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A physical st of the firefly,

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A PHYSICAL STQDYQE-PHE FIREFLY

Se

‘BY

_ WILLIAM W. COBLENTZ

WASHINGTON, D. ¢.

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

1912

A PHYSICAL STUDY OF THE FIREFLY

BY

WILLIAM W. COBLENTZ

WASHINGTON, D. C.

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

1912

CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 164

Copies of th: Book

ware first isstse

APRS 1912

PRESS OF GIBSON BROS.
WASHINGTON, D. Cc.

A PHYSICAL STUDY OF THE FIREFLY.

CONTENTS.

Page. Page.

I. Introduction............ 00... cece eee e ee eee 3 VII. Spectral Energy Curves of the Light
II. Habits of the Various Species Investigated... 5 emitted by Various Species of Fireflies.. 18

ILL, Composition of the Light of Fireflies........ 6 ; VIII. Luminous Efficiency and Candle-power
IV. The Function of the Light emitted by Living Measurements. ....... 06.0. cc sees eae eees 26
Organisms s cixisisc'son selector aa ensinee 9 IX. Radiationand Temperature Measurements. 29
V. Histology of the Light Organs.............. 10 X. The Fluorescent Substance in Fireflies.... 37
VI. Methods of Investigation of the Light XI. Infra-red Absorption Measurements....... 40
GMb O Deca iss dome csscetessemisind aessunciancnannians 14 XII. Nature of the Light emitted.............. 43

I, INTRODUCTION.

During the latter part of the summer of 1908 the first attempt was made
to measure the radiation from the local species of firefly. The season was
late and but little was accomplished beyond the acquisition of some knowl-
edge of the habits of these interesting insects. The following summer, in
collaboration with Dr. H. E. Ives,* the spectral energy distribution of the
light from the species Photinus pyralis was obtained by photographic pho-
tometry. Last year an attempt was made to obtain photographically the
spectral energy distribution of the species Photuris pennsylvanica, which
comes earlier in the year. As mentioned elsewhere,} the work was begun
too late in the season, and in the course of only a few days this species had
disappeared for the year. However, in spite of the lack of sensitiveness of
the photographic plates in the red, there was considerable evidence that
thelight fromthe Photuris, which to the eye appears a greenish blue, is quite
monochromatic, being deficient in the yellow and red rays which are promi-
nent in the light of the Photinus. Whether the maximum emission is very
different in these two genera remained undetermined until this year. ‘The
work requires an intimate knowledge of the habits of the various species,
the lack of which has been the cause of most of the delays in reaching some
conclusion in regard to the various questions under discussion.

One of the foremost problems which now occupies the attention of investi-
gators is the improvement of our methods of illumination. It is recognized
that our present methods of light production are of the most primitive type,
and yet we seem helpless in the matter. Recognizing our helplessness, we
should not hesitate to turn to other animals having photogenic systems
which apparently are far more efficient than our own, and try to learn their
methods of operation. It may be that in the ultimate analysis the efficiency

*Bull. Bur. Standards, vol. 6, p. 321, 1909.
fElectrical World, 56, p. 1012, Oct. 27, 1910.

4 A PHYSICAL STUDY OF THE FIREFLY.

of their lighting systems is no higher than our own (but their fuel supply is
evidently different from ours) and to learn that much of theirsecret may be
of value.

Aside from this utilitarian question, there are others of equal importance.
For example, we know that closely related species of fireflies emit differently
colored light, variously described as blue, green, orange, etc. Is this color-
variation a subjective phenomenon, resulting from the variation in color
sensibility of the eye with variation in intensity,* or is it an objective
reality? If the latter, then what are the laws governing the light emission?
It is almost too much to hope to solve this problem in the near future, but
it seems possible to settle definitely the question whether or not the light
differs in composition and hence is “blue,’”’ ‘“‘green,” or ‘orange,’ as
observed. ‘This knowledge is obtained from the spectral energy curves of
the light from various fireflies, to be described presently.

Experimenters have become so accustomed to thinking of the artificial
production of light as being accompanied by a large amount of invisible
radiation that no experiments would be complete without a repetition of
Langley’s search for infra-red radiation. Accordingly this question has
also been given attention in this paper.

In view of the doubt in many investigators’ minds as to whether the color
of the light from the firefly is due to a variation in intensity rather than to
anactual variation in composition, it seemed worth while to attempt to settle
the question by making a thorough study of the light emitted by various
species of this insect. Accordingly, the photographic work was continued
for about four weeks, during which time 152 photographic exposures were
made, on two spectrometers differing widely in dispersion, the time of
exposure varying from 30 seconds to 5 hours. The time consumed in actual
photography, in holding the insects on the spectrometer slit, was over 56
hours.

The histological data is introduced for clearness in discussing the present
work, as well as its bearing upon the whole subject. It permits also a
discussion of the plausibility of the functions assigned to certain parts of
the photogenic organs as viewed from a physical standpoint and more
especially from the optical properties of materials.

While much has already been done on the light of the firefly, much more
remains undone, all of which no doubt will have direct bearing on the manner
of light production in animals. Among the important problems awaiting
solution are (1) whether, in animals having luminous organs emitting light
of different colors, the maxima of emission are different; and (2) whether
the maximum emission in any one photogenic cell is the same when the
light is emitted from the living animal and when the photogenic material is
excited to light emission by an oxidizing agent after the luminous organs
have been dried.

Heretofore we have been led to think that “light”? must be accompanied
by infra-red radiations; and we seem utterly unable to turn away from this
idea. The various investigations of the firefly have without doubt opened
up a new line of thought in regard to the emission of heat and light.

*Knab., Canadian Entomologist, 37, p. 238, 1905. Molisch, Leuchtende Faancen,
Jena, 1904.

HABITS OF THE VARIOUS SPECIES INVESTIGATED. 5

I]. HABITS OF THE VARIOUS SPECIES INVESTIGATED.

At first thought it may seem aside from the main problem to discuss such
details, but successful work can be done only after learning the habits of
these insects. Not being familiar with the subject at the start, the results
given here are the outgrowth of four summers of personal experiences in the
field, and it is hoped that they may be useful to others who may be led to
undertake similar work. A recent paper by McDermott* is in practical
agreement with the experiences herein described.

The insects were caught by means of a net made of white cheesecloth
attached to a light wooden handle about1.5 meters long. The whiteclothis
easily seen and avoided by theinsects, butitis more easily handled in the dark.

The habits of the Photuris pennsyluanica are markedly different from the
various species of Photinus. he Photuris inhabits damp ground along’
wooded brooks and rivers, and in the daytime it seems to rest in the trees
as well as in the grass, from which it does not emerge untilafter it has become
quite dark. It flies high, at great speed, gives a quick flash, and the next
moment may be repeating the flash 10 to 20 meters away. ‘The color of
the light is a rich bluish-green. Because of the darkness, it usually is
impossible to see these insects except when they flash. At times, however,
when in flight, a faint glow is visible which is easily followed with the net.
Late at night they become more quiet and often may be found, flashing, on
grass stems and on golden-rod. When struck to the ground, they run about
flashing violently (just the opposite from the Photinus) and hence are easily
located. Late in the season they seem to rest, without flashing, among the
underbrush. Many specimens were caught by beating the brush and taking
advantage of their flashing when disturbed and on the ground.

One compensation for the difficulties in catching the Photuris was in find-
‘ng that they are strong and hardy, not being easily crushed; and that they
could be kept for days in large glass beakers (12 X18 cm.) containing clean
moist sod, the top being covered with cheesecloth. ‘They are carnivorous,
2zating their dead comrades. In captivity they kill and eat the Photinus.
The Photinus pyralis are not so easily kept, but are more plentiful and more
2asily obtained. In the previous work on the latter, they were caught and
che photographs made the same night. In the present work, half the night
was spent in catching the Photuris, and usually the photographing of the
ight was done in a dark room on the following day.

In the Photuris the light organs of the female are almost as large as those
xf the male. The females outnumber the males by about 15 to 1, so that
me catches but few of the latter.

The Photinus pyralis is plentiful everywhere in this locality. It comes
nut at dusk, when it can be seen and easily caught. It flies low, hovers
ibout the grass, apparently searching for the female. ‘This species is more
lelicate than the Photuris, is easily injured, and is not so easily kept in
captivity. The light organ in the female is very small, and from her habit of
‘emaining in the grass, occasionally flashing, and from the fact that the males
yutnumber the females by about 15 to 1, the captured specimens of this species
vere mostly males. The flash is a long fulmination of yellowish light.

*McDermott, Canadian Entomologist, Nov. 1910.

6 A PHYSICAL STUDY OI THE FIREFLY.

The Photinus scintillans and the Photinus consanguineus were found
together among shrubbery, in small isolated spots of an acre or less in area.
The scintillans is only about 3 to 5 mm. long and is therefore very difficult
to handle. On the spectrometer slit it would flash but a few times, when
fortunately it would emit a rich glow. A female scintillans was caught while
flying about during the daytime. As in the female pyralis, the light organ
occupies one-third of the ventral area of the third abdominal segment. Both
male and female have the appearance of diminutive pyralis, the color of the
wings being more grayish.

The consanguineus is somewhat larger than the scintillans andcan bemade
to flash for a longer time before it begins to glow. Both of these species
become active early in the evening, but cease their flashing some time before
dark. ‘The flash is a beautiful orange-red, which is emitted at long intervals.
The fewness of these two species and the infrequency of the flash made it
difficult to obtain specimens in sufficient number for the requirements of the
present work. In work like the present it is necessary to have alarge number
of insects to last three hours. Only once was work undertaken (this on the
small spectrograph to be described subsequently) with less than a dozen
insects, and of this number only two were very active.

III. COMPOSITION OF THE LIGHT OF FIREFLIES.

The question of the composition or “color”’ of the light emitted by vari-
ous species of fireflies is of considerable interest, and may prove of great
importance in the theory of radiation. ‘The color of the light may vary
according to the species that produces it and from different parts of the body
the same animal may emit
different-colored light. For
example, Photinus pyralis
has two small light organs,
L’ L’, Fig. 1, on the last ab-
dominal segment which emit
light of a decidedly more
greenish color than the light
emanating from the rest of
the luminous organs.

It would be interesting to
know whether the maximum
emission is different in the
two luminous organs or
whether this greenish color

Fic. 1.—Photinus pyralis. in L’ L’ is simply a question
A=male. B=female. L and L’=luminous organs. of intensity. Inthe P hotinus

pyralisit appears to bealow-
intensity effect. Whether there is an actual variation in the composition of

the light emitted by any one luminous organ is of considerable interest, as
this would show that not only the act of emitting the light, but also the com-
position of the light, is under control of the insect. Whether there is such

a variation in the composition, subject to the control of the insect, would
be difficult to establish. =

COMPOSITION OF THE LIGHT OF FIREFLIES. 7

It is evident, from visual observations, that the range of variation in color
of the light of any one species, whether or not it is under control, must be
very limited in extent. Occasionally, however, one finds specimens of a
given species in which the light seems to differ from the accustomed color.
This is especially true of the Photinus consanguineus and scintillans. For
example, while photographing the light from Photinus consanguineus a speci-
men was found in which the flash appeared a deeper reddish-yellow than
that usually observed. To the eye it appeared as though the photogenic
material extended into the adjacent dark segment, and as though the color
of the light emanating from the line of intersection of the second luminous
segment with the adjoining dark segment was of a more reddish tinge than
the light coming from the other parts of the luminous segments. This was
probably caused by absorption of the yellow and blue light in passing
through the slightly brownish integument joining the segments. The exten-
sion of the luminous organs under the dark segments would explain the red-
dish light sometimes observed in insects. Several other specimens were
observed in which, while photographing, the light changed from an orange
to a decidedly reddish-yellow hue. ‘The illumination was so weak that it
was extremely difficult to keep the insects in place on the spectrometer slit,
and it is therefore evident that this change in color was not due to the physi-
ological effect of low intensity on the retina; for in that case the light should
have appeared greenish.

