of the species. It is during the imaginal stadium that
the sexes pair, and the females lay their eggs. With many species
this is done very soon after the last ecdysis ; but with others the egg-
laying is continued over a long period; this is especially true with
females of the social Hymenoptera.
h. HYPERMETAMORPHOSIS
There are certain insects, representatives of several different orders
that exhibit the remarkable peculiarity in their development that the
successive larval instars represent different types of larvae. Such
insects are said to undergo a hypermetamorphosis.
The transformations of several of these insects will be described
later in the accounts of the families to which they belong; and for
this reason, in order to avoid repetition, are not discussed here. The
more striking examples are Mantispa, Meloe, Stylops, and Platy-
gaster.
I. VIVIPAROUS INSECTS
There are many insects that produce either nymphs or larvas
instead of laying eggs. Such insects are termed viviparous. This
term is opposed to oviparous, which is applied to those insects that lay
eggs that hatch after exclusion from the body.
It has been pointed out in the discussion of the reproductive organs that, from
the primordial germ -cells, there are developed in one sex spermatoza and in the
other eggs; and it should be borne in mind that the germ-cells produced in the
ovary of a female from the primordial germ-cells are eggs. These eggs grow and
mature; in some cases they become covered with a shell, in others they are not
so covered ; in some cases they are fertilized by the union of a spermatozoan with
them, and in others they are never fertilized; but in all these cases they are eggs.
We may say, therefore, that all insects are developed from eggs.
A failure to recognize this fact has introduced confusion into entomological
literature. Some writers have termed the germ -cells produced by agamic aphids
pseudova or false eggs. But these germ-cells are as truly eggs as are those from
which the males of the honeybee develop; they are merely unfertilized eggs.
The term pseudovum conveys a false impression; while the phrase, an unfer-
tilized egg, clearly states a fact.
192 AN INTRODUCTION TO ENTOMOLOGY
Some writers make use of the term ovoviviparous indicating the production
of eggs that have a well -developed shell or covering, but which hatch within the
body of the parent; but the distinction is not fundamental, since viviparous ani-
mals also produce eggs as indicated above.
Among viviparous insects there are found every gradation from
those in which the larvae are born when very young to those in which
the entire larval life is passed within the body of the parent. There
also exist examples of viviparous larvae, viviparous pupae, and vivi-
parous adults. And still another distinction can be made; in some
viviparous insects the reproduction is parthenogenetic ; in others it
is sexual.
Viviparity with parthenogenetic reproduction. — In certain vivipar-
ous insects the reproduction is parthenogenetic; that is, the young are
produced from eggs that are not fertilized. This type of reproduction
occurs in larvae, pupae, and apparently in adults.
P&dogenetic Larva. — In 1862 Nicholas Wagner made the remark-
able discovery that certain larvae belonging to the Cecidomyiidae give
birth to living young. This discovery has been confirmed by other
observers, and for this type of reproduction the term p&do genesis,
proposed by Von Baer, has come into general use. This term is also
spelled pedogenesis; the word is from p&do or pedo, a child, and genesis.
The phenomenon of paedogenesis is discussed later in the accounts
of the Cecidomyiidaa and of the Micromalthidae.
P&dogenetic pupce. — The most frequently observed examples of
paedogenetic reproduction are by larvae ; but that pupae also are some-
times capable of reproduction is shown by the fact that Grimm ('70)
found that eggs laid by a pupa of Chironomus gtimmii, and of course
not fertilized, hatched.
Anton Schneider ('85) found that the adults of this same species of
Chironomus reproduced parthenogenetically. This species, therefore,
exhibits a transition from paedogenesis to normal parthenogenesis.
Viviparous adult agamic females. — There may be classed under this
class provisionally, the agamic females of the Aphididae ; as these are
commonly regarded as adults. It has been suggested, however, that
the agamic reproduction of the Aphids may be a kind of paedogenesis ;
the agamic females being looked upon as nymphs. This however, is
not so evident in the case of the winged agamic generation. On the
other hand, the reproductive organs of the agamic aphids are incom-
pletely developed, as compared with those of the sexual forms, lacking
a spermatheca and colleterial glands.
THE METAMORPHOSIS OF INSECTS 193
This discussion illustrates the difficulty of attempting to make sharp distinc-
tions, whereas in nature all gradations exist between different types of structure
and of development. Thus Leydig ('67) found a certain aphid to be both ovipar-
ous and vivaprous; the eggs and the individuals born as nymphs being produced
from neighboring tubes of the same ovary.
Viviparity with sexual reproduction. — Although most insects that
reproduce sexually are oviparous, there are a considerable number in
which sexual reproduction is associated with viviparity.
Among these sexual viviparous insects there exist great differences
in method of reproduction ; with some the young are born in a very
immature stage of development, a stage corresponding to that in
which the young of oviparous insects emerge from the egg ; while with
others the young attain an advanced stage of development within the
body of the mother.
Sexual viviparous insects giving birth to nymphs or larvce. — That
type of viviparity in which sexual females give birth to very immature
nymphs or larva? exists in more or less isolated members of widely
separated groups of insects. As the assumption of this type of repro-
duction involves no change in the structure of the parent, but merely
a precocious hatching of the egg, it is not strange that it has arisen
sporadically and many times. In some cases, however, the change is
not so slight as the foregoing statement would imply ; as, for example,
in the case of the viviparous cockroach, which does not secrete
oothecae as do other cockroaches.
Among the recorded examples of this type of viviparity are repre-
sentatives of the Ephemerida, Orthoptera, Hemiptera, Lepidoptera,
Coleoptera, Strepsiptera, and Diptera.
Sexual viviparous insects giving birth to old larva. — The mode of
reproduction exhibited by these insects is doubtless the most excep-
tional that occurs in the Hexapoda, involving, as it does, very import-
ant changes in the structure of the reproductive organs of the
females.
With these insects the larvae reach maturity within the body of the
parent, undergoing what is analogous to an intra-uterine development,
and are born as full-grown larvae. This involves the secretion of a
"milk" for the nourishment of the young.
This mode of reproduction is characteristic of a group of flies,
including several families, and known as the Pupipara. This name
was suggested for this group by the old belief that the young are born
as pupae ; but it has been found that the change to pupa does not take
place till after the birth of the larva.
194 AN INTRODUCTION TO ENTOMOLOGY
The reproduction of the sheep-tick, Melophagus ovinus, may be
taken as an illustration of this type of development ; this is described
in the discussion of the Hippoboscidae, the family to which this insect
belongs.
The giving birth to old larvae is not restricted to the Pupipara.
Surgeon Bruce (quoted by Sharp, '99) has shown that the Tse-tse-fly,
Glossina morsitans, reproduces in this way, the young changing to
pupae immediately after birth.
