The larva and pupa of the caddisfly genus Setodes in North America (Trichoptera: Leptoceridae)

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Life Sciences
Occasional Papers

Royal Ontario Museum
November 26, 1971

No. 19

The Larva and Pupa of the

Caddisfly Genus Setodes in North America

(Trichoptera: Leptoceridae)

by Dorothy Merrill1 and Glenn B. Wiggins2

Among the leptocerid caddisflies of North
America there are many species which are
not known in the larval stage. All but one
of the genera, however, contain a sufficient
number of species for which larva-pupa-
adult associations have been established to
permit the construction of fairly reliable
generic keys to the immature stages, such as
those by Ross (1944, 1959). The genus
Setodes is the exception. For none of the
Nearctic species have immature and mature
forms been associated. In Ross' keys to the
genera of leptocerid larvae, specimens char-
acterized chiefly by strongly developed sclero-
tized plates on the caudal face of the anal
prolegs were tentatively assigned to Setodes,
because these unusual larvae clearly do not
fit readily into any of the other North Ameri-
can genera.

Descriptions of Setodes larvae in Europe
in recent years have left little doubt that
Ross' tentative assignment was correct: Se-
todes hungarica Ulmer (Botosaneanu, 1959);
Setodes punctata Fabricius (Botosaneanu
and Sykora, 1963). Larvae of both species
were associated with mature pupae, and
both have the sclerotized plates on the caudal
face of the anal prolegs as illustrated by
Ross {op. cit.). The larva of a third Euro-
pean species was described by Murgoci
(1959); the identity is not clear, but Boto-

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saneanu and Sykora (1963) suggested that
this is the larva of S. argentipunctella Mc-
lachlan. Hickin (1967) provided a descrip-
tion of the larva of S. argentipunctella, but
details of the plates of the anal prolegs were
not given. Illustrations of the larva of a
Japanese species assigned to Setodes by
Akagi (1957) show that similar larval
characteristics occur in the Asian fauna.

Knowledge concerning the comparative
morphology of larval and pupal stages of
Setodes from various parts of the world has,
then, been steadily accumulating, although
few observations have been offered on the
biology of the group. For the North Ameri-
can fauna nothing has been added since the
original tentative allocation of these larvae
to Setodes by Ross in 1944. In North
America, six species are known, and only
from the adult stage. These are confined to
the eastern half of the continent, extending
as far west as Oklahoma: Setodes incerta
(Walker), S. stehri (Ross), S. oxapia
(Ross), 5. guttata (Banks), S. oligia (Ross),
and S. epicampes Edwards.

In the summer of 1969, larvae possessing
the sclerotized anal plates were collected in
Michigan (by D.M.). Rearing them to the
adult stage established that the species was
Setodes incerta (Walker). Additional obser-
vations were made in the summer of 1970.

1. The University of Michigan Biological Station, Pellston, Michigan.
Present address: Western College, Oxford, Ohio, 45056.

2. Department of Entomology and Invertebrate Zoology, Royal Ontario Museum.

Based upon that association, this paper
offers the first detailed information on the
morphology of the larva and pupa of this
genus in North America (by G.B.W.), and
on the behaviour of the larva as well (by
D.M.). Diagnostic genitalic characters for
the female of S. incerta were not previously
known and are illustrated. The larva of
another species of Setodes from South Caro-
lina is briefly characterized, although no
specific association has yet been established
for it.

The larvae of S. incerta were collected in
the Ocqueoc River, near the northern tip of
Michigan's lower peninsula, in a region of
strong current, upstream from Ocqueoc
Falls. The initial collection was made by
examining rocks taken from a depth of less
than 2 feet. Laboratory observations indi-
cated that the larvae were burrowers, and
in subsequent collecting, screening of sand
from the lee of rocks provided a larger
number of larvae in a given time.

After transportation to a laboratory at the
University of Michigan Biological Station,
the animals were maintained in individual
stender dishes, mostly without aeration, at
room temperature ( 20-25 °C). Plant mate-
rial was present in all containers and the
water was changed every 2 or 3 days.