As so much importance has been attached to the physiological effect of
lights of different intensities upon the retina, notes were made of the appear-
ance of the light emitted by various species. After sitting in a dark room
from two to four hours, in which fireflies of all degress of activity were
handled, it was an easy matter to study this effect. The observations
extend over several weeks and are in complete agreement. They show that
for the same genus at low intensities the light is more bluish than at high
intensities, which is the physiological phenomenon. Examined side by side,
after beingin darkness for three hours, the glow of Photinus pyralis appeared
more yellow than that of Photuris pennsylvanica, and the flash of the latter
appeared bluer than the glow of the former. This evidently can not be
explained on the basis of physiological optics. The glow from Photinus
consanguineus was at all times greenish to yellowish, while the flash was a
reddish-yellow. This evidently is the physiological effect of variation in
intensity. However, when we compare the latter with Photinus pyralis
and find that its light is reddish-yellow in spite of the fact that it is barely
sufficient to illuminate the spectrometer slit, while the light of Photinus
pyralis is greenish-yellow and is of sufficient intensity to read the time on a
small watch dial, it is evident that there is an actual difference in the light
emitted by the various genera and species of fireflies.

After one has done field work for several summers, has handled hundreds
of these insects, and has observed the viciousness with which the Pho-
turis pennsylvanica, in captivity, kill and devour the smaller species, e. g.,
pyralis and scintillans, one concludes that, if for no other purpose, the
difference in the color of the light is useful as a distinguishing mark of the
different species.

8 A PHYSICAL STUDY OF THE FIREFLY.

In photogenic bacteria the color of the light* appears to depend upon the
environment and the culture medium; and in any one medium the light is
described as silvery white, bluish, greenish, or tinged with orange, depending
upon the species. Among the mushrooms there are similar variations, one
species emitting a bluish, another a greenish, and still others a whitish light.

In addition to the fireflies investigated, through the kindness of Mr. F.
A. McDermott (then connected with the Hygienic Laboratory of the U.S.
Marine Hospital Service), an opportunity was granted to examine a large
glow-worm (female of Phengodes laticollis) which along its sides emitted
(in spots of about 1 mm. diameter) a rich greenish light; also an Elater
(Pyrophorus noctilucus) from Cuba, of which the eye-spots (1 mm. diam-
eter) showed a weak greenish glow; and a culture of photogenic bacteria
(Pseudomonas lucifera Molisch), which appeared a rich green in color though
weak in intensity. Because of the weakness of the light, which would have
required an exposure of eight to ten hours, using the large spectrograph,
no attempts were made to photograph the light of these animals. From
the data to be discussed presently, it will be possible to give an estimate of
the quality of the light emitted by these animals without subjecting it to
rigid examination.

Some insects are described as emitting a ‘“‘red light.”’ If this is the ‘ car-
dinal”’ or the “‘scarlet’’ one usually thinks of when reading such a descrip-
tion, the color must be very remarkable and deserves investigation. First
of all, it would be desirable to know whether the description is correct and
whether the “‘red”” may not in reality be the ‘‘orange red’”’ observed in
Photinus scintillans.

Tests have been madet to decide on the color of the light emitted by
fireflies by examining them in the light ofa kerosene or anincandescent lamp.
‘The writer has tried the same experiment and concluded that such observa-
tions are meaningless, for the reason that the light observed is composed of
that emitted by the firefly and of that emitted by the lamp, which, by
internal reflection in the photogenic cells, is returned to the eye. The
resultant color of the combination can hardly be considered a criterion for
judging the color of the (unmixed) light emitted by the insect.

The popular notion is that the maximum emission of the sun and the
maximum color sensitivity of the eye coincide; and that this is the result of
evolution of the eye so as to utilize the sunlight to the best advantage. In
the same manner comments have been made on the supposed coincidence of
the luminosity curve of the eye and the luminosity of fireflies. In the
attempt to harmonize all phenomena, the fact is overlooked that the struc-
ture of the (compound) eye of an insect is entirely different from the human
eye. It remains to be shown that the insect eye is sensitive to color with a
maximum of sensibility such as obtains in the human eye.

*Molisch, Leuchtende Pflanzen, Jena, 1904.
}Knab, Canadian Entomologist, 37, p. 238, 1905.

THE FUNCTION OF THE LIGHT EMITTED BY LIVING ORGANISMS. 9

IV. THE FUNCTION OF THE LIGHT EMITTED BY LIVING
ORGANISMS.

Various explanations have been offered to account for the presence of the
light emitted by living organisms. For example, the light from fungi would
serve as an illuminant to worms and insects.*

The light emitted by insects, or fishes, would serve as a bait to attract

( prey: to illuminate the way; to blind its prey and thus prevent its escape;
to blind its enemies and thus serve as a defense; to serve as an adornment in
more highly organized forms; or, most plausible of all, to serve as signals
between male and female of the same species. {

- The light of the firefly is evidently intended as a means of attracting one
another. For example, during the work of photographing the light of the
firefly, on at least three different occasions fireflies, which had escaped into
the room and were flying about flashing, came to where an insect was being
held upon the spectrometer slit, and were caught. Further evidence was
accidentally found in the field work. The insects were caught and placed
in test tubes. Later they were transferred to a large wide-mouthed bottle
covered with cheesecloth. Leaving such a bottle in the field occasionally
attracted Photuris pennsylvanica. On returning to the laboratory, carrying
the bottle of insects, on two occasions Photuris alighted on the writer’s
shoulder and were caught. In view of the difficulties usually encountered
in catching this species, these experiences can hardly be considered acci-
dental. The habit of the female Photinus pyralis remaining in the grass,
occasionally flashing, and of the male pyralis hovering about the ground,
rising up and down and flashing, is further evidence that the light is useful
as an attraction for these insects, where in others the same end is accom-
plished by the emission of sounds. This habit of rising a few feet above the
ground, remaining stationary and fluttering the wings, is to be observed also
in the large yellow-winged grasshoppers.

To the writer most of these explanations seem far-fetched. I'or example,
if the Photuris desired to hide when molested, why does it run about the
ground flashing violently, instead of dropping to the ground on the slightest
provocation and disappearing without a flash, which is the habit of the
various species of Photinus. Again, one species of firefly in the larval state
has photogenic organs, but in the adult state has none. In some localities
the mating of the glow-worm (Phengodes) is said to occur in the daytime;
hence the light emitted can not be of great service in mating for that
purpose.

*Piitter, Leuchtende Organismen, Zeitschr. f. Allg. Physiol., 5, 1905.
TMangold, ‘Die Produktion von Licht,” Winterstein’s Handb. der Vergleich. Physi-
ologie, Band 3, 1911.

10 A PHYSICAL STUDY OF THE FIREFLY.

V. HISTOLOGY OF THE LIGHT ORGANS.

For clearness in understanding and ease in describing the thermal and
other measurements given on a subsequent page, the general topography of
the ventral side of a firefly (Photinus pyralis) is given in Fig. 1. The dark
areas, L, represent the regions of the luminous organs. It will be noticed
that in the male the luminous region includes the whole of two seg-
ments (the second and third from the end), while in the female only
about a third of the third segment is luminous. In Photinus pyralis there
are two additional points of light, L’L’, which are of a more greenish hue
than the light emitted from the rest of the body. These points of light are
of interest in connection with the temperature measurements to be discussed
presently. In the male Photuris pennsylvanica the luminous organs are
situated as in Fig. 1. In the female the dark area, illustrated in Fig. 1, is
expanded so that it occupies about three-fourths of each of the two segments
(the second and third from the end), Hence the female is almost as lumi-
nous as the male.

In Fig. 2 is given a transverse section through the entire abdomen of the
firefly Photinus mar-
ginellus, studied by
Townsend,* and in
Fig. 3 is given a de-
tailed view of the
general structure of
the luminous organ.
Fig. 4 gives illustra-
tions of transverse
sections of Photuris
pennsylvanica and
Photinus pyralis re-
cently studied by McDermott and Crane.f

In all these illustrations, L is the photogenic tissue, P is the pigment
layer, and R is the so-called ‘'reflecting layer,’’ the properties of which will
be mentioned presently. From a physical standpoint these comparisons
are of great interest, because the structure of the photogenic organs is prac-
tically the same in all three insects, although the Photuris is a distinct genus,
widely separated from the Photinus in the scale of development. Evidently
the structure of the luminous organs has nothing to do with the great differ-
ence in the color of the light emitted by these two insects.

A further illustration of the great similarity of the essential parts of the
photogenic organs in different animals is given in Fig. 5, which shows a
transverse section{ through a luminous organ situated on the ventral side
of a marine fish, Maurolicus pennanti. Here again we see photogenic mate-
rial, d. K., backed by the so-called ‘‘reflecting layer, i. R. ‘The whole light
organ is inclosed with a layer of black pigment, P, on the inner wall of which

Fic. 2. Fic. 3.

L=luminous tissue. R=reflecting layer. T=trachezx.

*Townsend, American Naturalist, 38, p. 127, 1904.
tMcDermott and Crane, Amer. Naturalist, 45, p: 306, 1911.
fMangold, Winterstein’s Handb. der Vergleich. Physiologie, vol. 3, 1911.

HISTOLOGY OF THE LIGHT ORGANS. II

is a layer of highly insoluble material, which, as already mentioned, is called
the “reflecting layer.” The light is generated in the glandular material,
the ‘‘driisenkorper,”’ d. K., and passes through the funnel-shaped opening,
finally emerging through the lens, /, into the air.

S IIT. Transverse section of Photinus pyralis (dia-
™ / gramumatic).

Rot isee—-R ‘The right-hand half shows the arrangement and

‘ distribution of the trachex, while the left-hand

half shows the modification of the luminous

S tissue at the point of attachment of the breath-

ing muscles.
G, testes; I, intestine; L, true photogenic tissue;
R, reflecting layer; S, spiracle or stigmatum;
L Sp, spiral organs (function uncertain); T,
main trachez to photogenic organ; Tb, main
respiratory trachez; M, muscle fibers.

Fic. 4.

This description seems a little fantastic, for optically the system is
extremely inefficient. ‘The low reflecting power which must obtain in the
so-called ‘reflecting layer’’ and the roundabout way for the light to reach

A, Section through the ventral region of a fish, Maurolicus
pennanti.

B, Section through alight organofafish. (After Mangold.)

Fic. 5.

the lens (granting that the lens and the reflector have sufficient figure to give
directive power) does not carry conviction to the idea that this is a miniature

12 A PHYSICAL STUDY OF THE FIREFLY.

searchlight, which projects the light in a fixed direction. The pocket shape
of the photogenic cell might be brought about by gravity. The refractive
index of the material composing the so-called “reflecting layer”’ cannot differ
greatly from the water and other constituents in the photogenic cell and
hence there can not be a distinct reflecting surface. As stated elsewhere,
a careful examination of a firefly, Photinus pyralis, shows that the whole
dorsal region of the abdominal segments containing the photogenic material,
except the narrow median line P, Fig. 4, II, which is filled with dark pigment,
is aglow (in fact flashes strongly) when the insect emits its flash. This
could not occur if the so-called “reflecting layer” performed, to any appre-
ciable extent, the function attributed to it.

Although from the earliest times the phenomena of light emission by
animals, “biological light,” has attracted the attention of observers, only
within the past century has any serious investigation been made of the
photogenic organs. According to Townsend (1. c.) there now seems to be
a general agreement among experimenters that in the male lampyrids the
photogenic organs are composed of two well-defined layers: the dorsal,
chalky, opaque layer, R, and the ventral or true photogenic layer, L (see
Figs.1 to 4). The latter is composed of two distinct elements: the tracheal
structures and the intermediate areas of parenchyma. ‘The main trachee
(see illustration, Fig. 4, III) of the photogenic segments send vertical
branches down through the light organs. These trachez (T, Figs. 3 and 4)
appear to connect with the main breathing system; and from this evidence
some hold to the belief that thelight is produced by oxidation. “Theentire
system suggests that the air is drawn in through the breathing tracheee, and
forced through the fine passages in the true photogenic tissue, where the
oxygen of the air is consumed in a biologic oxidation.’’*

While this seems a highly plausible explanation, the details as to the
manner in which the light production is regulated do not appear so simple.
For, if the data on the temperature of the luminous segments, presented on
a subsequent page, has any significance, the oxidation seems a continuous
process, while the light production (the intense flashes) is controlled at will.
One peculiarity in the control of the light emission is the manner in which
the flashes may be emitted in one segment or in one small part of the lumi-
nous segment while all the rest of the luminous organs are emitting a bright
glow. Aslight pressure on the luminous segment apparently paralyzes that
region and it begins to emit a strong glow, while at intervals the usual
flashes are emitted from the uninjured, non-glowing parts.