An intermediate type of development is illustrated by Hylemyia
strigosa, a dung-frequenting fly belonging to the Anthomyiidae.
This insect, according to Sharp ('99), produces living larvae, one at a
time. "These larvae are so large that it would be supposed they are
full-grown, but this is not the case, they are really only in the first
stage, an unusual amount of growth being accomplished in this
stadium."
/. NEOTEINIA
The persistence with adult animals of larval characteristics has
been termed neoteinia* or neotenia. When this term first came into
use it was applied to certain amphibians, as the axolotle, which retains
its gills after becoming sexually mature; but it is now used also in
entomology.
The most familiar examples of neoteinic insects are the glow-
worms, which are the adult females of certain beetles, the complemen-
tal females of Termites, and the females of the Strepsiptera.
II. THE DEVELOPMENT OF APPENDAGES
In the preceding pages the more obvious of the changes in the
external form of the body during the metamorphosis of insects and
some deviations from the more common types of development have
been discussed. The changes in the form of the trunk that have been
described are those that can be seen without dissection; but it is
impracticable to limit a discussion of the development of the appen-
dages of the body in this way, for in the more specialized types of
metamorphosis a considerable part of the development of the appen-
dages takes place within the body-wall.
*Neoteinia: neos (^os), youthful; teinein (reiveiv}, to stretch.
THE METAMORPHOSIS OF INSECTS 195
0. THE DEVELOPMENT OP WINGS
Two quite distinct methods of development of wings exist in
insects; by one method, the wings are developed as outward project-
ing appendages of the body; by the other, they reach an advanced
stage of development within the body. The former method of
development takes place with nymphs and naiads, the latter with
larva?.*
i. The Development of the Wings of Nymphs and Naiads
In insects with a gradual or with an incomplete metamorphosis the
development of the appendages proceeds in a direct manner. The
wings of nymphs and naiads are sac-like outgrowths of the body-wall,
which appear comparatively early in life and become larger and larger
with successive molts, the expanding of the wing-buds taking place
immediately after the molt ; an illustration of this has been given in
the discussion of gradual metamorphosis, page 175.
2. Development of the Wings in Insects with a Complete
Metamorphosis
Although there are differences in details in the development of the
wings in the different insects undergoing a complete metamorphosis,
the essential features are the same in all. The most striking feature
is that the rudiments of the wings, the wing-buds, arise within the
body and become exposed for the first time when the last larval skin
is shed. The development of the wings of the cabbage butterfly
(Pontia rapes) will serve as an example of this type of development of
wings. The tracing of that part of this development which takes
place during the larval life can be observed by making sections of the
body-wall of the wing-bearing segments of the successive instars of
this insect.
The first indication of a wing-bud is a thickening of the hypo-
dermis; this thickening, known as a histoblast or an imaginal disc,
has been observed in the embryos of certain insects, in the first
larval instar of the cabbage butterfly it is quite prominent (Fig.
216, a). During the second stadium, it becomes more prominent
and is invaginated, forming a pocket-like structure (Fig. 216, 6).
During the third stadium a part of this imagination becomes
thickened and evaginated into the pocket formed by the thinner
*Only the more general features of the development of wings are discussed
here. For a fuller account see "The Wings of Insects" (Comstock 'i 8, a).
196
AN INTRODUCTION TO ENTOMOLOGY
portions of rne invagination (Fig. 216, c). During the fourth
stadium, the evaginated part of the histoblast becomes greatly
extended (Fig. 216, d).
It is this evaginated
portion of the histo-
blast that later be-
comes the wing. Dur-
ing the fifth stadium
the wing-bud attains
the form shown in
JjjjjJI I* Figure 216, e, which
represents it dissected
out of the wing-pocket
At the close of the last
larval stadium, the
fifth, the wingis pushed
out from the wing-poc-
ket, and lies under the
old larval cuticula dur-
ing the prepupal sta-
dium. It is then of
the form shown in
Figure 216, /. The
molt that marks the
beginning of the pupal
stadium, exposes the
wing-buds, which in
the Lepidoptera be-
come closely soldered
to the sides and breast
of the pupa. Imme-
diately after the last
molt when the adult
emerges, the wings
expand greatly and
assume their definitive
form.
While this increase in size and changes in form of the developing
wing are taking place, there occur other remarkable developments in
its structure. A connection is made with a large trachea near which
the histoblast is developed, shown in cross-section in the first four
Fig. 216. — Several stages in the development of the
wings of a cabbage butterfly (After Mercer).
THE METAMORPHOSIS OF INSECTS 197
parts (a, b, c, and d) of Figure 216; temporary respiratory organs,
consisting of bundles of tracheoles, are developed (e and/) ; and later,
near the close of the larval period, the tracheae of the wing are devel-
oped, and the bundles of tracheoles disappear. During the later
stages in the development of the wing the basement membranes of the
hypodermis of the upper and lower sides of the wing come together,
except along the lines where the veins are to be developed later, and
become- united. In this way the wing is transformed from a bag-like
organ to a sheet-like one. The lines along which the two sides of the
wing remain separate are the vein cavities ; in these the trunks of the
wing-tracheas extend. During the final stages of the development of
the wing, the walls of the vein-cavities are thickened, thus the wing-
veins are formed ; and the spaces between the wing-veins become thin.
By reference to Figure 216, c and d, it will be seen that the histo-
blast consists of two quite distinct parts, a greatly thickened portion
which is the wing-bud and a thinner portion which connects the wing-
bud with the hypodermis of the body-wall, and which constitutes the
neck of the sac-like histoblast, this is termed the peripodal membrane,
a term suggested by the similar part of the histoblast of a leg ; and the
enclosed cavity is known as the peripodal cavity.
In the more specialized Diptera, the peripodal membranes are
very long and both the wing-buds and the leg-buds are far removed
from the body-wall. A condition intermediate between that which
exists in the Lepidoptera, as shown in Figure 216, and that of the
more specialized Diptera was found by Kellogg ('07) in the larva of
Holorusia rubiginosa, one of the
crane-flies (Fig. 217).
b. THE DEVELOPMENT OF LEGS
The development of the legs
proceeds in widely different ways
in different insects. In the _
Fig. 217. — Wing- bud in the larva or the
more generalized forms, the giant crane-fly, Holorusia rubiginosa;
legs of the embryo reach an *?* hypodermis; pm peripodal mem,
brane; /, trachea; wb, wing- bud (After
advanced stage of development Kellogg).
before the nymph or naiad
leaves the egg-shell, and are functional when the insect is born; on
the other hand, in those specialized insects that have vermiform larvae,
the development of the legs is retarded, and these organs do not
become functional until the adult stage is reached. Almost every
conceivable intergrade between these two sxtremes exist.