Enchytraeid worms were provided daily
as food until the animals closed their cases
for pupation. At that time the cases were
transferred to clean stender dishes until the
larval sclerites were extruded by the pupa.
The sclerites were then placed in labelled
vials and the pupal cases transferred to rear-
ing cages for capture of adults upon emer-
gence. Some of the insects were preserved
in the larval and pupal stages to provide
adequate material for morphological study.

Larval behaviour was observed daily, and
some aspects were recorded on motion pic-
ture film, using Kodak Double-X negative
film in a Bolex H-16 Rex 3 camera. With
the aid of extension tubes on a Switar 25 mm
lens, magnifications of 1 to 2.4 were ob-
tained. The illustrations of larval behaviour
(Fig. 15) were printed from this film.

Specimens are deposited in the collections
of the Department of Entomology and In-
vertebrate Zoology, Royal Ontario Museum.

Setodes incerta (Walker)

Leptocerus incertus Walker 1852, p. 71.

Setodes incerta McLachlan 1863, p. 158,

163.

Setodes vernalis Betten 1934, p. 277, pi. 36,

figs. 7-9.

Setodes incerta Betten and Mosely 1940,

p. 69, fig. 33.

Setodes incerta Ross 1944, p. 302, figs. 737

(S. vernalis), 872.

larva — (Figs. 1, 2, 8-11). Full-grown
larva 5-6 mm in length, uniformly whitish
in overall colour, sclerotized portions pale
yellow, affording little contrast in colour
with the unsclerotized portions. Head with-
out contrasting colour pattern, and with
several structures typical for the Mystacidini:
gular sclerite rectangular, extending posterad
to occipital foramen; antennae long and
stout; labrum with only primary setae; man-
dibles with teeth arranged around edge of
a central cavity, but with a patch of stout
spines in this central area on left mandible;
maxillary palpi not projecting conspicuously
beyond anterior edge of labrum. Thorax as
illustrated; trochantin of fore legs rather
truncate anteriorly, as in Mystacides; hind
legs without dense row of swimming hairs,
tibia of hind legs with constriction and
secondary suture, apparently dividing it into
two equal parts as in Mystacides and
Triaenodes; metasternum with transverse
row of about 17 dark setae, terminating on
each side in a patch of about a dozen setae.
Abdomen with pair of single posteroventral
gills on segments n to vn inclusive; first ab-
dominal segment with lateral humps rough-
ened with tiny spines, and bearing a short
sclerotized bar, line of 6-8 dark setae be-
tween lateral and dorsal humps, sternum with
curved line of about a dozen dark setae; seg-
ment vni with row of about 30 small sclero-
tized points along each side; lateral line of
very short setae on segments in to vn inclu-
sive; segment ix with sclerotized, but lightly
pigmented, plate bearing 4-5 pairs of moder-
ate to stout setae along posterior margin; cau-
dal face of anal prolegs with sclerotized con-
cave plate on each side of anal opening, each I
plate extending into a median ventral lobe
and an ovoid lobe arising near base of anal
claw, all plates armed around margins with

Figures 1-3, Setodes incerta, larva
1. Head and thorax, dorsal view. 2a. Larva, lateral view. 2b. Right metathoracic leg, lateral view. 2c.
Trochantin and base of prothoracic leg, lateral view. 2d. Lateral hump of first abdominal segment, lateral
view. 2e. Sclerotized plates on base of anal prolegs, caudal view. 2f. Claw of anal proleg. 3a. Larval case.
3b. Posterior opening of larval case.

short spines; anal claws each with two small
dorsal accessory hooks.

Case (Fig. 3) of rock fragments, slightly
curved, but with little taper; posterior end
open, diameter of opening not reduced with
silk or rocks; length, for full-grown larvae,
6-7 mm, scarcely longer than larva itself.

pupa — (Figs. 4, 5, 6). Length 6 mm. Head
unlike that in Mystacides and Triaenodes in
having prominent tubercle arising between
bases of antennae, and with two additional
tubercles on each side between base of an-
tenna and mandible; labrum with patches of
stout setae and pair of thorny protuber-
ances; mandibles well sclerotized, much as
in Mystacides (Yamamoto and Wiggins,
1964, fig. 10), blade fairly straight, with
single row of minute teeth extending full
length, blade not curved as in Triaenodes.
Abdominal sclerites different from Mysta-
cides and Triaenodes in that one additional
segment, vn, bears a pair of hooked plates,
and plate 5P is narrower and bears only
three hooks, in contrast to 6-10 in the other
genera; anal appendages with prominent
dorsal and ventral spiny lobes, roughly mid-
way between origin and apex, seen to best
advantage in lateral view; proximal portion
before these lobes longer in male than in
female.