The dorsal layer (R, Fig. 3) is composed of fairly regular polygonal cells,
filled with white crystalline material, which is supposed to be urate salts
(either guanine or ammonium urate, or both). This is the so-called ‘‘reflect-
ing layer,’ the idea to be conveyed being that this material is present to
shield the central nervous system, lying directly above it, from the light
produced in the layer below it, and more especially to act as a reflector to
increase the illumination in the direction desired. From an optical stand-
point this does not appear very convincing; for the optical properties of a
granular mass of material such as described, and of the thickness indicated,
would have a low reflecting power and would be but little more than a dif-

*McDermott and Crane, Amer. Naturalist, 45, p. 306, 1911.

HISTOLOGY OF THE LIGHT ORGANS. 13

fusing screen transmitting almost as much as it reflects, and causing the
glow which is visible throughout the whole abdominal region when the flash
occurs. An examination of the dorsal side of the abdomen shows that only
along the median line, where the integument is dark, is the light absorbed.
Evidently the absorption is caused by the dark pigment. ‘The rest of the
abdomen is all aglow during the flash. This could not occur if the ‘‘reflect-
ing layer” were efficient.

The more radical views of Dubois* may be given equal weight. He has
found the eggs of Pyrophorus noctilucus to be luminous even before they
were laid, so that the light appears to be transmitted in unbroken continuity
from one generation to the next. By following the development of the light
organs through all the different stages occurring from the beginning of seg-
mentation of the egg to the emerging of the adult insect, he has reached the
conclusion that the photogenic tissue in the light organs is derived directly
from the underlying hypodermis, and that in the development of the light
organs a transformation takes place in the protoplasm of the cells, the older
cells toward the upper surface of the light organs becoming filled with
opaque, chalky granules forming the aforesaid ‘‘reflecting layer.”

Dubois has shown that the mollusk Pholas dactylus produces photogenic
matter in the form of a liquid which remains luminous after filtering. The
photogenic liquid contains fine granulations that giveit a cloudy appearance.
‘These granulations are of a colloidal nature which is lost in passing into the
crystalline, when the light emission ceases. he Orya barbarica of Algeria
secretes a luminous fluid which, under the microscope, shows photogenic
granulations that are said to become magnificent crystals during the light
emission. The theory of Dubois is that the light is produced as the result
of the action of an “oxidase” (oxidizing agent) upon another substance,
also of unknown composition. According to Townsend, the view generally
accepted is that the light results from the oxidation of a substance produced
by the metabolism of the light organs. The nature of the substance is
unknown. ‘That its photogenic property is independent of the life of the
cell is proven by the fact that when the organs are dried and reduced to a
powder the light reappears under the influence of air and moisture.

The thickness of the “reflecting layer’’ is about the same in both species,
but in the Photuris the layer of true photogenic tissue (Fig. 4, L) is much
thinner than in the Photinus. It will be shown presently that the light
from the Photinus is much richer in orange-red radiation than obtains in
the Photuris, which emits bluish light. Whether this is owing to a variation
in the thickness of the radiating layer is a question requiring further study.

On a subsequent page it will be shown that after a firefly has done a
certain amount of flashing it suddenly ceases flashing and (if the photogenic
organs do not at once begin to emit a bright glow which soon decreases in
intensity, when the insect succumbs) it is useless afterwards. It would be
desirable to learn whether the insect stores a certain amount of energy or
“fuel,’’ when in the larval state, which is consumed in flashing when in the

*Dubois, Compt. rend.,'Ass’n franc, avanc. Sci., Sess. 22, p. 298; Lecons de Physiologie
P. générale et comparée, Paris, 1908.

fIn this connection see Bongardt, Zeitschr. f. Wissensch. Zoél., 75, p. 1, 1903. Kastle
and McDermott, Amer, Jour. Physiology, 27, p. 122, 1910.

14 A PHYSICAL STUDY OF THE FIREFLY.

adult state. It would also be interesting to examine sections of Photuris
after the insect has flashed for an hour and a half or longer, as described
elsewhere, and of a similar specimen which has not undergone such a period
of flashing, to see whether the ‘reflecting layer”’ is markedly different in
the two samples. Of course numerous examples of each (of the same age)
would have to be examined to obtain an average value.

VI. METHODS OF INVESTIGATION OF THE LIGHT EMITTED.

As with other sources of radiation, the best method of investigating
the composition of the light of the firefly appeared to be one involving the
determination of the spectral energy curve. Because of the weakness and
intermittence of the radiation it is not possible to use a bolometer or a spec-
trophotometer, and the only satisfactory method is the one involving pho-
tography. While this seems to be an indirect method to obtain a spectral
energy curve, it will be noticed presently that it is not so cumbersome as it
appears. The photographic plate, being integrative in its action, is well
adapted to the investigation of weak and intermittent sources, but it must
be of a special kind which is sensitive to all parts of the visible spectrum.
The nearest approach to this condition is the Wratten and Wainwright
“Panchromatic’’ plate, which is sensitive, in a variable degree, to all the
frequencies from the ultra-violet far into the red. The method is really a
species of spectrophotographic photometry in which the light of the firefly
and that of the standard source are photographed, after which the “densi-
ties’’ of the negatives are compared.

In ordinary spectrophotometry one compares intensities. In the photo-
graphic method the effects of these intensities on the photographic plate are
compared, the element of time being the unit of measure and the effect of a
given intensity upon the photographic plate being taken proportional to
the time of exposure. This, of course, is not true for long exposures. For
a limited range the density of the plate is proportional to the time of expos-
ure; beyond this range saturation begins and, still farther away, reversal
takes place. It is therefore necessary to determine the exposure equivalent
of each density measured; that is to say, the ‘‘characteristic curve’’ of the
plate must be determined. This is accomplished by making a series of
exposures of known relationship of a standard source in which the densities
cover the same range as those obtained on the unknown source, e. g., the fire-
flies. Such a series of exposures are shown in Plate 1, A, the standard
source being acarbon glow-lamp operated on 4 watts per candle. Thelamp
was placed at a fixed distance from the spectrograph slit, which was covered
with a ground-glass plate, the voltage was kept constant, and a series of
exposures was made corresponding to 2, 4, 6, 8, 10, 12, 20, 30, 60, 120, and
240 seconds. For densities on the firefly negatives, in which there were no
corresponding density measurements on the glow-lamp, the characteristic
curve of the latter was drawn for the particular wave-length in question,
and the firefly density value was determined by interpolation.

Since the energy value, ‘‘mechanical equivalent,” is not the same for all
rays, it is necessary to know the spectral energy distribution of the carbon
glow-lamp. This was determined anew for the present research by means of
a large vacuum spectrobolometer, fluorite prism, and mirror spectrometer.

METHODS OF INVESTIGATION OF THE LIGHT EMITTED. 15

The photographic plates were imported fresh from the factory about a
month before work was begun. It is, of course, impossible to obtain all the
glow-lamp and firefly exposures upon one plate, and since they were all from
the same batch as sensitized in the factory it was assumed that all the plates
have the same “characteristic curves” as those obtained from the spectrum
of the carbon lamp photographed upon one plate. That this assumption
was permissible is shown by the excellent agreement of the spectral energy
curves of the light of the firefly as obtained from different plates taken from
the same box and from different boxes.

More important than the difference in the plates is the question of devel-
opment. All plates were developed, under identically the same conditions,
by time, in complete darkness, the developer and fixing bath being kept cool
to prevent the softening of the gelatin. The negatives should therefore be
comparable without all the exposures being made on one plate. In fact,
when using the large spectrograph, there was usually a long and a short
exposure of a given species of firefly, from which it was possible to obtain the
complete energy curve from one plate. Experience showed that, although
the photographing was done in a well-darkened room, when the plate was
in the holder for from 4 to 6 hours it became “‘fogged,’’ and hence, when
photographs were taken during the daytime, no attempt was made to obtain
more than two negatives on one plate. Such plates, when developed, were
entirely free from “‘fog.’’ No risks were taken in developing the plates,
fresh developer (metol-guinol) being made up each time. The agreement of
the photometric curves of the light of the same insect, obtained from differ-
ent plates, indicates that but little would have been gained by attempting
to photograph different species on the same plate.

In the vicinity of the Bureau of Standards, District of Columbia, the
flight of the Photuris occurred the first week in June, and no Photinus pyralis
could be found until the last week in June, when but few examples of Pho-
turis were to be seen. However, by bringing Photinus pyralis from some
distance* (in a different part of the city of Washington some 3 miles away,
where the pyralis appeared in the middle of June), it was possible to obtain
the short-time exposures of various species on the same plate (Plate 1, D),
using the small spectrograph to be described presently.

The proper timing of the exposures was found impossible, other than the
rough estimate of exposing the plate to the pyralis from 1 to 2 hours, and
about four times as long to obtain the same impression from the Photuris.
An excellent example of the uncertainty in the matter is shown in Plate 1,
B, spectra 4and 7. The former was obtained in an hour and the latter in
3% hours. It was intended to have the two negatives widely different in
densities, but they are identical in density (see Fig. 7, ‘‘plate 5:27:'11’’).
On one occasion a 53-hour exposure did not give as dense a negative as the
one recorded in Fig. 7, ‘plate 5:26:11." This is owing to the fact that after
being in captivity for a few days the activity of the insect is decreased and its
flash does not seem to be so intense.

‘Two spectrographs were used in obtaining the photographs of the firefly
light. The one was a large quartz-lens instrument having a focal length of

*Especial acknowledgment is due to Mr. U. F. Rosen for his faithfulness in catching
these specimens, and for valuable aid throughout this season’s work.

16 A PHYSICAL STUDY OF THE FIREFLY.

1 meter. When used with a light flint-glass prism the dispersion in the
yellow was sufficient to separate the yellow mercury by 0.5 mm., the total
length of the spectrum between the green (\=o0.5014) and the red (A= 0.6671)
helium lines being 37 mm. ‘The firefly light falls within this same region.
A small spectrograph, of triple achromatic lenses 6 cm. diameter and 18
em. focal length, which had a much greater light-gathering power, was
used in photographing the weak radiations in the red. ‘The same prism (of
light flint-glass) was used on both instruments. In the large spectrograph
the slit opening was 0.75 mm. and in the small instrument it was 0.4 mm.
The collimator slit was covered with a small piece of white cardboard con-
taining an opening 5 mm. high and 2 mm. wide, which enabled the operator
to center the fireflies over the slit, by means of their own light; for, asalready
mentioned, the photographing was done in a completely darkened room.

‘The method of procedure consisted in holding the insects in the fingers,
one or two at a time, over the spectrographic slit. The constant struggle of
the insect to get away caused it to flash more frequently than is its accus-
tomed rate. If it became quiet and ceased to flash, a movement of the
fingers, or allowing the insect to move about, increased the frequency of the
flashing. Frequent flashing is an important item when it comes to making
exposures to weak sources like the firefly, requiring from 1 to 5 hours to
obtain a satisfactory negative.