198 AN INTRODUCTION TO ENTOMOLOGY
I. The Development of the Legs of Nymphs and of Naiads
In insects with a gradual metamorphosis and also in those with an
incomplete metamorphosis the nymph .or naiad when it emerges from
the eggshell has well-developed legs, which resemble quite closely
those of the adult. The changes that take place in the form of the
legs during the postembyronic development are comparatively slight ;
there may be changes in the relative sizes of the different parts ; and
in some cases there is an increase in the number of the segments of the
tarsus ; but the changes are not sufficiently great to require a descrip-
tion of them here.
2. The Development of the Legs in Insects with a Complete Metamor-
phosis
It is a characteristic of most larvae that the development of their
legs is retarded to a greater or less extent. This retardation is least
in campodeiform larvae, more marked in cruciform larvae, and reaches
its extreme in vermiform larvae.
The development of the legs of insects with campodeiform larvae,—
Among the larvae classed as campodeiform the legs are more or less
like those of the adults of the same species ; there may be differences
in the proportions of the different segments of the leg, in the number
of the tarsal segments, and in the number and form of the tarsal claws ;
but these differences are not of a nature to warrant a discussion of
them here. These larvae lead an active life, like that of nymphs,
and consequently the form of legs has not been greatly modified from
the paurometabolous type.
The development of the legs of insects with cruciform larvae. — In
caterpillars and other cruciform larvae the thoracic legs are short and
fitted for creeping ; this mode of locomotion being best suited to their
mode of life, either in burrows or clinging to foliage. This form of leg
is evidently an acquired one being, like the internal development of
wings, the result of those adaptive changes that fit these larvae to lead
a very different life from that of the adults.
In the case of caterpillars the thoracic legs are short, they taper
greatly, and each consists of only three segments. It has been com-
monly believed and often stated that the three segments of the larval
leg correspond to the terminal portion of the adult leg; but studies of
the* development of the legs of adults have shown that the divisions
of the larval leg have no relation to the five divisions of the adult leg.
THE METAMORPHOSIS OF INSECTS 199
It has been shown by Gonin ('92), Kellogg ('01 and '04), and
Verson ('04) that histoblasts which are the rudiments of the legs of the
adult exist within the body-wall of the caterpillar at the base of the
larval legs. . Late in the larval life the extremity of the legs of the
adult are contained in the legs o f the caterpillar. It has been shown
that the cutting off of a leg of a caterpillar at this time results in a
mutilation of the terminal part of the leg of the adult.
The development of the legs of the adult within the body of cater-
pillars has not been studied as thoroughly as has been the develop-
ment of the wings ; but enough is known to show that in some respects
the two are quite similar ; this is especially true of the development of
the tracheoles and of the tracheae.
The development of the legs in insects with vermiform larvae. — In
vermiform larvae the development of the entire leg is retarded. The
leg arises as a histoblast, which is within the body and bears, in its
more general features, a resemblance to the wing-buds of the same
insect. The development of the legs of vermiform larva? has been
studied most carefully in the larvae of Diptera. During the larval
life the leg becomes quite fully developed within the peripodal cavity;
in Corethra, they are spirally coiled ; in Musca, the different segments
telescope into each other. At the close of the larval period, the
evagination of the legs takes place.
C. THE DEVELOPMENT OF ANTENNAE
i. The Transformation of the Antenna of Nymphs and of Naiads
In the case of nymphs and of naiads the insect when it emerges
from the eggshell has well-developed antennae. The changes that
take place during the postembryonic development are, as a rule, com-
paratively slight; in most insects, an increase in the number of the
segments of the antennae takes place ; but in the Ephemerida, a reduc-
tion in number of the antennal segments occurs.
2. The Development of the Antenna in Insects with a Complete
Metamorphosis
One of the marked characteristics of larvae is the reduced condition
of the antennae; even in the campodeiform larvae of the Neuroptera,
where the legs are comparatively well-developed, the antennae are
greatly reduced.
In cruciform larvae the development of the antennae follows a
course quite similar to that of the legs. The larval antennae are small •
200
AN INTRODUCTION TO ENTOMOLOGY
the antennae of the adult are developed from histoblasts within the
head and during the latter part of the larval life are folded like the
bellows of a closed accor-
dian; at the close of this
period they become eva-
ginated, but the definitive
form is not assumed until
the emergence of the adult.
A similar course of devel-
opment of the antennas
takes place in vermiform
larvae (Fig. 218).
i mx
d. THE DEVELOPMENT OF
THE MOUTH-PARTS
Great differences exist
insects with refer-
Fig. 218. — Sagittal section through headof old
larva olSimulium, showing forming imaginal
head parts within. Ic, larval cuticula; id,
imaginal head- wall; la, larval antenna; ia,
imagmal antenna; i-e, imaginal eye; Imd,
larval mandible; imd, imaginal mandible; ence to the comparative
Imx larval maxilla; imx, .imaginal maxilla; structure of their mouth-
Ih, larval labium; u%, imaginal labmm (From
Kellogg). parts in their immature
and adult instars. In
some insects the immature instars have essentially the same type of
moueh-parts as the adults ; in most of these cases, the mouth-parts are
of the biting types, but in the Homoptera and Heteroptera both
nymphs and adults have them fitted for sucking; in many other
insects, the mouth-parts of the larvae are fitted for biting while those of
adults are fitted for sucking; and in still others, as certain maggots, the
development of the mouth-parts is so retarded that they are first
functional in the adult insect. Correlated with these differences are
differences in the method of development of these organs.
In those insects that have a gradual or incomplete metamorphosis
and in -the Neuroptera, the Coleoptera, and the Hymenoptera in part,
the mouth-parts of the immature and adult instars are essentially of
the same type. In these insects the mouth-parts of each instar are
developed within the corresponding mouth-parts of the preceding
instar. At each ecdysis there is a molting of the old cuticula, a
stretching of the new one before it is hardened, a result of the growth
in size of the appendages, and sometimes an increase in the number
of the segments of the appendage. In a word, the mouth-parts of the
adult are developed from those of the immature instar in a compara-
tively direct manner. In some cases, however, where the mouth-
THE METAMORPHOSIS OF INSECTS 201
parts of the larva are small and those of the adult are large, only the
tips of the developing adult organs are within those of the larva at the
close of the larval period, a considerable part of the adult organs being
embedded in the head of the old larva.
In a few Coleoptera and Neuroptera (the Dytiscidae, Myrme-
leonidae, and Hemerobiidae) the larvae, although mandibulate, have
the mouth-parts fitted for sucking. In these cases the form o the
mouth-parts have been modified to fit them for a peculiar metho d of
taking nourishment during the larval life. The mouth-parts of the
adults are of the form characteristic of the orders to which these
insects belong.