Case closed at both ends with rock frag-
ments and silk, leaving a round central hole,
lip of the hole somewhat raised (Fig. 7);
length 5-7 mm.

adults — The diagnostic features of the fe-
male genitalia of 5. incerta are illustrated
here for the first time (Fig. 14). They show
a clear generic affinity in structure to those
of the other three North American species
for which the female is known: S. stehri
(Ross) (Ross, 1941, fig. 80); 5. oligia
(Ross) (Ross, 1944, fig. 871); S. epicampes
Edwards (Edwards, 1956, figs. 4, 5). Com-
parison of the diagnostic details in these
figures indicates that the female of S. incerta
is distinguished by having segment x trun-
cate posteriorly in lateral view. Segment x
of S. incerta also bears a deep median notch,
and is subtended at the base on segment ix
by a pair of raised ridges. These characters
are not, however, illustrated for the other
species. A noteworthy structural detail of

S. incerta is the pair of sclerotized, invagi-
nated pockets in the sternum of segment
vin, presumably to receive the ends of the
long claspers of the male during copulation.
Similar sclerotized pockets are shown for
S. stehri (Ross, 1941, fig. 80) but it is not
clear whether or not these structures occur
in the other two species.

The diagnostic characteristics of the male
genitalia in 5. incerta were described and
illustrated previously (Betten and Mosely,
1940, p. 69, fig. 33; Ross, 1944, fig. 872),
but are included here (Fig. 13) to make
the present paper a more useful reference
for identification of S. incerta in its larval,
pupal, and adult stages.

MATERIAL EXAMINED MICHIGAN, OcqueOC

River, Presque Isle Co., 20 June 1969, 12
larvae; 1 July 1969, many larvae, reared
and preserved as pupae and adults; all col-
lected by D. Merrill.

Setodes sp.

larva — Larvae of this genus were also col-
lected from the Chattooga River in South
Carolina. These larvae were burrowing in
accumulations of sand lying in the lee of
large rocks in the riffle areas of the river.
Depth of water was about 1 foot. The col-
lection comprises two types of larvae: one
having a light yellowish head with no dis-
tinguishing colour pattern, as in S. incerta;
the other having a dark brown head, with
some indistinct lighter patches and some
darker patches on the pronotum (Fig. 12).
No other distinctions among these two larval
types and the larva of S. incerta from Michi-
gan were found. All have similar sclerotized
plates on the anal prolegs, and a subdivided
hind tibia. The light-coloured larvae may be
conspecific with those of 5. incerta, but the
dark-headed larvae very likely belong to a
different species.

The larval cases of both types of South
Carolina specimens are also of sand grains,
but many of these are black and have a
flattened flaky texture. The posterior opening
is the full diameter of the case as in 5.
incerta.

MATERIAL EXAMINED — SOUTH CAROLINA

Chattooga River at Burrell's Ford Camp-
ground, near Route 107, Oconee Co., 18-19

Figures 4-11, Setodes incerta

4a. Head of pupa, dorsal view. 4b. Head of pupa, lateral view. 5a. Mandibles of pupa, dorsal view. 5b.
Labrum of pupa, dorsal view. 6a. Abdomen of pupa with sclerites enlarged, dorsal view. 6b. Anal appen-
dage of pupa, dorsal view. 6c. Anal appendage of pupa, mesal view. 7. Pupal case, anterior opening.
8. Labrum of larva, dorsal view. 9. Head of larva, ventral view. 10. Labium and maxillae of larva, ven-
tral view. 11. Mandibles of larva, dorsal view.

Figure 12, Setodes sp.

Head and thorax of larva, dorsal view.

May 1970, many larvae, G. B. Wiggins and
T. Yamamoto.