In the small spectrograph only from 1 to 60 minutes were required to
obtain a uniformly exposed, dense negative. On the small apparatus a
single flash from the Photinus pyralis was sufficient to cause a streak across
the plate (see Plate 1, D, V 12; and VII 5, 6, and 8), and in order to obtain a
uniformly exposed negative, the slit was covered with a piece of ground
glass. In Plate 1, D, V 2 the negative was obtained in 2 minutes from 50
flashes of pyralis through ground glass; III 2 was similarly obtained from 8
flashes; III 5 from 20 flashes; III 6 from 35 flashes; III 7 from 60 flashes;
III 8 from 5 flashes; and III 9 from too flashes or about 4 minutes. On the
other hand, the negative directly below this one, III 10, required 30 minutes
exposure (without glass) to a male Photuris pennsylvanica which flashed at
the rate of about 100 times per minute.

As indicated elsewhere, the longer time required for exposure to the
Photuris is not because of the lack of intensity, but because of the shorter
(as compared with pyralis) duration of the flash. Although the luminous
organs in the two species are different, those of the Photuris being the smaller,
the area covered on the spectrograph slit is not markedly different in the
two insects.

On the spectrometer slit, the Photinus scintillans and consanguineus could
be made to flash about 4 to 5 times per minute. The Photinus pyralis
flashed about 20 times per minute when at its best, but the flash lasts longer
and therefore (aside from its greater volume) has a greater effect on the plate
than the flash of the Photuris pennsylvanica. Inthe Photuris pennsylvanica
the flash, accompanied by a “twinkling effect,’ often occurs at the rate of 2
to 3 times per second, which in some male specimens was sometimes con-
tinued for an hour or more with but few interruptions.

Only a few specimens of Photuris glowed vigorously during the photo-
graphing. The Photinus pyralis easily tires of flashing and then emits a

METHODS OF INVESTIGATION OF THE LIGHT EMITTED. rE]

strong glow which usually is the signal of the end of its usefulness. If,
however, it is placed in an inclosure and allowed to rest, it often resumes the
flashing. As stated elsewhere, the flashing is under the control of the insect,
unless injured or overexerted when the luminous organs begin to glow.
This glow may be in only one segment. Sometimes the glow extends over
one whole segment and part of the other segment. The non-glowing part
of the latter will then be found to emit its flash as usual; and in one case it
was found that this local flashing continued for about 10 minutes. The
application of a slight pressure caused a continuous glow to be emitted also
from this part.

The Photinus scintillans is so small that it is handled with great difficulty.
Its flashes are few, but it emits a strong glow. Only one photograph was
made, Plate 1, D, I 14, inwhich the light was entirely of the scintillans. In
Plate 1, D, I 11, a few glowing scintillans were used, the negative being
finished with consanguineus.

In order to make certain that at least some of the negatives would prove
suitable for demonstrating the great difference in the light emissivity, the
method adopted was to take one or two exposures of the Photuris and to fill
in the rest of the plate with the light of the Photinus pyralis, which required
less time to obtain an exposure. In this manner it was possible to obtain
negatives of the pyralis which were both more and less dense than the photo-
graphs of the Photuris. From these photographs it was easy to see that the
light of the Photuris does not extend so far into the red as does the Photinus.
This is well illustrated in Plate 1, B 4 and 7 (Photuris) and 5 and 6 (Photinus).
If it were simply a question of “density” or time of exposure, then 5 should
have extended farther into the blue than 4, just as it doesin the red. ‘This
is better illustrated in Fig. 7, “plate 6:19:’11"’ and ‘‘plate 5:27:’11,”’ which
gives the photometric densities of Plate 1, B 5 and 6,and 4and 7, respectively.
Here it is shown that, for the same density in the blue, the Photinus shows a
far greater abundance of radiation in the red than obtains in the Photuris.
In Plate 1, D, the scintillans, I 14, and the pennsylvanica, I 13, are of the
same density (by actual measurement) in the yellow, yet the former lies the
farther toward the red. Similarly, Plate 1,D,the pyralis, II 7 and 8, penn-
sylvanica, II 9 and 10, and consanguineus, II 11, have closely the same den-
sity in the yellow; and a mere visual inspection shows that the maximum of
the radiation of the pennsylvanica lies farther toward the blue than obtains
in the other species. All these negatives show that the less dense Photuris
extends farther toward the violet than does a more dense Photinus, which
would not be possible if the light of the two species had the same composition
and differed merely in intensity. So much emphasis is placed upon this
point for the reason that, among those with whom the writer has discussed
the matter, the predominating opinion is that the whole effect upon the eye
is, and should be, the result of a difference in intensity.

18 A PHYSICAL STUDY OF THE FIREFLY.

VII. SPECTRAL ENERGY CURVES OF THE LIGHT EMITTED BY
VARIOUS SPECIES OF FIREFLIES.

The photographs just discussed show that some fireflies emit light which
is much richer in the red rays than obtains in the light of other species. It
is important to know whether this is due simply to adifference in the shape
of the spectral energy curve, or whether the maximum emission is different
in the various species. ‘To obtain this information it is necessary to elimi-
nate the unequal sensitiveness of the plate for different wave-lengths, and
also the change in the density exposure relationship for different colors.
This is accomplished by finding the “densities” or ‘‘blackening’’ of the
photographic plate, for which purpose a Martens polarization photometer
wasemployed. ‘Che photometer was substituted for one of the microscopes
on a small comparator, as shown diagrammatically in Fig. 6. A beam of
light from the standard lamp, J, passed directly into the photometer, Q.
A second beam of light, on reflection from the mirror, M, passed through
plate, N, to be examined, through the slit, R, and into the photometer.

i

Fig. 6.—Side view of] photometer used in measuring densities of the photographic plates.

Although this was not absolutely necessary, a standard plate, P, was kept
on the comparator, for reference in case of accident to the adjustments.
The slit, R, was placed as close as possible (about 0.2 mm.) to the plate, 1,
so as to take in the width desired and no more. ‘The width of the slit, R,
was 0.5 mm. for the small spectrometer and 1 mm. for the large instrument.
Lengthwise on the negative, NV, was placed a strip of black paper with a slit
in it, which was somewhat longer than the photograph of the spectrum.
This slit was 1.5 mm. wide for the large spectrometer, and 1 mm. wide for
the smaller instrument. ‘he microscope and scale, S, were used to make
settings on the different parts of the negative. In thismanner, for the nega-
atives obtained with the large spectrograph, strips 1.5 mm. high and 1 mm.
long were compared against the clear unexposed plate. In the same manner
for the negatives obtained with the small spectrograph, strips 1 to 1.2 mm.
high and 0.5 mm. wide were photometered. One of the helium lines, illus-
trated in Plate 1, B, 1 was used as a reference standard and the points of
measurement were the same on all the plates.

SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 19

The “densities,” i. e., the amount of light absorbed, were than plotted to
a scale as indicated in Fig. 7. In this illustration the heavy lines (x X,
x ») give the effect of the light of Photuris pennsyluanica, of Photinus
pyralis (o- —- --°- —- -©) andof the carbon glow-lamp (-—-—, 4”, 8”, etc.)
as recorded on the large spectrograph negatives. In the same manner,
Fig. 8 shows several density curves of Photuris pennsyluanica (@——* *),
Photinus consanguineus (e——e———®), and of the glow-lamp (-—-—
------ ), as measured on the negatives obtained with the small spectro-
graph. In all cases the Photinus density curves extend farther into the red.
Evidently the time of exposure, i. e., effect on the photographic plate, has
nothing to do with the question of obtaining an impression in the red. In
other words, the lack of ‘‘density”’ in the negative of the red end of the
spectrum is owing to a deficiency in red rays, in the light emitted by the
Photuris; it is not a lack of intensity of the emitted light.

T T T T T T T T T 7 T T

100% Plate 6, 26,11 s ‘ 4
60-------Zt mor SOS) sons pea: soe 5, 27,711 -,x
al a ae ae hs eaeshe-== | 6, 18,7 oa 4
eal a vi ES ce _ Suen as 6, 20,'11 85, 6.6 |
7 or AS Cn Q lar

—
i=}
T

Opacity or Density of photographic plate.

«“

nN w
Q 5
T T

3
T

It
SO 1 2 38 4 55 6 7 8 8 60 ' 2 3 4 S&S G6n
Photuris pennsylvanica { * a
Photinus pyralis o----9-~~--o---~©
Glow ‘lamp Sie eS Sees eee co ae ee

Fic. 7.—Spectrum density curves of photographic plates.
The two heavy continuous lines are for Photuris pennsylvanice.

This illustration in itself is an ample demonstration that the light of the
Photinus pyralis is much richer in red and yellow rays than is the Photuris;
for the curves intersect, and all those of the Photinus, whatever the density,
lie to the right of those of the Photuris of equal density. But we can go
a step farther and eliminate the inequalities of sensitiveness for different
wave-lengths, by comparing the densities of the negatives. It is assumed
that the density, or the effect of the light upon the photographic plate, is
proportional to the time of exposure. This is of course not true for very
long exposures, and hence the highest parts of the curves are not used in
this work. For convenience the 4 seconds density curve of the glow-lamp
was taken as the unit of comparison. Hence the 8 seconds density curve
represents 2 units, the 12 seconds density curve represents 3 units, etc.

20 A PHYSICAL STUDY OF THE FIREFLY.

Turning now to the firefly curves we see that the upper, heavy density
curve of Photuris pennsylvanica intersects the 4 seconds density curve of the
glow-lamp at 0.520 and at 0.592y; i. e., at these two points the two curves
have the same density, which in magnitude on our arbitrary scale is 1 unit.
At 0.532u and at 0.5724 this same Photuris curve intersects the 8 seconds
density curve of the glow-lamp and hence, on our arbitrary scale, its photo-
metric value at these two points is two units. At the intersections with the
20 seconds density curve of the glow-lamp, the photometric value is 5 units;
and soon. ‘These photometric values, 1, 2, 5, etc., or ‘‘ratio of densities,”’
are plotted to scale as ordinates in Fig. 9, and the corresponding wave-
lengths as abscissee. ‘he dotted curve in this illustration is plotted from
Langley’s photometric values of Pyrophorus noctilucus. ‘The firefly curves

100%

wo
oO

@
oO

~
o
T

aD
oO

a
fo}

aN
°

Opacity or “Density” of photographic plate.

i n 1 1 ! l i 1 1

SO 2 4 6 8 60 2 #* 6 8 70M

Photuris pennsylvanica »——~»e——.
Photinus consanguineus e—~o——e
Glow lamp -------+------

Fic. 8.—Spectrum density curves of photographic plates.

have now been compared against a standard source, and their maxima are
found to be entirely different. It is of interest to note that the glow-worm
(larva of the Photuris pennsylvanica) has its maximum at practically the
same place as has the adult insect.

In these curves, Fig. 9, the various circles (0, 4, 6 etc.) indicate that
the resultant curve is the composite of numerous “ratio of densities” curves
(see Fig. 7), obtained by multiplying each “‘ratio of density” curve by a suit-
ablefactor. ‘The photographic plate is very sensitive in the region of 0.590
and it is difficult to eliminate this effect in the firefly curves. The hump in
the curve of the light from the glow-worm and of Photinus consanguineus at
0.59 is therefore not considered of real significance. ‘I'he curves of these

SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 21

two samples were obtained from the photographs taken with the small
spectrograph, Fig. 8. Knowing the distribution of energy in the spectrum
of the glow-lamp given in Fig. 10, it is possible to determine the spectral
energy distribution of the firefly by multiplying the energy values of the

low-lamp by the ratio of densities = aeny EBA from Fig. 9, at each
8 Pe ’ glow-lamp light ’ oe
wave-length. The resultant curves are given in Figs. 10 and 11 and tabu-
lated in Tables 1 and 2.

In Fig. 10 the spectral energy curve of Photuris pennsylvanica and Pho-
tinus pyralis are plotted to the same scale in the blue-green.