In those insects in which the larvae have biting mouth-parts and
the adults those fitted for sucking, the development is less direct. In
the Lepidoptera, for example, to take an extreme case, there are great
differences in the development ot thf. different organs; within the
mandibles of the old larvae there are no developing mandibles, these
organs being atrophied in the adult; but at the base of each larval
maxilla, there is a very large, invaginated histoblast, the developing
maxilla of the adult; these histoblasts become evaginated at the
close of the larval period, but the maxillae do not assume their defini-
tive form till after the last ecdysis.
The extreme modification of the more usual course of development
of the mouth-parts is found in the footless and headless larvae of the
more specialized Diptera. Here the mouth-parts do not appear
externally until during the pupal stadium and become functional only
when the adult condition is reached. See the figures illustrating the
development of the head in the Muscidae (Fig. 220).
It should be noted that the oral hooks possessed by the larvae of the
more specialized Diptera are secondarily developed organs and not
mouth-parts in the sense in which this term is commonly used. ' These
oral hooks serve as organs of fixation in the larvae of the CEstridae and
as rasping organs in other larvae.
e. THE DEVELOPMENT OF THE GENITAL APPENDAGES
The development of the genital appendages of insects has been
studied comparatively little and the results obtained by the different
investigators are not entirely in accord ; it is too early therefore to do
more than to make a few general statements.
In the nymphs of insects with a gradual metamorphosis rudimen-
tary genital appendages are more or less prominent and their develop-
202 AN INTRODUCTION TO ENTOMOLOGY
ment follows a course quite similar to that of the other appendages of
the body.
In insects with a complete metamorphosis the genital appendages
are represented in the larvae by invaginated histoblasts ; the develop-
ing appendages become evaginated in the transformation to the pupa
state and assume their definitive form after the last ecdysis.
III. THE DEVELOPMENT OF THE HEAD IN THE
MUSCID.E
In the more generalized Diptera the head of the larva becomes,
with more or less change, the head of the adult ; the more important
of these changes pertain to the perfecting of the organs of sight and the
development of the appendages, the antennas and mouth-parts.
But in the more specialized Diptera there is an anomalous retard-
ing of the development of the head, which is so great that the larvae
of these insects are commonly referred to as being acephalous. This
retarded development of the head has been carefully studied by Weis-
man ('64), Van-Rees ('88) and Kowalevsky ('87). The accompanying
diagrams (Fig. 220) based on those given by the last two authors illus-
trate the development of the head in Musca, which will serve as an
illustration of this type of development of the head.
The larvae of Musca
are conical (Fig. 219) ; and
the head-region is repre-
sented externally only by
the minute apical segment
Fig. 219— Larva of the house-fly, Musca of the conical body. It
domestica (After Hewitt).
will be shown later that
this segment is the neck of the insect, the developing head being
invaginated within this and the following segments. This invagina-
tion of the head takes place during the later embryonic stages.
In Figure 220 are given diagrams, adapted from Kowalevsky and
Van Rees, representing three stages in the development of the head of
Musca. Diagram A represents the cephalic end of the body of a
larva; and diagram B and C, the corresponding region in a young and
in an old pupa respectively; the parts are lettered uniformly in the
three diagrams.
The three thoracic segments (1,2, and 3) can be identified by the
rudiments of the legs (/l, /2, and /3). In the larva (A) the leg-buds
are far within the body, the peripodal membrane being connected with
THE METAMORPHOSIS OF INSECTS
203
the hypodermis of the body-wall by a slender stalk-like portion. In
the young pupa (B) the peripodal membranes of the histoblasts of the
legs are greatly shortened and the leg-buds are near the surface of the
body; and in the old pupa (C) the leg-buds are evaginated. The
wing-buds are omitted in all of the diagrams.
In the first two segments of the body of the larva (A) there is a
cavity (pti) which has been termed the "pharynx" ; this is the part in
which the oral hooks characteristic of the larvae of the Muscidae
develop. The name pharynx is unfortunate as this is not a part of the
alimentary canal; it is an invaginated section of the head, into the
base of which the oesophagus (oe) now opens.
In the figure of the larva (A) note the following parts: the
oesophagus (ce) ; the ventral chain of ganglia (vg), the brain (!>)', and a
Fu
. 220. — Development of the head in the Muscidae. A, larva; B, young pupa ;
', old pupa (From Korschelt and Heider after Kowalevsky and Van Rees).
sac (ba) extending from the so-called pharynx to the brain. There are
two of these sacs, one applied to each half of the brain, but only one of
these would appear in such a section as is represented by the diagram.
These sacs were termed the brain-appendages by Weismann. In each
of the "brain-appendages" there is a disc-like thickening near the
brain, the optic disc (od) ; this is a histoblast which develops into a
compound eye ; in front of the optic disc there is another prominent
histoblast ; the frontal disc (fd) , upon which the rudiment of an antenna
(at) is developed.
In the larva the brain and a considerable part of the "brain-
appendages" lie within the third thoracic segment. In the young
pupa (B) these parts have moved forward a considerable distance;
and in the old pupa (C) the head has become completely evaginated.
The part marked p in the two diagrams of the pupa is the rudiment
of the proboscis.
204 AN INTRODUCTION TO ENTOMOLOGY
By comparing diagrams B and C it will be seen that what was the
tip of the first segment of the larva and of the young pupa (+ + )
becomes the neck of the insect after the head is evaginated.
IV. THE TRANSFORMATIONS OF THE INTERNAL
ORGANS
Great as are the changes in the external form of the body during
the life of insects with a complete metamorphosis, even greater changes
take place in the internal organs of some of them.
In the space that can be devoted to this subject in this work, only
the more general features of the transformation of the internal organs
can be discussed; there is an extensive and constantly increasing
literature on this subject which is available for those who wish to study
it more thoroughly.
In insects with a gradual or with an incomplete metamorphosis
there is a continuous transformation of the internal organs, the changes
inform taking place gradually ; being quite comparable to the gradual
de velopment of the external organs ; but in insects with a complete
metamorphosis, where the manner of life of the larva and the adult
are very different, extensive changes take place during the pupal
stadium. The life of a butterfly, for example, is very different from
that it led as a caterpillar; the organs of the larva are not fitted to
perform the functions of the adult ; there is consequently a necessity
for the reconstruction of certain of them ; hence the need of a pupal
stadium. Pupae are often referred to as being quiet; but physiologi-
cally the pupal period is the most active one in the post-embryonic
life of the insect.
In those cases where a very marked change takes place in the
structure of internal organs, there is a degeneration and dissolution of
tissue, this breaking down of tissues is termed histolysis.