Cases and Case-building — The slightly
curved cylindrical cases of the larvae were
constructed of fine, flattened sand grains
closely fitted together. The size of the grains
was variable, but rarely exceeded 0.5 mm
in their largest dimension. The majority of
the grains measured 0.3 mm or less. The
outside diameters of the larval cases aver-
aged 1 mm, and the lengths, at the time of
collection, ranged from 3 to 7 mm. Pupal
cases ranged in length from 5 to 7 mm, and
were just sufficient to enclose the insect.

Five larvae were evicted from their cases
and were placed in stender dishes with sand,
filamentous algae, and Utricularia. As the
naked larvae crawled on the sand, they
applied silk to the grains, and thus formed
masses of sand grains loosely bound to-

gether. Within a short time, each larva bur-
rowed into the sand, making further obser-
vations impossible. By the following day
each larva had a new case, the definitive
portion of which was 3 to 5 mm in length.
Two of the cases had a few millimeters of
loosely organized sand at the posterior end.

One can infer that as the larvae burrowed
they continued to apply silk to the grains
around them, forming a rough provisional
case in this manner. As this tube was
lengthened to encompass more of the body,
the building pattern progressed to greater
selectivity of grains and more careful attach-
ment, ultimately giving rise to the definitive
case, which was essentially identical to the
larva's original case. The progression from
provisional to definitive building has been
described elsewhere for several species of
caddis larvae (Bierens de Haan, 1922;
Gorter, 1929; Fankhauser and Reik, 1935;
Copeland and Crowell, 1937; Neilsen, 1942,
1948).

On only one occasion was a larva ob-
served in the process of adding a sand grain
to the rim of its case. A high degree of selec-
tivity on the part of the larva was indicated
by the large number of sand grains that were
picked up, tested in several positions against
the rim, and finally rejected before a suitable
grain was found.

Burrowing — The five larvae that had built
new cases continued to burrow in the sand
even after their case-building activity had
apparently ceased. When five additional lar-
vae were provided with sand in their con-
tainers, they too proceeded to burrow with
their cases. Daily observations on the 10
larvae indicated a strong tendency to posi-
tion themselves with one end of the case
buried in the sand and the other end nearly
flush with the surface (Fig. 15a). This posi-
tion was seen in 88 out of 112 observations.
In a majority of the observations, the case
position differed from one day to the next,
indicating some degree of mobility even
under these unnatural conditions.

Occasionally at the time of an observa-
tion, a larva had apparently reversed its
position in the case and was beginning tc
burrow, its posterior end clearly visible

through the opening of the case at the sur-
face of the sand substrate. During some of
these observations it was possible to insert a
worm or another small object through the
opening. Invariably the larva moved back-
ward rapidly and ejected the intruding ob-
ject.

It is possible that the open posterior end
of the case is associated with the animal's
burrowing habits, permitting reversal of
larval direction without the necessity of re-
versing the case. The unusual sclerotization
at the larva's posterior end appears to com-
pensate for the vulnerability resulting from
the open-ended case.

Adaptations to Current — Because of its ob-
vious protective advantage, burrowing may
also be an adaptation for survival in rapid
currents. Another behaviour pattern ob-
served in larvae of S. incerta that may be an
adaptation to the habitat was a tendency to
fasten both ends of the case to the substrate
when they were not actively moving about.
This was particularly noted among larvae in
containers without sand. Larvae of many
species fasten the anterior ends of their
cases to the substrate when at rest (Lestage,
1921), but attachment of both ends gener-
ally indicates the approach of pupation. In
5. incerta, attachment at both ends appears

14c

Figures 13-14, Setodes incerta

13a. Male genitalia, lateral view. 13b. Male genitalia, dorsal view. 13c. Male genitalia, ventral view. 14a.
Female genitalia (abdominal segments in Roman numerals) lateral view. 14b. Female genitalia, dorsal
view. 14c. Female genitalia, ventral view.

to provide additional protection against
being swept away by the current. This be-
haviour by the larvae of Setodes probably is
unusual because most other caddis larvae
inhabiting streams and building portable
cases cannot extend the head from either end
of the case.