An integration
- T T T T
* Plate 5,26,'Il
S279 Ih =
Tr
6,19, lod
620,711 &
6 2
ia, fo)
>| B Ey
aie ce
O lew rm
=
HES ;
1) ®
=)
i e.
ds :
E i
‘a
Ae 3
£2]
fo)
°
=
<
G 2 4
|

rl 1
52 4 6 8 60 2 abe

Fic. 9.—Ratio of densities of photographic plates (firefly light+glow-lamp light)
obtained from the preceding illustrations.

of these two curves shows that for the same emissivity in the blue the energy
curve of the Photinus pyralis is 2.83 times that of the pennsylvanica. To
the eye it is apparent that the illuminating power of the Photinus is far
greater than that of the Photuris. Whether the Photuris pennsylvanica
curve is asymmetrical, as indicated in Fig. 10, or symmetrical is difficult to
decide because of the far greater sensitivity of the plate for the region of
0.59u, and the fewness of the negatives. The same irregularity occurs in
the Photinus pyralis, but is somewhat obliterated by the nearness of the

maximum of emission.

22 A PHYSICAL STUDY OF THE FIREFLY.

In Fig. 11 no special significance is to be attached to the tilted appearance
of the energy curve of the Photinus consanguineus. A slight increase in the
ordinates of the spectral energy curve of the glow-lamp at 0.60p to 0.654
would obliterate this effect. ‘Then, too, the negatives are too small, and any
slight shift from the point of reference in making the photometric measure-
ments would change the symmetry of the curve. The first published curve*
of Pholinus pyralis was more asymmetrical than the present one, owing to
the fact that the spectral energy curve of the glow-lamp, as then used, was
steeper in the red than the one used in thepresentcurves. ‘The energy curve
of the firefly then published does not superpose nicely upon the present one,
being too flat at the top. From the present work it appears that this is due
to the use of too greatly overexposed and underexposed photographs in

TABLE 1.
Photuris pennsylvanica. Glow-worm (larva of Photuris pennsylvanica).
Ratio of quate of i
densities: 0 nsities:
Wave- cha aistee ton Pind Wave- : seein disceination
length. | Firefly of carbon oF i fl von!) Jength. | Glow-worm | of carbon of
Glow-lamp | glow-lamp. MECH Glow-lamp | glow-lamp. | glow-worm.
(Fig. 9). (Fig. 9).
0.520n 1.00 3.30 3.30 0.531h 1.50 3.88 5.82
-528 1.50 3.70 5.55 -534 2.00 3.98 17.96
-5305 2.00 3.82 7.64 -542 3.00 4.47 13.41
©5325 2.50 3.98 9.95 .550 4.00 | 4.92 19.68
5345 3.00 4.02 12.06 2555 3.70 | 5.20 19.24
541 5.00 4.39 21.95 561 3.00 | 5.61 16.83
-547 5.75 4.72 27.14 568 2.00 , 6.13 12.26
-557 5.00 5.33 26.065 -578 1.50 6.92 0.38
568 3.00 6.10 18.30 598 1.00 8.74 8.74
+5715 2.50 6.36 15.90 612 -50 10.14 5.07
576 2.00 6.74 13.48
«581 1.50 7.15 10.73
«583 1.33 7-32 9.74
589 1.00 7.87 7.87
596 .50 8.50 4.25
600 50 8.90 4.45 '
610 25 9.90 2.47 |

making up the composite curve. ‘This made no difference in locating the
maximum, which is practically the same as in the present determination, but
the energy curve is distorted and flattened at the top. The irregularities
in the old curve are similar to those obtained in the present work.

The “density” curve of Photinus consanguineus (the lower curve, Fig. 8)
is irregular at 0.564 and 0.58y, being the highest at thelatter point. Another
curve showed this same irregularity, but its highest point was at 0.56u. It
is therefore doubtful whether there are two emission maxima in the consan-
guineus, although indicated by dotted lines in Fig. 11. In Fig. 8 the density
curves of the carbon lamp found with the large spectrograph were used, while
the photographic curves of the insects were obtained with the small spectro-
graph. This might make some difference in eliminating the small irregu-

larities of the plate, but it can not have a marked effect upon the position
of the maximum.

“Ives and Coblentz, Bull. Bur. Standards, 6, a 321, 1909.

SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 23

In view of the fact that the color of the light of Photinus consanguineus and
Photinus scintillans appears to be more variable than that of other fireflies,
it would be of interest to determine whether they emit light composed of two
bands, of which the one is yellow and the other is orange-red. A large dis-
persion would be required; and plates sensitized so as to be more uniform
in their action in the red and yellow would be useful.

TABLE 2.
Photinus pyralis. Photinus consanguincus.
i pate of Pies tio of
ensities: D lensities: ener, A
ae | Firefly distribution di see ot wave caswrig aigtribation Gece

length. | oO length. fe of carbon f

Glow-lamp glow lanip: of firefly. Glow-lamp | glow-lamp. of firefly.
| (Big. 9). (Fig. 9).

.0523u | 0.25 3.45 0.86 0.522p 0.2 3.40 0.68
527 1.00 3.05 3.65 525 5 3.53 1.77
-530 .50 3.78 1.89 -530 1.5 3.78 5.67
534 1.50 3.98 5.97 -531 2.0 3.88 7.76
-535 | 1.50 4.05 ,08 534 3.0 3.98 11.94
-537 2.00 4.18 8.36 535 43 4.05 17.42
-544 | 3.00 4.57 13.71 537 5.0 4.18 20.90
+540 . 3.70 4.68 17.32 541 755 4.39 32.93
553 5.00 5.07 25.35 543 6.9 4.53 31.26
555 5.55 5.20 28.86 545 9.3 4.64 43-15
560 7.00 5.55 38.85 548 15.0 4.80 72.00
-570 7.20 6.24 44.92 -549 13.5 4.87 65.75
-576 5.55 6.74 37-41 555 18.3 5.22 95-93
-577 5.00 6.82 34.10 -560 21.6 5.54 119.66
581 3.70 7.15 20.45 , +570 28.0 6.30 176.40
585 3.00 7.52 22.56 -570 20.7 6.30 130.41
587 2.78 7.68 21.35 -576 23.0 6.74 155.02
589 2.00 7.87 15.74 . 580 26.5 7.09 187.89
- 590 1.85 7.90 14.73 585 25.1 7.53 189.00
.592 1.50 8.12 12.18 . 589 23.42 7.89 183 .05
-598 1.25 8.72 10.90 -590 23.0 7.97 183.31
601 1.00 9.03 9.03 595 15.0 8.44 126.60
605 75 9.40 7.05 596 13.8 8.53 117.71
615 38 10.45 3.97 598 9.2 8.72 80.20
620 25 11.05 2.76 .600 7.5 8.90 66.78
623 13 11.35 1.48 601 6.9 9.03 62.31

-605 5.0 9.80 49.00
.610 4.5 9.91 44.60
611 3.0 10,02 30.06
-619 2.3 10.91 25.09
620 2.0 11.05 22.10
624 1.5 11.50 17.25
628 1.0 12,02 12.02
.634 1.0 12,80 12.80
638 5 13.30 6.65
-648 2 14.61 2.92

Having but one density curve (Fig. 8, 6,—%,—%) of Photinus scintillans,
and this one being too much overexposed to work out the central part of the
curve, no exact value of the maximum emission of this insect has been
obtained. The density curve is symmetrical with a similar one of the
Photinus consanguineus, and, in view of the fact that to the eye there is no
marked difference in the color of the light (the flash) emitted by these two
species, the maximum emission of scintillans is taken to be the same as that

24 A PHYSICAL STUDY OF THE FIREFLY.

of consanguineus. It is possible that when subjected toa thorough analysis
the maximum emission of the scintillans will be found still farther toward the
red, say 0.58u. From the photometric curve, Fig. 9, obtained by Langley
and Very,* the maximum emission (not applying the energy curve of the
comparison source) of the Cuban firefly, Pyrophorus nochilucus, occurs at
0.5384. ‘Two determinations by Dubois,{ using a grating, gave \=0.53y as
the point of maximum emission. ; :

In connection with the much-discussed question of the physiological effect
on the eye, it is of interest to note the description, by Dubois, of the varia-
tion in the color of the light emitted by Pyrophorus. He says that when the
light diminishes in intensity the red and orange rays disappear and at last

T T T are | T

Plate 5,26,’ll 4

5,27,7ll_
6,19, 1! OO 4
6,20,’11 6

Emissivity
oO

} i

! \
52 4 6 8 60 EyT}
Fic. 10.—Spectral energy curves of fireflies and glow-lamp.

only the green rays persist; and that when the animal begins to emit light
the green rays first appear, then the color extends farther and farther into
the red, when the light attains its maximum intensity.

In concluding this section it may be added that for the first time we have
substantial evidence that the color of the light of various fireflies is different,
the maximum emission varying from o. 55u in the bluish light of the Photuris
pennsylvanica to 0.584 in the orange-red light of the Photinus consan guineus.
Evidently, in these extreme cases the eye made no serious mistake in assign-

*Langley and Very, Amer. Jour. Sci. (3), 40, 1890. Smithsonian Mis. Collections
No. 1258, 1901,

tDubois, Legons de Physiologie générale et comparée, Paris, 1898.

SPECTRAL ENERGY CURVES OF LIGHT EMITTED BY VARIOUS SPECIES. 25

ing the color values which correspond with these maxima. A mere glance
at the photographic curves of the light of the same insects, obtained with

T T T
19 : : i

Emissivity

5 L
we 4 6 8 -60 2 aL
Fic. 11.—Spectral energy curves of firefly and glow-worm.

different apparatus, as shown in Figs. 7 and 8, should suffice to demonstrate

this point. The emission maxima of the various insects studied are shown
in Table 3.

TABLE 3.
Animal. | Maximum of emission.
| -_
Pyrophorus noctilucus....................... . 0,538 to0.540
Photuris pennsylvanica...................... | +552
Glow-worm (larva of Photuris pennsylvanica) . 552
Photinus: pyralisy ioc ccsud cog aaicingee ase eee da 507
Photinus consanguineus...................... | 578
Photinus scintillans.................00.00000- | 2 ae to 0.580

26 A PHYSICAL STUDY OF THE FIREFLY.

VIII. LUMINOUS EFFICIENCY AND CANDLE-POWER
MEASUREMENTS.

In the experiments on the Cuban firefly, Pyrophorus noctilucus, Langley*
and his assistants found that to the eye the insect gave one-eighth as much
light as an equal area of the candle flame, and that the actual candle-power
of the insect was 1/1600 candle. ‘The light from this insect appears to be
emitted continuously, but to fluctuate somewhat in intensity.

Accurate measurements ona fluctuating source, such as the flashes emitted
by the local species of fireflies, is, of course, impossible; but it is highly
desirable to have some idea of the magnitude of the light emission. A
simple ‘grease spot” photometer was therefore provided, the comparison
source being a standard sperm candle.

In the measurements on the glow of the firefly, the photometer screen was
placed at 5 meters from the comparison source and the insect, with the
luminous segments turned toward the screen, was moved back and forth
until the neutral point was reached. This distance varied from 1.0, 1.5,
to 2.2 cm., depending upon the intensity of the glow and the size of the
insect. Some specimens for a short time glowed very strongly, 1/50,000
candle, then suddenly decreased to 1/150,000 candle or Jess. ‘The weakest
measurements were 1/250,000 candle; but these were strong as compared
with the faint glow observed while photographing the spectrum of the
Photinus scintillans.

Measurements were made of 8 to 10 different specimens, the average
intensity of the glow being of the order of 1/50,000 candle. Because of the
inability to cause the insect to flash rapidly for any length of time, the
measurements were more difficult, and hence uncertain. In this case the
insect was held over a glass plate at a fixed distance (about 7.5 cm. from the
photometer screen), and the latter was moved back and forth before the
comparison source. The main difficulty encountered was the variation in
intensity of the flash. A fresh specimen sometimes emitted several strong
flashes which would suddenly become weak.