In the course of histolysis some cells, which are frequently leu-
cocytes or white blood corpuscles, feed upon the debris of the disin-
tegrating tissue ; such a cell is termed a phdhgocyte, and the process is
termed phagocytosis. It is believed that the products of the digestion
of disintegrating tissue by the phagocytes pass by diffusion into the
surrounding blood and serve to nourish new tissue.
After an organ has been more or less broken down by histolysis,
the extent of the disintegration differing greatly in different organs
and in different insects, there follows a growth of new tissue; this
process is termed htsto genesis.
THE METAMORPHOSIS OF INSECTS 205
The histogenetic reproduction of a tissue begins in the differentia-
tion and multiplication of small groups of cells, which were not affected
by the histolysis of the old tissue; such a group of cells is termed an
imaginal disc or a histoblast. They were termed imaginal discs on
account of the disc-like form of those that were first described and
because they are rudiments of organs that do not become functional
till the imago stage ; but the term histoblast is of more general appli-
cation and is to be preferred.
The extent of the transformation of the internal organs differs
greatly in different insects. In the Coleoptera, the Lepidoptera, the
Hymenoptera, and the Diptera Nemocera, the mid-intestine and
some other larval organs are greatly modified, but there is no general
histolysis. On the other hand, in the Diptera Brachycera, there is a
general histolysis. In Musca all organs break down and are reformed
except the central nervous system, the heart, the reproductive organs,
and three pairs of thoracic muscles. Regarding the extent of the
transformations in the other orders where the metamorphosis is com-
plete we have, as yet, but little data.
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INDEX
Figures in bold-faced type refer to pages bearing illustrations.
Abdomen, 75; appendages of the, 76;
segments of the, 75
Acalles, 88
Accessory circulatory organs, 122
Accessory glands, 162
Accessory veins, 68
Acerentomidae, 26
Acerentomon doderoi, 25
Acetabula, 52
Acone eyes, 141
Acrida turrita, 134
Adaptive ocelli, 135, 136,
Adelung, 150
Adipose tissue, 123
Adventitious veins, 70
Air-Sacs, 118
Akers, Elizabeth, 78
Aleyrodidae, 177
Alimentary canal, 107
Alitrunk, 49
Alula, the, 60
Alulet, 60
Alveolus, 32
Ambient vein, 74
Ametabola, 174
Ametabolous development, 174
Amphipneustc, 115
Anal angle, 60
Anal area, 75; the veins of the, 65
Anal furrow, 73
Anastomosis of veins, 70
Androconia, 100
Anepimerum, 51
Anepi sternum, 51
Angles of wings, 60
Anobium, 79
Anosa plexippus, head of, 109
Antecoxal piece, 54
Antennae, 40, 41 ; the development of,
199
Antennal sclentes, 39
Anterior arculus, 72
Anthony, Maude H., 113
Anuricia, 47
Anus, 113
Aorta, 122
Apex of the wing, 60
Aphididae, 177
Apodemes, 95, 98
Apophyses, 31
Appendages, the development of, 194
Apposed [image, 143
Arachnida, 9
Arculus, 72
Arolium, 58
Arthropoda, i
Articular membrane of the setae, 32
Articular sclerites of the legs, 53; of
the wings, 54, 55
Ateuchus, 88
Atropos divinatoria, So
Auditory pegs, 147
Audouin, 49
Axillaries, 54
Axillary cord, the, 60
Axillary excision, 61
Axillary furrow, 74
Axillary membrane, the, 60
Basement membrane, 31, 109, 118
Bear Animalcules, 11
Bedbug, 103
Bellesme, 92
Berlese, 25, 106, 113, 128, 132, 1 33,134.
151,155
Blastophaga, 59
Blepharocera, 144
Blood, 122
Blood-gills, 114, 120
Body-segments, 34
Body-wall, 29, 34
Bombyxmori, 128
Boophilus annulatus, 2
Bothropolys multidentatus, 21
Brachypauropodidae, 19
Brindley, 172
Bullae, 74
Burgess, 109, 160
Bursa copulatrix, 1 59
Buzzing of flies and bees, 91
Cabbage butterfly, development of the
wings of, 196
Caecum, 113
Callosamia promethea, cocoon of, 1(89
Caloptenus italicus, 149
•.Campodea, 157, 161
Campodeiform, 184
•Cantharis vesicatoria, larva of, 117
Capitate, 41 .
Capnia, 179
Carabus auratus, alimentary canal of,
110
Cardo, 44
Carlet, 89, 90
Carolina locust, 82
Carpenter, 17
Carriere and Burger, 103
Cells of the wing, terminology of the, 72
Centipedes, 20
213
214
INDEX
Ceratopogon, 136
Cerci, 24, 77
Cervical sclerites, 40
Chelophores, n
Chemical sense organs, 130, 132
Cheshire, 102
Chiasognathus, 88
Child, C.M., 153, 154
Chilopoda, 20
China wax, 102
Chironomus, 120, 147, 148
Chitin, 30
Chitinized tendons, 95
Chordo tonal ligament, 147
Chordotonal organs, 145, 146, 147,
148; of the Acridiidae, 148, 149;
of the Locustidaeandof Gryllidae, 149
Choruses, 93
Chrysalid, 186
Chrysalis, 186
Chrysopa, 170, 171
Chylestomach, in
ChylifiC ventricle, 1 1 1
Cicada plebeia, 89
Cicadas. 177
Cicada, the musical organs of a, 89, 90
Cicindela, maxilla of, 45
Circulation of the blood, 122
Circulatory system, 121
Clavate, 41
Clavola, 41
Clisiocampa americana, 170
Cloeon, head of, 144
Closing apparatus of the tracheae, 116
Clothilla pulsatoria, 80
Clothing hairs, 33
Club, 41
Clypeus, 38
Coarctate pupae, 191
Coccidae, 177
Cockroach, head of a, 38; head and
neck of a, 39; internal anatomy of,