Camouflage Behaviour — Frequently a larva
was observed holding a large grain of sand
(hereafter called a rock) over the mouth of
the case. Sometimes the rock was loosely
attached to the rim of the case and was
seized by the larva when there was a change
in the light pattern or a disturbance in the
water. This practice appeared to provide ex-
cellent camouflage as long as the animal re-
mained motionless. If a worm was placed
near the larva under these circumstances, the
larva continued to hold the rock, sometimes
manipulating it, until the worm was within
reach. Then it released the rock and seized
the worm (Fig. 15 c, d, e).

This behaviour was seen in 10 out of 12
larvae observed daily during July 1970. In
80 observations, 42 were made under condi-
tions where camouflage behaviour might
occur, i.e. one end of the case flush with the
surface and the animal facing upward.
Camouflage behaviour actually occurred
during 24 of these observations. In eight
others, the larva was manipulating a rock
but did not hold it still.

It is possible that this behaviour is derived
from the manipulation of sand grains for
purposes of case-building, but the large size
of the grain and its loose attachment to the
rim of the case suggest that it is a special
adaptation of the more primitive behaviour
pattern.

Feeding — Filamentous algae (Spirogyra)
from the collecting site were provided initi-
ally until the supply was depleted. The prin-
cipal food provided for the larvae was en-
chytraeid worms. The larvae seized and ate
the worms without hesitation, even when
the worm was nearly as large as the larva
and writhed violently in the larva's grasp.

Usually while the animal was feeding, its
respiratory current was reversed, propelled
anteriorly instead of posteriorly. This phe-
nomenon has been observed in several spe-

cies of caddis larvae, and is currently under
investigation by one of us (D.M.).

No attempt was made to examine stomach
contents of freshly caught larvae, although
fecal pellets in the containers were collected
and examined the first morning after the
capture of the larvae. Most of the contents
were filamentous algae and other plant
material, but several minute head capsules of
insects were also found. Thus, it seems that
these larvae are omnivorous. The avidity
with which they seized and ate the worms
leaves little doubt that animal food is an im-
portant component of their diet.

Cannibalism sometimes occurred when
larvae were kept in the same container. Ex-
periments on a small number of larvae indi-
cated that this tendency can be suppressed
by provision of an adequate food supply.

General Conclusions — 1. There seems to be
general agreement in the morphological de-
tails of larvae and pupae among the North
American, European, and Asian species of
Setodes for which these stages are known. In
the larvae differences exist at the specific
level in the colour pattern of the head and
thorax, the shape of the sclerotized plates of
the anal prolegs, and the presence or absence
of abdominal gills.

2. Ulmer (1955), in discussing the inter-
relationships of the genera comprising the
Mystacidini, assigned Setodes to a group in
which the posterior tibia of the larva is not
secondarily divided, and in which the dark,
sclerotized ridge on the lateral hump of the
first abdominal segment is absent. No infor-
mation on North American larvae of Setodes
was available to Ulmer. It is noteworthy,
then, that with this information now avail-
able, Setodes emerges as a genus comprising
species with a clearly divided hind tibia (Fig.
2b), and a lateral sclerite on the first ab-
dominal segment (Fig. 2d), along with spe-
cies evidently lacking these features. There
appears to be no mention in the recent litera-
ture of a subdivided hind tibia in the Euro-
pean larvae of Setodes, although a clear con-
striction in the tibia is shown in an illustra-
tion of Setodes sp. by Murgoci (1959, fig.
5c). Whatever may be the functional signifi-
cance of this character, it seems that the sub-
divided hind tibia of the larva can no longer

be regarded as a consistent generic charac-
ter in the way that Ulmer did. Another pos-
sibility is that further study of all stages of
the species now assigned to Setodes will
show consistent groupings, and may ulti-
mately require recognition of an additional
:nus. This discordance may, however, be
another example of the residue of characters
in the Leptoceridae which apparently cannot
be completely aligned with a generic classifi-
cation. The diverse shapes taken by the gu-
lar sclerite in species of Athripsodes appears
to be one such set of characters.