As in the measurements on the glow, the specimens examined were of two
kinds, viz, fireflies that had been in captivity for over a week and fireflies
that had just been caught in the fields. Several other simple tests were
tried, but no satisfactory measurements were made of the flash, due princi-
pally to the great fluctuations in intensity. The measurements on the
candle-power of the flash indicated a variation from 1/50 candle to 1/400
candle, the predominating values being 1/400 candle. In all, about a dozen
measurements of the intenstity of the flash were made on three healthy
specimens of Photinus pyralis. It would have been interesting to make
similar measurements of the Photuris, but it was too late in the season. As
mentioned elsewhere, male specimens of the latter are frequently obtained
which emit a “twinkling” flash 3 times per second, and will sometimes con-
tinue to do so for 1 to 13 hours without serious interruption.

In the previous investigation the radiant efficiencyf of the Photinus pyralis
was found by multiplying the spectral energy curve by factors representing

*Annals Astrophys. Obs., 2, p. 5, 1902.
tIves and Coblentz, Bull. Bur. Standards, 6, p. 330, 1909.

LUMINOUS EFFICIENCY AND CANDLE-POWER MEASUREMENTS. 27

the visibility of the radiation. In the meantime these factors have under-
gone revision,* and in the present work the spectral energy curves given in

Figs. 9 and 10 were multiplied by these new factors, the numerical values

for high intensities being used. The resulting spectral energy curve, Fig.

12, is similar to the one previously published.| The ratio of the area of

this curve (the ratio of shaded area to the total area of the spectral energy
_. curve), which represents the most advantageous possible distribution of light

to the eye, to the area of the spectral energy curve of all the light emitted

(i. e., energy radiated X visual sensibility the total radiated energy) is the

reduced luminous efficiency. An integration of the curves thus obtained

indicated that this reduced luminous efficiency of the Photinus consan-

guineus is about 79 to 80 per cent, that of the Photinus pyralis is about 87

per cent, and that of the Photuris pennsylvanica is about 92 percent. The

latter emits the bluer light, corresponding more nearly with that part of the
spectrum for which the eye has its maximum
sensibility (at 0.54u) and hence should be the
more efficient.

The higher value, 96.5 per cent, previously
obtained for the Photinus pyralis, would be
decreased by using the new visual intensity
factors. Furthermore, as already mentioned,
the difference in the densities of the photo-
graphic negatives was too great, and the denser
one was too much over-exposed, to obtain a
proper multiplying factor which would give
the proper intensities
of the central (highest)
part of the curve repre-
senting the ratios of fire-
fly light to glow-lamp
Sa x light. This tends to
give too high values for
the radiant efficiency.

To the writer this
method of estimating the luminous (radiant) efficiency seems misleading,
for the reason that individual eyes vary too much{ to attempt to define
this quantity in terms of the ‘‘average eye.’ Then, too, in the ultimate
analysis, the question of real interest is the total cost of production and
the total quantity of the commodity produced. ‘The aforesaid value of
the radiant efficiency of the firefly is obtained on the assumption that
there is no infra-red radiation emitted. It will be shown presently that
thus far it has been impossible to detect infra-red radiation, and hence the
high numerical values obtained for the radiant or luminous efficiency (ratio
of ‘‘light’’ to total radiation) seem-plausible; but it is misleading if we over-
look the question of the energy input, about which we have no information.

q

NW

Fic. 12.—Luminous efficiency of firefly (shaded area to
total area). This curve is taken from Fig. 10.

*P, G. Nutting, Visual sensibility of the eye, Bull. Bur. Standards, 7, p. 235, 1911.
Circular Bur. Standards 28, p. 8.

{Bull. Bur. Standards, vol. 6, No. 3, p. 331.

tIves, Trans. Illum. Eng. Soc., 6, p. 258, rg1I.

28 A PHYSICAL STUDY OF THE FIREFLY.

From his four summers’ experience with these insects, during which time
hundreds of them were caught, and a great number of the stronger ones
were handled on the spectrometer slit, the writer has come to the conclusion
that, in emitting the light, considerable energy is expended in some unac-
countable manner. After a period of flashing (the frequency of which is
stimulated by the mere effort of the insect’s struggling to escape) the light
emission suddenly decreases in intensity. After a short rest the flashing
may again be obtained; but after repeated trials the flash ceases and the
glow may begin. If the insect now be agitated, the characteristic glow
appears, and after a short time life becomes extinct, apparently from
exhaustion.

The Photuris is very strong and furnishes good examples. After being
held on the spectrometer slit, and after having flashed until tired and then
placed away, it was found that they never recovered, when examined the
next day. On the other hand, samples caught the same evening, but not
used until the next day, were then very active and flashed as frequently and
sometimes as intensively as those caught and used on the same evening. A
specimen that had flashed from two to three times per second for half an
hour without tiring was given a rest of an hour, after whichit flashed with the
same rapidity for another hour, then suddenly ceased flashing. In the
meantimeit had remained quiet over the spectrometer slit and hence escaped
the continual handling usually necessary to keep a specimen in place. This
specimen was then placed alone in comfortable quarters, but it never recov-
ered, life being extinct the next day, although quite active (except in light
emission) when given its freedom. It was apparent that the insect had
not ceased flashing because of any injury received, but because of the exer-
tions in emitting the light. Whether this exertion was entirely muscular
effort and whether the light ceased as the result of the exhaustion of the
fuel supply are pertinent questions.

In concluding this phase of the subject it should be noted that if the
radiant efficiency had been rated as was done by other experimenters (viz,
all light, no invisible rays, and not applying the correction for visual sensi-
bility) the numerical value would be 100 per cent, as found by Langley.

To the writer this manner of rating the luminous efficiency is far from
satisfactory. For example, it has just been noticed that bluish-green light
from the Photuris has the highest luminous efficiency of the three species
examined and one might conclude that it would be the most economical
light. From the standpoint of actual illumination this does not necessarily
follow. For example, in Fig. 10 (where the spectral energy curves of the
Photuris and the Photinus pyralis are plotted to the same scale in the green,
0.54u), it may be noticed that when the intensity of the light is the same in
the blue-green the total amount of light obtained from the Photinus pyralis
is 2.83 times that of the Photuris. It would be interesting to determine the
specific emissivity of the light organs of these insects, in spite of its great
variability. ‘The aforesaid curves simply show that if we obtain the same
emissivity in the blue (by varying the relative distance of the two sources
or the size of the luminous organs or by varying the specific emissivity of
the luminous organs) the Photinus pyralis sends out at least 2.8 times the
“light” (visual sensation) emitted by the Photuris, even though it is not

RADIATION AND TEMPERATURE MEASUREMENTS. 29

brought about with the same efficiency as regards the effect upon the retina.
This appears very interesting viewed from the standpoint of the needs of
the human eye. But how do we know that the insect’s eye requires light
fulfilling our specifications; and if it does, why not say that the color of the
light varies for esthetic reasons, rather than the more plausible one that it
serves as a mark of distinction between the various species?

IX. RADIATION AND TEMPERATURE MEASUREMENTS.

The more recent attempts of Langley* and his assistants to measure the
radiation from the Cuban firefly show that if there are radiations lying
beyond the visible spectrum their heating value is immeasurable. The
earlier measurements of Duboist seemed to indicate the presence of infra-red
radiation, but in view of the greater radiometric sensitivity now attainable
and the greater precautions taken to exclude extraneous radiations, the older
measurements must be assigned but little weight. The measurements of
Dubois indicate a very considerable amount of radiation (the galvanometer
deflection being 1.8°) from the luminous organs of the Cuban firefly. The
dark parts of the body of the insect emitted radiation causing a deflection
of 0.95°. Itis difficult to understand how, with the facilities now available,
these radiations, if present, can escape detection.

The present measurements were first undertaken on samples of the Cuban
firefly (Pyrophorus noctilucus). Out of the two dozen in the original ship-
ment, only one survived in transit, and this one was too weak for radiation
measurements. ‘Thinking that the lack of intensity might be compensated
by using a larger radiating area, measurements were undertaken on the
luminous organs of the local firefly, when stimulated so as to emit a rich
glow. As mentioned elsewhere, a slight pressure on the luminous segments
will cause a strong glow. A vacuum thermopile, exhausted by means of
liquid air and charcoal (after preliminary evacuation with an oil pump) was
used as aradiometer. The sensitive parts consisted of a fine platinum wire
0.1 mm. in thickness, and a strip of bismuth rolled to 0.02 mm. thickness,
6 mm. long, and of the shapeindicated in Fig.13,B. The platinum wire was
used because of the ease with which it could be “tinned” and soldered to the
bismuth with Wood’s alloy. The latter ordinarily makes a poor high-
resistance connection, but this is easily remedied by coating the wire (plati-
num or iron) with solder. The wide circular area, constituting the receiver,
was blackened with a mixture of lamp-black and platinum-black. The out-
side of the glass tube was covered with tin-foil, except openings on opposite
sides of sufficient size to admit radiation on the sensitive junction and for
the purpose of adjustment. This thermopile was entirely free from the lag
usually found in suchinstruments. The heating attained its maximum in
less than 2 seconds, which was the time required for the galvanometer to
reach the maximum deflection.

It will be noticed that in certain work, for example where the insect was
near the instrument, radiation fell on both junctions, and hence not operat-
ing at its maximum efficiency. On actual trial it was found that it made

*Annals Astrophys. Obs., 2, p. 5, 1902.
tDubois, Bull. Soc. Zool. France, parts 1, 2, and 3, 1886.

30 A PHYSICAL STUDY OF THE FIREFLY.

no appreciable difference to expose the two junctions or only the dark june-
tion to radiation. ‘This is due in part to thesmallness of the bright junction.
The radiating of such a thermopile by the candle test has but little meaning
because of the absorption by the glass walls, which absorb perhaps 40 to 50
per cent of the incident radiation. The test was applied, however, giving
a deflection of 2.5 cm. for a sperm candle at 2 meters and galvanometer
sensitivity of 7=3 107? amp., or 10 cm. for a candle at 1 meter.

In actual work on fireflies the galvanometer sensitivity was increased to
i=1.5X10- amp. to 7X10~ amp. or 5 times as sensitive as for the
candle test. In his measurements Langley found that:

A portion of the flame of a standard sperm candle, equal in area to the bright part of
the insect, gave, under the same circumstances, a bolometric effect of such magnitude
that had the heat of the insect between 1/80,000 been as great as this from the candle, it
would certainly have been recognized.

‘The sensitivity of the present apparatus, under conditions similar to
those just quoted, was such that radiation 1/60,000 as great as that of the
candle would have caused a deflection of 1 mm. The thermopile in vacuum

A
C
Cu OCT
% To Bi
sx Galv.
Cu \ 2008
B
Pt. ON Cu i SS et
a iy N E
BI Cu a } 2202.
Pt. i,

Fic. 13.—Thermopiles for measuring the temperature and radiation of fireflies.

is far more steady than a bolometer in air and covered with a glass window, so
that while the present sensitivity appears to have been less than that used
by Langley and his assistants, the efficiency or reliability was no doubt con-
siderably greater. ‘That the sensitivity of the instrument was very high is
indicated by the fact that when a firefly was placed on the glass walls over
the sensitive junction, or within 5 to 10 mm. of the same, the galvanometer
indicated a cooling of this junction which attained its maximum in about
I minute.

It will be shown presently that the temperature of the firefly is some-
what less than the surrounding air; hence in the heat interchange the pile
would radiate to the insect. However, the radiation from the pile could not
pass through the glass, and the explanation offered is that the glass was
cooled (principally by conduction) by the insect and in turn the dark junc-
tion radiated to the glass; whence the slowness of the operation. This
observation of a cooling effect when the insect was near the thermopile came
as a surprise, and for a while it appeared that there might be an endothermic
action taking place in the photogenic cells instead of the long-sought-for

RADIATION AND TEMPERATURE MEASUREMENTS. 31

heat radiation. However, it is not considered a true endothermic phenome-
non caused by the absorption of heat by the photogenic organs, but is rather
a slow conduction of heat from the walls of the radiometer to the insect,
which is at alower temperature. If the phenomenon is one of a true trans-
fer of radiant energy, then the galvanometer should have reached its maxi-
mum deflection in 4 seconds instead of 60 to 80 seconds, and it should have
remained at a maximum instead of slowly drifting back after the expiration
of 1.5 to 2 minutes. The crucial test would be to use a rock-salt window
which would permit the long-wave radiation from the thermo-junction to
pass directly to the insect, instead of impinging on the glass walls of the
radiometer. ‘This cooling effect led to the thermo-couple explorations to be
described presently.