107 ; labium of a, 46 ; tentorium of a
96; the base of a leg of a, 53
Cocoon, 1 88; modes of escape from
the, 1 88
Colleterial glands, 160
Collophore, 76
Colymbefes, eyes of, 143
Commissure, 125
Complete metamorphosis, 1 80
Compound eyes, 134, 139; absence of,
135; dioptrics. 141
Comstock and Needham, figures from,
84, 85, 86
Concave veins, 73
Conjunctiva, 34
Connectives, 123
Conpcephalus, 86, 87
Cotiopx, Wing of, 60
Convex veins, 73
Corethra, 121, 134
Corethra culiciformis, 154
Cornea, 138. 139
Corneagen, 138
Corneal hypodermis, 138 139
Corneas of the compound eyes, 36;
of the ocelli, 37
Corrugations of the wings, 73
Corydalus, 62. in, 119, 125, 126, 136;
head of 39; head of a larva of 38,
137
Cossus ligniperda, 104, 105
Costa, 64
Costal margin, 60
Coxa, 56
Coxal cavities, 52
Crampton, 40, 49, 52
Cray-fishes, 6
Cremaster, 1 , 87
Cricket head of a 37, 40, 136; part
of the tentorium of a 96
Crista acustica, 152
Crop, up
Cross- veins, 64, 71
Crotch, 88
Crura cerebri, 123
Crustacea, 6
Crystalline cone-cells, 140
Cubito-anal fold, 73
Cubitus, 64
Cucujo, 165
Culex, 153
Cuticula, 30
Cuticular nodules, 31
Cyclops, 6
Cylisticus convexus, 7
Cypridopsis, 6
Cyrtophyllus concavus, 93
Dactylopius. 28
Damsel-fly, tracheal gill of a, 120
Daphnia, 6
Darwin, Charles. 88, 181
Datana, 28
Day-eyes, 142
Death-watch, 80
Decticus verrucivorus, 150, 151, 152
Definite accessory veins, 69
De Meijere, 58
Dermis, 31
Deutocerebrum, 47, 124
Development without metamorphosis,
174
Dewitz, 101
Diapheromera, 169
Digitus, 45
Diplopoda, 15
Discal cell, 74
Discalvein, 74
Di&sgsteira Carolina, 82
Distal retinula cells, 140
INDEX
215
Divided eyes, 144
Dorsal diaphragm, 121, 162
Doyerc, 12, 13
Dufour, no
Dyar, 33, 173
Ecdysis, 1 7 1
Ectoderm, 29
Egg, 166
Egg-burster, 171
Egg-calyx, 159
Egg-follicles, 158
Egg- tooth, 171
Ejaculatory duct, 162
Elytra, 59
Empodium, 58
Endo-skeleton, 95
Endothorax, 97
Eosentomidae, 26
Ephemera varia, 178
Ephemerida, 180; ocelli of, 139
Epicranial suture, 37
Epicranium, 38
Epidermis, 31
Epimerum, 51
Epipharnyx, 46
Epipleurae, 74
Episternum, 51
Epithelium, 109, 118; of mid-intestine,
112
Eruciform, 184
Eucone eyes, 141
Eurypauropidae, 20
Eurypauropus ornatus, 19
Eurypauropus spinosus, 19, 20
Eusternum, 52
Exarate pupae, 190
Exner, 141, 143
Exuviae, 171
Eyes of insects, two types of, 134; with
double function, 143
Femur, 57
Fibula, 62; of Corydalus, 63
Filiform, 41
Fixed hairs, 31
Flask-like sense-organ, 131
Follicular epithelium, functions of the,
159
Folsom 43, 47
Fore-intestine, 108. 109
Frenulum, 61
Frenulum hook, 61
Front, 37
Froth-glands of spittle insects, 102
Funicle, 41
Furcae, 98
Furrovv-s of the vsting, 73
Gahan, 88
Gastric caeca, JI2
Genae, 39
Geniculate, 41
Genital appendages, the development
of the, 20 1
Genital claspers, 76
Genitalia, 76
Geophilus flavidus, 21
Germarium, 158
Glands, 98 ; connected with setae, 99
Glandular hairs, 33
Glomeris, 16
Glowworms, 194
Gnathochilarium, 16
Gonapophyses, 76
Gcnin, 199
Graber, 146. 148, 149, 150
Gradual metamorphosis, 175
Grassi, 157, 172
Grimm, 192
Gryllotalpa borealis, chirp of, 93
Gryllus, 83 ; ventral aspect of the
meso- and metothorax of, 98
Guenther, 132
Guilbeau, 102
Gula, 39
Gynandromorph, 156
Hagen, 113, 171
Halteres, 59
Hammar, 125, 126
Hamuli, 61
Hansen, 23, 24, 43
Harpalus, labium of, 45, 52
Harvestmen, 9
Hatching of young insects, 171
Hatching spines, 171
Hautsinnesorgane, 130
Head, 36
Head measurements of larvae, 173
Hearing, organs of, 145
Heart, 121
Hemelytra, 59
Hemimetabola, 179
Hemimetabolous, development, 178
Henneguy, 117, 124
Hepialid, wings of a, 62
Hess, 136, 137, 139, 146, 147, 148
Heterogamy , 177
Hewitt, 202
Hexapoda, 26
Heymons, 174
Hicks, 155
Hilton, 128, 129, 132, 133
Hind-intestine, 108, 112
Hispopria foveicollis, 88
Histoblast, 195, 205
Histogenesis, 204 .
Hochreuter, 155
Hoeck, n
Hofer, 127
Holmgren, 99
210
INDEX
Holomctabola, 180
Holometabolous development, 180
Holorusia rubiginosa, 197
Homochronous heredity, 181
Homologizing of the sclerites, 35
Honey-bee, 158
Horseshoe-crabs, 8
House-fly, larva of the, 202
Humeral angle, 60
Humeral cross- vein, 71
Humeral veins, 74
Huxley, 40
Hydrophilus, egg sac of, 170; embryo
of, 76; maxilla of, 44
Hyper metamorphosis, 191
Hypodermal glands, 98
Hypodermal structures, 95
Hypodermis, 29
Hypopharynx, 47
Hypopygium, 75
Hypothetical tracheation of a wing of
the primitive nymph, 63
Hypothetical type of the primitive
wing-venation, 62
Imaginal disc, 195, 205
Imago, 191
Imperf orate intestines, 108
Incomplete metamorphosis, 178
Inner margin, 60
Insect?, 26
Instars, 172
Intercalary veins, 69
Intermediate organ, 152
Internal anatomy, 94
Internal organs, the transformations of
the, 204
Internal skeleton, 95 ; sources of the, 95
Intersegmental plates, 40
Intima, 109, 117
Invaginations of the body-wall, 95
Janet, 87
Japyx, 161
Johnston, Christopher, 152
Johnston's organ, 152
Judeich and Nitsche, 116
Jugular sclerites, 40
Jugum, 6 1 ; of a hepialid, 63
Julus, 16
Katepimerum, 51
Katepisternum, 51
Katydid, chirp of the, 93
Kellogg, 100, 197, 199, 200
Kenyon, 18, 19
King-crabs, 8
Kirby and Spence, 97