3. Most of the information now available
indicates that Setodes larvae inhabit cool
running waters, although S. argentipunctella

is reported to inhabit lakes as well (Hickin,
1967). Our observations indicate that these
larvae are strongly inclined to burrow in the
sand of river bottoms, but thus far there is
little to support this view from the European
observations. Murgoci (1959) assigned lar-
vae of Setodes sp. collected in rivers in Ro-
mania to "la biocenose petricole du biotype
lotique," where they were evidently taken in
company with larvae of Psychomyia, Hy-
dropsyche, Lepidostoma, Brachycentrus,
and Sericostoma. This same author regarded
the concave sclerotized plates of the anal
prolegs as an adhesive organ by which the
larva maintains its case on rocks in the cur-
rent, aided by secretion from a pair of glands

j^. , §w. m £OK« wmmmmmJKKM

Figure 15, Setodes incerta larva and case; case diameter is 1 mm. 15a. Partially buried in sand, anterior
end uppermost. 15b. Posterior end of case showing wide opening and plates on bases of anal prolegs.
15c, d, e. Camouflage behaviour of larva in case; c = case; r = rock; arrow indicates head of larva
protruding from case. 15c. Rock held over mouth of case. 15d. Rock manipulated to a different position.
15e. Rock cast aside, larva feeding on worm.

situated near the ventral edge of the sclero-
tized plates. Evidence supporting this inter-
pretation is, however, lacking.

Our observations prompt us to offer a
different interpretation of the function of
these sclerotized plates of the anal prolegs,
which are so remarkable among case-build-
ing caddis larvae. Larvae of Setodes have
a second unusual feature — a behavioural
pattern of case-making that produces a case
with little taper from end to end, with the
openings at both ends unrestricted with silk
or sand grains, and equal to the full inside
diameter of the case (Figs. 3, 15b; Murgoci,
1959, fig. 3c). Although there appears to be
some diversity in the extent to which the
cases of different species of Setodes taper
posteriorly, the larval case in species such
as 5. incerta contrasts strikingly with the
larval cases of most Trichoptera that build
portable cases. In other genera the case has
a definite taper from front to rear, and the
posterior opening is usually further reduced
with silken secretion and other materials.

The intepretation offered here is that the
sclerotized plates and the case with two
open ends are components of an integrated
morphological-behavioural-character com-
plex enabling the larva to extend its head,
thorax and legs from either end. The sclero-
tized plates on the caudal face of the anal
prolegs protect the larva against intrusions
through whichever opening harbours its pos-
terior end. Our observations demonstrate
that the larva does actively repel, with these
plates, intrusions through the distal end of
its case. Evidence indicates that Setodes lar-
vae do not always burrow in the beds of the
rivers they inhabit, but that they also do
occur on the rocks of the river bed. This
character complex seems to serve the larvae
well in both habitats by permitting them: to

burrow in the bottom, advancing from either
end; to burrow to the depth of the case, and
take up a feeding position with the head
level with the river bed; or to fasten both
ends of the case to a rock for added stability
while feeding in strong currents. This inter-
pretation is consistent with the evidence now
available, and is offered as a conceptual basis
for further observations on the biology of
these remarkable insects.

Acknowledgments — We are indebted to Dr.
L. Botosaneanu of Bucharest for comments
on the manuscript. All line drawings are the
work of Mr. Anker Odum, scientific illustra-
tor in the Department of Entomology and
Invertebrate Zoology, Royal Ontario
Museum.

Summary — This paper provides morpholog-
ical details for the larva and pupa of Setodes
incerta (Walker) from Michigan, the first
North American species in this genus for
which this information is available. The male
and female genitalia of this species are also
illustrated, the latter for the first time. The
larva of another species of Setodes from
South Carolina, unidentifiable to species, is
also briefly described and illustrated. Larvae
of these two North American species are
shown to be discordant with European lar-
vae of Setodes in having a secondary division
in the hind tibia, a character previously con-
sidered to be generic.

Observations on aspects of the behaviour
of the larva of S. incerta are presented: case-
building, burrowing, adaptations to current,
camouflage behaviour, and feeding. Arising
from these is an overall interpretation of the
function of the unusual morphological and
behavioural characteristics of larvae of the
genus Setodes.

10

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