The radiometric tests of the flash and the glow of both Photuris pennsyl-
vanica and Photinus pyralis extend over a period of 3 weeks, during which
time four distinct series of measurements were made; each series took from
1 to 3 hours, during which time various experiments were tried, using red
absorption glass, etc. In the earlier measurements on the Photuris the
insect was attached to a thin wooden rod about 15 cm. long, held horizon-
tally and suspended over the thermo-junction. ‘The tests were made in a
darkened room and shields were provided so that no stray light could get
into the thermopile case. The flashes were as frequent as 3 times per
second, but in neither the flash nor the steady glow could any radiation be
detected with certainty other than the cooling effect (conduction of heat)
which will now be discussed.

If it were not for the novelty of the phenomenon and the averseness with
which one views data of this type, after having been taught to search for a
heating effect rather than the opposite, it would be possible to urge that the
emission of light is accompanied by an absorption of heat. For example, a
strong, active Photuris, flashing 80 to 90 times per minute, caused a cooling
effect of 5 to 6 cm. deflection (galvanometer period 4 seconds, therefore very
steady and no ‘‘drift); and later on, when the flashes became less frequent,
in two distinct series of measurements the maximum (cooling) deflection
was in each case only 3.5 cm. Another specimen gave a (cooling) deflection
of 1.5 cm. and then began ‘‘flashing brighter”’ (as the records have it), when
the deflection increased to 3.1 cm. Still another specimen of Photuris,
‘flashing 3 times per second,” gave a maximum (cooling) deflection of 6 to 7
cm., and finally only 2.2 cm. deflection. The records show several similar
cases. Whether these measurements have any significance it is difficult to
say. ‘The closer to the radiometer the greater was the cooling, but it is not
known whether the insect was accurately at the same distance from the walls
of the radiometer when flashing and when it became less active.

If the insect generates only the radiations which are visible to the eye, the
heating value of these rays will be small. It is evident that but little infra-
red radiation can be generated, for so much would be absorbed that unless
it has some cooling device the temperature would rise to a high value and
the insect would be speedily desiccated. It is therefore possible to have the
radiation (heating) effects on the radiometer masked by the thermal con-
duction effects (cooling) if the temperature of the insect is less than that of
the thermopile.

32 A PHYSICAL STUDY OF THE FIREFLY.

This apparent increase in the cooling effect in the more active (flashing)
state of the insect is a complex phenomenon requiring further investigation.
As it stands, it could not be verified by the iron-constantan thermo-couple
measurements on the temperature of the different parts of the body of the
insect. ‘The thermo-couple shows, Fig. 13, A, that the temperature of the
insect is lower than the air (this is in agreement with the radiometric obser-
vations), and that, used differentially, the temperature of the luminous seg-
ments is higher than the rest of the body; but it was not possible to show
with certainty whether the temperature of the luminous segments varied
(apparently decreased by the radiometric test) during the act of flashing.

After trying out these heat-conduction effects with the insect close to the
vacuum thermo-element, the radiometric apparatus was arranged so that a
mirror, 15 cm. in focal length and 10 cm. diameter, projected an image of an
opening (2.5 mm. diameter) in a heavy black cardboard upon the active
thermo-junction. ‘The whole was covered so that no radiation, except that
which passed through the opening in the cardboard, could fall wpon the
thermo-element. The firefly was then held with the luminous segments
over the opening in the cardboard. Both glowing and flashing specimens
were examined, but in only one instance was any deflection (a kick) observed
that might be interpreted as possibly being due to the light (flash) from the
firefly. It was therefore concluded that, since the glass wall of the bismuth-
platinum thermo-element absorbed the long waves, no radiations less than
3u were observable. The radiations which are greater than 3u would fall in
the category of ‘‘animal heat,” which will now be discussed.

Langley avoided the effect of “animal heat’’ by interposing a glass screen,
which prevented an interchange between the insect and the radiometer of
radiations greater than 3 to 4u. It has just been noticed that under certain
conditions, when the insect was near (or touching) the glass walls of the
thermopile, the deflection of the galvanometer indicated that the sensitive
junction was losing heat by radiation; and, from the slowness of the action,
that this was due to the local cooling of the glass wall surrounding it. Evi-
dently conditions might be brought about whereby a heating effect, caused
by a true radiation from the insect, might be counterbalanced by a cooling
effect (conduction) due to a difference in temperature between the insect
and the radiometer, the latter being at the higher temperature. Further-
more, if the light production is a biochemical process, as some believe it to be,
it is important to know whether this low temperature is due to the fact that
we are dealing with a “cold-blooded” animal, or due to an endothermic
reaction. ‘Thermometric measurements were accordingly undertaken to
determine the nature of the heat distribution in the body of the firefly. The
first tests were made radiometrically with the vacuum thermo-element just
discussed. It was found that the glowing luminous segments, when sepa-
rated from the rest of the insect and placed upon the vacuum thermopile,
exerted a certain cooling effect which decreased when life was extinct. An
insect on its back caused a cooling, but it was impossible to determine by the
vacuum thermo-element whether there was a difference in the temperature
of the dark and luminous segments.

‘These experiments were continued by exploring the temperature distri-
bution in the segments of the insect. For this purpose, thermo-couples of

RADIATION AND TEMPERATURE MEASUREMENTS. 33

iron-constantan were employed. ‘T'wo types of couples were tried. The
couple shown in Fig. 13, A, consisting of wires 0.005 mm. in diameter (the
constantan wire being about 3 cm. long) attached to heavy copper wires
mounted upon a wooden base, proved the more satisfactory. In this instru-
ment the junctions were fine globules of solder hammered flat and black-
ened. In the second type the iron wire (0.2 mm. in diameter) was filed to
a point to pierce the outer integument. The great thickness of the iron
introduced heat conductivity, which sometimes introduced irregularities,
either heating or cooling, when only one of these effects was supposed to be
present. It was difficult to operate and hence was discarded for the couple
made of fine wires just mentioned. The latter was laid on several folds of
cheese-cloth, which reduced the effect of air-currents. The experiments
were carried on in a well-darkened room. ‘The effects observed being so
marked, it was necessary to reduce the sensitivity of the galvanometer (half
period usually 1 second; i=7 10° ampere) and keep a resistance vary-
ing from 200 to 700 ohms in series with it.

The insect was attached to a thin wooden rod (15 cm. long, and held hori-
zontally) in such a manner that it could be made to touch either one or both
of the thermo-junctions. Covering either one or both of the junctions by
means of a bit of cork or a dead insect, attached to the rod in the same
manner, produced no effect. It is therefore evident that the thermal meas-
urements made on live active insects are realities and not due to instrumen-
tal errors. Furthermore, the galvanometer deflections were decisive, not a
few millimeters, but usually from 15 to 20 cm. and frequently “‘ off the scale.”
The results are not based upon a series of measurements in which the mean
of the positive deflections outweigh the mean of the negative deflections, or
vice versa.

While the magnitude of the deflections varied enormously, owing to poor
contact, one could continue for 10 to 30 minutes at a time with rarely a false
reading. For example, starting with the first or second abdominal (the
luminous) segment, Fig. 1, touching the upper thermo-junction, Fig. 13, A,
the galvanometer would indicate a cooling of this junction which was usually
“off the scale.” Rotating the insect about a vertical axis through 90°, with
the luminous segment over the junction and with the third (adjacent) dark
segment on the lower junction, the galvanometer would go off the scale in
the opposite direction, indicating that the upper junction is now the hotter.
In other words, while the insect is cooler than the air in the room, the dif-
ferential measurements show that the luminous segments are hotter than
the dark segments. Reversing the operation by starting with one of the
luminous segments touching the lower junction (cooling), then with one of
the dark segments on the upper junction, the galvanometer deflection indi-
cated that the lower junction was now the hotter.

Sometimes only the heating effects would be tested, starting, say, with a
luminous segment on the upper junction and a dark segment on the lower
junction, then the reverse, and the test would be kept up in rapid succession
for 10 to 15 minutes, when another insect was tried. In this series there
should always have been a heating. Sometimes there would be a cooling
effect and, on examination, it would be found that the dark segments were
not touching the thermo-junction. While such a series was in progress, and

34 A PHYSICAL STUDY OF THE FIREFLY.

everything going smoothly, variations would suddenly be introduced. For
example, the two couples would be brought close together, so as to touch a
single segment, luminous or dark, when no effect would be observed—the
reader at the galvanometer not being aware of the change.

Turning the insect on its back and proceeding in the above-described
manner, it was found that the region above the luminous segments was
hotter than the region of the dark segments, just as was observed for the
ventral side of the insect; but the difference in temperature between the
region of the luminous segments and the dark segments was Jess than when
measured on the ventral side.

Placing one junction on a luminous segment and the other junction on the
dorsal part of the same segment (dorsal-ventral temperature) the measure-
ments were subject to greater fluctuations, hence not very conclusive.
There was some evidence that the region directly over the luminous organs
is hotter than the dorsal part of the same segment. ‘This test was applied
only to the Photinus pyralis, the Photuris being too active for any measure-
ments of this kind. In one very concordant series of measurements on the
Photinus, of the difference in temperature between the dark and the lumi-
nous segments, it was observed that on the dorsal side the temperature dif-
ference was only about half as great as on the ventral side, where the lumi-
nous organs are close to the surface. Comparing different species, it was
found that the Photinus pyralis seemed hotter (the temperature difference
between the bright and dark segments) than the Photuris pennsylvanica.

Dry specimens had no effect; but in dead specimens in which the lumi-
nous organs had begun to glow (putrefaction) there appeared to be a small
heating. A freshly killed specimen seemed to come to room temperature
(no cooling) very quickly; but the glowing luminous segments were always
hotter than the dark segments. After a time this heating effect decreased,
but on stimulating the luminous organs to a bright glow the usual (“‘off the
scale”) heating effect became more nearly normal.

In the female Photinus pyralis the luminous organ is a small circular spot
on the third abdominal segment from the end (see Fig. 1). On placing one
junction on the luminous spot and the other junction on the second segment
from the end, the galvanometer showed that the luminous spot was much
hotter than the other segments. In fact, in the first trial, the indication
was so decided that the galvanometer reading was thrown entirely off the
scale, requiring considerable time to bring it back into adjustment.

Both the male and female of Photinus pyralis have two small glowing
luminous points, Fig. 1, on the next to the last abdominal segment. ‘The
thermo-couple test on one of these glowing points, on the female just men-
tioned, indicated a higher temperature than the adjacent dark segment.

The complete thermo-couple test comprised four distinct series of meas-
urements made on as many different days, those on the Photinus pyralis
being made some 26 days later than the measurements on the Photuris. Of
the Photuris, two males and three females, and of the Photinus pyralis, eight
males and one female were subjected to thorough tests. Numerous other
specimens were discarded before completing a test, because of accidents or
inactivity.

These experiments are described in such detail because of their novelty,

RADIATION AND TEMPERATURE MEASUREMENTS. 35

and more especially because of the question of the reality of the results.
The tests were repeated when doubts arose at various times during an inter-
val of more than 3 weeks, the instruments having been dismounted in the
meantime, and they always led to the same result, viz, that the temperature
of the insect is less than the air and that the region of the luminous segments
is hotter than the dark segments.