Korschelt and Heider, 203
Kowalevsky, 202
Labial palpi, 46
Labium or second maxillae, 45
Labrum, 38, 43
Lace-like cocoon, 188
Lacinia, 45
Lamellate, 41
Landois, 91
Large-intestine, 113
Larva, the term denned, 180
Larvae, adaptive characteristics of, i?i ;
the different types of, 183
Lateral conjunctivas, 35
Latzel, 19, 21, 23, 24
Leach, 174
Legs, 56; the development of, 197
Lehr, 155
Lentigen layer, 138
Lepisma saccharina, 48, 78
Leucocytes, 122
Lienard, 125
Ligament of the ovary, 159; of the
testes, 162
Light-organs, 164
Limulus polyphemus, 8
Lingua, 47
Linguatula, 14
Lingua tulids, 14
Locusta viridissima, 128
Longitudinal veins, 64
Lubbock, 1 8, 48, 106
Lyonet, 104, 105, 106
Mclndoo, 155
Machilis alternata, 174
Machilis, ommatidium of, 139 ; leg of,
57; ventral aspect of, 77; the
tracheae of, 116, 117
.Malpighian vessels, 113; as silk-glands,
H3
Mandibles, 43
Marey, 81
Marginal accessory veins, 69
Margins of wings, 59, 60
Maxillae, 43
Maxillary palpus, 44
Maxillary pleurites, 40
Maxillnlae, 16, 43
May-beetle, heart of a, 121; leg of a,
106
May-fly, wings of a, 70
Mechanical sense-organs, 130
Media, 64
Medial cross- vein, 71
Median caudal filament, 78
Median furrow, 74
Median plates, 55
Median segment, 49
Median sutures, 35
Medio-cubital cross- vein, 71
Megalopyge opercularis, cocoon of, 189
INDEX
217
Melanoplus, 160; ental surface of the
pleurites of the meso- and meta-
thorax of. 96 ; head of, 97 ; tentorium
of, 97
Melolontha vulgaris, larva of; 185
Melophagus ovinus, 194
Mentum, 46
Mercer, 196
Mesenteron, 108, in
Mesonotum, 50
Mesophragma, 97
Mesothorax. 48
Metameres, 34
Metamorphosis of Insects, 166
Metanotum, 50
Metaphragma, 97
Metapneustic, 115
Metathorax, 48
Miall, 170
Micropyle, 167
Mid-intestine, 108, in
Milk-week butterfly, reproductive
organs of the, 160; transformations
of the, 187
Millipedes, 15
Milne-Edwards, 47
Mites, 9
Molting fluid, 172
Molting fluid glands, 99
Molting of insects, 171
Moniliform, 41
Morgan, 70
Mosaic vision, theory of, 141, 142
Mosquitoes, antennae of, 153
Mouth-parts, 42; the development of,
200
Muller, Fritz, 181
Muller, J., 141
Muller's organ, 149
Muscidae, development of the head in
the, 202
Muscles, 104
Musical notation of the songs of insects,
92
Musical organs of insects, 78
Music of flight, 80
Myriapoda, 15
Myrientomata, 24
Myrmecial wings of, 74
Myrmica rubral stridulating organ of,
87
Naiad, the term defined, 179
Naupliiform, 185
Needham, 112, 178
Nemobius, 84
Neoteinia, 194
Nerves, 123
Nervous system, 123
Neuronia, 56; lateral aspect of the
mesothorax of, 57
Neuropore, 130
Newport, 106
Nidi, 112
Night-eyes, 143
Nodal furrow, 74
Notostigma, 22
Notum. 49
Nurse-cells, 158
Nymph, the term defined, 176
Nymphon hispidum, 11
Obtected pupae, 191
Occiput, 39
Ocelli, 134, 135
Ocular sclerites, 39
Odonata, 180
(Ecanthus, 84, 85, 86
CEcanthus niveus, 93
(Enocytes, 163
CEsophageal sympathetic nervous sys-
tem, 125, 127
CEsophageal valve, 1 1 1
(Esophagus, no
Olfactory pore of Mclndoo, 155
Olfactory pores, 131, 154
Ommatidium, 135; structure of, 139
Oniscoida, 7
Onychii, 58
Onychophora, 4
Ootheca, 170
Oral hooks, 201
Organs of sight, 130
Orthesia, 102
Osmeteria, 101
Osmylus hyalinatus, wings of, 68, 69
Ostia of the heart, 121
Oudemans, 117
Outer margin, 60
Ovarian tubes, 157, 158
Ovaries, 156
Oviduct, 156, 159
Ovigerous legs, 1 1
Oviparous, 191
Ovipositor, 76
Packard, 149, 189
Paedogenesis, 192
Paedogenetic larvae, 192
Paedogenetic pupae, 192
Palaepstracha, 8
Palpifer, 44
Palpognaths, 21
Pamphilins, wings of, 67
Papilio thoas, 173
Papilio thoas, larva of, 101
Paraglossae, 43
Parapsides, 51
Paraptera, 51
Parasites, Respiration of, 120
Passalus, stridulating organ of a larva
of, 89
218
INDEX
Patagia, 50
Paurometabola, 176
Paurometabolous development, 175
Pauropoda, 18
Pauropodidae, 20
Pauropus huxleyi, 18
Pectinate, 41
Pedicel, 41
I'elobius, 1 20
Penis, 162
Pentastomida, 14
Pentatomidae, 103
Penthe, prothorax of, 53
Pe"rez, 92
Pericardial cells, 164
Pericardial diaphragm, 163
Peripatoides nova-zealandica, 4
Peripatus, I, 4
Peripheral sensory nervous system, 128,
129
Periplaneta orientalis, 107, 127
Peripneustic, 115
Peripodal cavity, 197
Peripodal membrane, 197
Peritoneal membrane, 109
Peritremes, 52
Peritrophic membrane, 111, 112
Phagocyte, 164, 204
Phagocytic organs, 164
Phagocytosis, 164, 204
Pharynx, 109
Phasma, 121
Phonapate, 88
Photinus marginellus, 165
Phragmas, 97
Pieces jugulaires, 40
Pigment cejls, accessory, 138, 140;
iris, 140
Piliferous tubercles of larvae, 35
Plasma, 122
Plecoptera, 136
Pleura, 34
Pleurites, 35
Pleurostigma*2i
Pocock, 17, 21
Poduridae, 115
Polyembryony, 168
Polyxenus, 16, 17
Ponitia rapa, 195
Pore- plate, 131
Porvcephalus, 14
Postembryonic molts, number of, 172
Posterior arculus, 72
Posterior lobe of the wing, 61
Postgense, 39
Postnotum, 50
Postphragma, 98
Postscutellum, 50
Poststernellum, 52
Praetarsus, 58
Praying mantis, eggs of the, 170
Preanal area, 75
Preepisternum, 51
Prephragma, 98
Prepupa, 185
Prescutum, 50
Presternum, 52
Primary ocelli, 135; structure of, 137,
138
Primordial germ-cells, 158
Prionoxystus, wings of, 70
Proctodaeum, 108
Prolegs of larvae, 78; the development
of, 182
Pronotum, 50
Prophragma, 97
Propneustic, 115
Propodeum, 49
Propygidium, 75
Prothorax, 48
Protocerebrum, 47, 124
Protura, 26
Proventriculus, no, 111
Pseudocone eyes, 141
Pseudo-halteres, 59
Pseudova, 191
Pteronarcys, 120; head of, 136
Pterostigma, 74
Ptilinum, 190
Pulsations of the heart, 122
Pulvilli, 58
Pulvinaria innumerabilis, 170
Pupa, 1 86
Pupae, active, 187; the different types
of, 190
Puparium, 190
Pupipara, 193
Pycnogonida, 10
Pygidium, 75
Radial cross- vein, 71
Radio-medial cross- vein, 71
Radius, 64
Rasping organs, 87
Rath, O. vom, 132
Rectum, 113
Redikorzew, 137
Regions of the body, 36
Reighardis, 14
Reproduction of lost limbs, 173
Reproductive organs, 156; of the
female, 157; of the male, 160, 161
Respiratory organs, the closed or ap-
neustic type of. 119; the open or
holopneustic type of, 114
Respiratory system, 113
Retina, 138
Retinula, 138, 140
Rhabdom, 137
Rhabdomere, 137
Rhyphus, a wing of 65
Riley C. V., 171, 177, 187
INDEX
219
Ring-joints, 41
Rolleston, 107
Ruptor ovi, 171
Salivary glands, 103, 104
Saltitorial Orthoptera, 177
Scape, 40
Scarabeiform, 184
Scent-glands of females, 100
Schiodte, 88, 185
Schneider, Anton, 192
Schwabc, 150, 151
Sclerites, 35
Scolopale, 146
Scolopendrella 23, 24
Scolopophore, 146
Scorpion, 9
Scorpions, lateral ocelli of, 137
Scudder, 92 ,
Scutellum, 50
Scutigera forceps, 22
Scutigerella, 24
Scutum, 50
Seaton, 139
Second antecoxal piece, 54
Secondary sexual characters. 157
Sectorial cross-vein, 71
Segmentation of the appendages, 34
Segmentation of the body, 34
Segments of the head, 47, 48
Seiler, 139
Seminal vesicle, 162
Sense- cones, 131
Sense-domes, 154, 155
Sense-hairs, 33
Sense-organs, classification of the, 129;
cuticular part of the, 130; of un-
known functions, 154
Sensillum ampullaceum, 131
Sensillum basiconicum, 131
Sensillum cceloconicum. 131
Sensillum chceticum, 131
Sensillum placodeum, 131
Sensillum trichodeum, 130, 132
Serial veins, 67
Serrate, 41
Setaceous, 41
Setae, 32 ; classification of, 33 ; taxono-
mic value of, 33
Setiferous sense-organs, 130
Sharp, David, figures from, 87, 89, 144;
quoted ,88, 93, 194
Siebold, 92
Siebold's organ, 152
Sight, organs of, 134
Silk-glands, cephalic, 103
Silkworm, 114; sense hairs of the, 133
Silvestri, F , 16,25, 113
Simulium, 120; head of larva of, 200;
larva of, 1 1 1
Small-intestine, 113
Smell, organs of, 132
Smynthurus, 115
Snodgrass, 49, 50, 55, 57, 98
Solpugida, 9
Somites, 34
Sow-bugs, 7
Spematheca, 159
Spermatazoa, 160
Spermathecal gland, 160
Spermatophores, 162
Spiders, 9
Spines, 32
Spiracles, 52, 113, 114; structure of,
116
Spiracular musical organs, 91
Spirostreptus, 16
Spring of the Collembola, 76
Spurious vein, 70
Spurs, 32
Squamae, 60
Squash-bug, egg-mass of the, 170
Stadia, 172
Stenobothrus, 82
Stenopelmatus, ventral aspect of the
meta thorax of, 98
Sternellum, 52
Sternites, 35
Sternum, 34, 52
Stigma, 74
Stigmata, 113
Stink-glands, 102
Stipes, 44
Stomach, in
Stomodaeum, 108
Straus Durckheim, 40, 106, 121
Strepsiptera, 194
Stridulating organs, 81 ; of the Acridii-
dae, 82; of the Gryllidae and the
Locustidae, 83
Styli, 56, 76
Subcosta, 64
Subcostal fold, 73
Subgalea, 44
Submentum, 46
Subcesophageal commissure, 125
Subcesophageal ganglion, 123, 124
Superimposed image, 143
Superlinguae, 43
Supplements, 70
Supra-tympanal or subgenual organ,
Suspensoria of the
thread-like, 163
Sutures, 35
Symphyla, 23
Tabanus, wing of, 66
Tapetum, 144
Tardigrada, 11, 12
Tarsal claws, 58
Tarsus, 57
viscera, 162;
220
INDEX
Taste and smell, organs of, 132
Tegmina, 59
Tegula, 54
Telson, 75
Tenent hairs, 58, 100, 101
Tentorium, 96
T -gites, 35
T\rgum, 34
Terminal filament, 158
Termites, 158, 194
Termitoxinia, 156
Testes, 160
Testicular follicle, structure of a, 161
Thalessa lunator, 169
Thorax, 48; diagram of, 50, 51
Thyridopteryx ephemerceformis, wings
of, 61
Thysanoptera, 178
Tibia, 57
Tipula abdominalis, larva of, 2
Touch, organs of, 131
Tower, 99, 172
Townsend, Miss, 165
Toxicognaths, 21
Tracheae, 113, 116; the structure of
the, 117
Tracheal gills, 119; the development
of, 182
Tracheoles. 113, 118
Transverse conjunctivas, 34
Tremex Columba, 169
Trichogens, 30
Trichopore, 32, 130
Tritocerebrum, 47, 124
Trochanter, 57
Trochantin, 53 ; of the mandible, 40
Tympana, 145
Ungues, 58
Vagina, 159
Van Rees, 202
Vas deferens, 156, 162
Venomous setae and spines, 100
Ventral diaphragm, 163
Ventral heart, 163
Ventral sympathetic nervous system,
127
Ventriculus, in
Verhceff, 49
Vermiform, 185
Verson, 114, 199
Vertex, 39
Viallanes, 47
Visual cell, structure of a, 137
Vitellarium, 158
Vitreous layer, 138
Viviparity. 192, 193
Viviparous insects, 191 ; adult agamic
females, 192
Vogel, 155
Von Siebold, 145
Wagner, Nicholas 192
Wax-glands, 102 '
Weisman, 202, 203
Wings, 58; the development of, 182,
195
Wings of the heart, 121, 162
Wing-veins, reduction of the number
of, 65; the chief branches of the, 64;
the increase of the number of. 68;
the principal, 64
Wollaston, 88
Xiphosura, 8
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