In these experiments no attempt was made to obtain exact measurements
of the difference in temperature of the luminous and the non-luminous seg-
ments. Indeed such data would be illusory. A rough estimate of the
temperature may be obtained from a knowledge of the galvanometer sensi-
tivity, the thermo-electric power of the junctions, and the resistance of the
circuit. Using these constants, and assuming a deflection of 20 cm., the
equivalent difference in temperature is 0.6°. Frequently the indicated
temperature was only 0.3° to 0.5°, while the “off the scale”’ deflections indi-
cated temperature differences as great as0.8°. The bright luminous points,
L’, Fig. 1, on the female pyralis indicated a temperature of 0.2° above the
surrounding dark regions. With the thermo-couples it was not possible to
determine with certainty whether the deflections were increased (a heating)
during the flash. The experiment showed that this temperature difference
was present continuously in healthy specimens. ‘This would seem to indi-
cate that even if the temperature difference is due to some biochemical
reaction, the light emission is under the control of the insect. The breathing
spiracles being over all of the segments, it is difficult to understand how the
influx of oxygen could produce a local effect in the luminous organs, unless
there is some chemical action in these organs.

The galvanometer deflections were of the same magnitude for the cooling
effect as they were for the heating effect. This seemed to indicate that the
temperature of the insect was not much below that of the room (26° C.) at
the time of the experiment. Hence, having been informed that the temper-
ature of these insects is about 17° to 18°, and that these measurements had
been made by placing a number of insects in a vessel containing a thermom-
eter, the writer undertook a similar experiment. Two similar glass speci-
men tubes, 15 cm. long, 2 cm. diameter, and two similar thermometers were
provided. The thermometers passed through corks fitted into the tubes.
These tubes were placed side by side in a vessel containing cotton batting.
More than 100 Photinus pyralis were caught and shaken directly from the
net into one of the glass tubes, which was then about half full of insects.
After replacing the tube in the insulating material and waiting 10 minutes,
the writer was surprised to find that the temperature had risen 1° above
the empty tube instead of dropping to alower temperature. Table 4shows
the course of events during the test. During the evening, as the activity
of the insects decreased, the temperature difference decreased. Inter-
changing the places of the test tubes showed no variation in the readings,
and the conclusion arrived at was that the temperature rise was due entirely
to the presence of the insects. In view of the fact that the body temper-
ature of these insects appeared to be almost as much below the room as the
temperature of the luminous segments was above that of the insect, it is not
unthinkable that in the thermally insulated vessel it was possible for the
temperature to rise as observed.

36 A PHYSICAL STUDY OF THE FIREFLY.

The preceding pages give an exact account of the observations, an expla-
nation of which is not attempted. However, in concluding this subject, it
may not be out of place to add that an explanation based on the assumption
that the dark segments are at a lower temperature than the luminous seg-
ments, due to their higher emissivity, does not appear satisfactory. As for
the oxidation theory, the present data do not refute it. Whether this tem-
perature difference hasanything to do with the photogenic processes remains
an undecided question. It seems to be present all the time in a healthy
insect, but is not observable on a freshly killed non-glowing specimen. As
stated elsewhere, several dead specimens were found which had begun to
decompose, and the luminous organs emitted a faint glow. Thermo-couple
meastirements on these samples showed but a slight warming, if any, over
the luminous segments.

TABLE 4.
. ‘Temperature Temperature
ee of empty pola Aig At Remarks.
test tube. fireflies,
fo ° o

8 20 30.6 30.6 o.o | Start; temperature read just before fill- |
ing test tube.

8 30 30.6 31.6 1.0

8 40 30.4 31.3 0.9 | Insects less active.

9 40 30.2 30.3 0.1 After return from field, nearly all insects
inactive at bottom of test tube.

g 48 30.2 30.7 o.5 | After shaking the test tube, covering the

thermometer with lively insects, and
waiting 8 minutes. No effect due to
shaking. All but about a dozen
insects inactive, active ones at top;
put thermometer there.

10 02 30.5 30.8 0.3

lo 20 | *30.5 30.8 0.3

10 25 30.3 30.6 0.3 | Interchanged places of test tubes.

10 30 30.3 30.7 0.4 | Agitated test tube; inserted thermometer
te Be: in living mass of insects.

10 33 30.4 30.8 0.4

10 35 30.4 30.8 0.4

As a final summary of this perplexing series of experiments, it may be
added that the temperature of the firefly is somewhat lower than the air,
but it is not known whether this body temperature has a fixed value; that
the luminous segments are hotter than the adjoining dark segments, this
temperature difference being most marked on the ventral side of the insect;
that it is uncertain whether this temperature difference is effected by the
flash (whether the flash is accompanied by an endothermic or exothermic
reaction); and that, if radiant energy (especially the infra-red) is emitted
by these insects, it is immeasurably small. No suggestions are offered as to
the cause of this difference in temperature between the luminous and the
non-luminous segments. Apparently there are slight discrepancies in the
two sets of measurements of the effect of the flash upon this temperature
difference; but a study of the data and of the functions of the two types of
instruments used in these measurements does not indicate that this is a

THE FLUORESCENT SUBSTANCE IN FIREFLIES. 37

serious matter. To measure such a temperature difference is no small
undertaking. To go one step farther and determine how this temperature is
modified, as for example by the flash, is a different problem requiring further
investigation.

In Fig. 13, C, is shown a type of (vacuum) bismuth-silver thermopile with
which it is purposed to make further radiation measurements. This pile
consists of 10 junctions of silver wire 0.019 mm. in diameter and bismuth
wire 0.04 mm. in diameter, blackened with a composition of platinum black
and lamp black. The central junctions are insulated with shellac, placed
in contact and backed with a receiver of pure tin, 0.01 mm. thick and 1 mm.
indiameter. In air it is one-half as sensitive as a 20-element Rubens linear
thermopile. It is about 4 times as sensitive as the radiometer used to
measure heat from stars, when used with a galvanometer of the same period
as the radiometer. Its resistance is 19.3 ohms.

X. THE FLUORESCENT SUBSTANCE IN FIREFLIES.

During the photographic work two years ago, in cleaning the test tubes
which had contained fireflies, the writer noticed that the wash water had a
peculiar faint bluish tinge, which he considered due to some fluorescent
material. In order to demonstrate to others that it was not “blue sky,”
““decomposition bacteria,” etc., an attempt was made to obtain a pure solu-
tion and photographs were made of the fluorescent material.*

The usual procedure to obtain a pure solution was to place the insects in a
strong solution of alcoholand water. Thealbuminous material was precip-
itated with lead acetate and the solution was concentrated by boiling.
The solution was then neutralized with potassium oxalate to prevent tur-
bidity while photographing the fluorescence spectrum, which was obtained
with the large spectrograph employed in photographing the light emitted by
the firefly. Such a photograph of the fluorescent spectrum is shown in
Plate1,C. The cadmium spark produced a greater fluorescence than either
zinc or aluminum, and was therefore used as an exciting source. The ultra-
violet light tends to produce a cloudiness in the solution which must then be
filtered. The fluorescence spectrum shows no structure and seems to be of
the same nature as is commonly observed in many substances. The main
point of interest is the extent (from 0.38 to 0.51) of the fluorescence spec-
trum, which is complementary with the light emitted by the firefly. (See
the right-hand side of Plate 1, C, which givesthe spectrum of the lightemitted
by Photinus.) However, no special significance is attached to this fact.
The maximum emission of the fluorescence spectrum lies at about 0.414.
The absorption in the ultra-violet appears to be very strong, as indicated by
the fact that, unless the solution is very dilute, the fluorescence extends but a
few millimeters into the solution. When concentrated the solution becomes
yellowish and but little fluorescence is noticeable. On dilution, however,
the fluorescence reappears. The yellowish color is attributed to the exten-
sion of the ultra-violet absorption, of the fluorescent substance, into the
visible spectrum on concentrating the solution. The material decomposes

*Coblentz, Phys. Zeit., 10, p. 955, 1909; Bull. Bur. Standards, 6, p. 321, 1909,

38 A PHYSICAL STUDY OF THE FIREFLY.

in the light, the solution obtained two years ago showing but little fluores-
cence at the present time.

After searching for some time for some one versed in the chemical side of
the subject, who was sufficiently interested to investigate the chemical prop-
erties of this material, Mr. F. A. McDermott has undertaken the subject
and has recently published some of the results of his preliminary tests.*

This fluorescent material is found in great abundance in Photinus pyralis,
P. consanguineus, and P. scintillans. It is also present, in somewhat less
abundance, in the non-luminous genus Ellychnia corrusca, a single speci-
men of which is sufficient to show fluorescence in a small-sized tube contain-
ing the alcoholic solution. Using the cadmium spark, there is no difficulty
in observing the fluorescence. ‘The fluorescent material appears to be
present in only a small amount in the Photuris pennsylvanica, and is not
observable immediately after placing insects in a strong solution of alcohol
and water. However, after standing a few hours, the fluorescence is visible,
especially when excited by the cadmium spark. All previous tests for the
fluorescent material in the various species made two years ago were verified
this present summer.

The rotary power of the solution of fluorescent material was tested by
Messrs. Jackson and Snyder with an ordinary quartz-wedge saccharimeter.
It was the extract of about 250 specimens of Photinus pyralis in 250 c.c. of
water and alcohol. Using a 20 cm. column of the solution (after treating
with lead acetate), the rotation was +0.04° Ventzke. The solution was
then boiled to about one-fourth to one-fifth its original volume and filtered
through four thicknesses of filter paper. The solution was a clear yellow,
but on standing a precipitate was formed. ‘The rotation of a 20 cm. tube
of the concentrated solution was 0.21° Ventzke. Using a 4o cm. tube, the
rotation was twice this amount. Since 100° Ventzke=34.657° for the D
lines of sodium, the actual rotation was 0.073° for the D lines of sodium. It
wasconcluded that, since the concentration was evidently very weak as com-
pared with an ordinary test solution of sugar, the natural rotary power of
the solution containing the fluorescent material of the firefly was very high.

The rotary power of this solution was again measured about five weeks
later, the solution in the meantime having been kept in complete darkness.
The small amount of precipitate present was removed just before examina-
tion. ‘The observed rotation was 0.22° V., from which it appears that the
rotary power of thesolution had changed but little, if at all, during this time.

SUPPLEMENTARY NOTE.

Since writing this paper, a recent note by Dubois (Compt. Rend., 153, p.
208, 1911) has come to my notice in which he claims priority in the discovery
of a fluorescent substance in animals, e. g., in Elaters and Lampyrids. He
says he has gone a step farther than Ives and Coblentz, and McDermott, by
defining its réle physiologically and by showing that “les insectes se servent
naturellement, depuis bien des siécles sans doute, de l'emploi que l’homme
vient seulement de faire industriellement de substances fluorescentes pour
augumenter le rendement des appareils d’éclairage, en améliorant les quali-

“McDermott, Jour. Amer, Chem. Soc., 33, p. 410, 1911.

THE FLUORESCENT SUBSTANCE IN FIREFLIES. 39

tés de Ja lumiére par transformations de radiations inutiles, et méme nuisi-
bles, en clarté agréable a l’oeil et favorable a la vision.”

No doubt the priority can be claimed by Dubois, who says he found the
fluorescent substance in the “‘ Luciola italica.”’ ‘To the writer, the mere dis-
covery has been of no interest. Furthermore, the presence of the fluores-
cent substance in one species of firefly is not a proof that it is to be found in
allspecies. For example, in the Photuris pennsvlvanica there is but little of
the fluorescent material, which may easily escape detection if the examina-
tion is made in white light.

Concerning the functions of the fluorescent substance, which Dubois
claims is present, to modify the quality of the light emitted by the firefly in
order to make it agreeable to the eye and more favorable to vision, further
information must be obtained to make this assumption tenable. As men-
tioned elsewhere, we do not know that the eye of the insect has the same
luminosity curve as has the human eye; hence we can not say that the insect
can use, the most efficiently, the same quality of light that is required by
the human eye.

According to the theory of Dubois, the quality of the light emitted by the
insect will depend upon the amount of violet light excited in the fluorescent
substance by the light generated in the photogenic cells. This would be
analogous to the recently developed mercury-vapor lamps backed by a dif-
fusing screen of some phosphorescent substance, in which the color of the
light of the combination is said to be decidedly different from that of the
mercury arc. According to this theory the bluish light of Photuris pennsyl-
vanica would be due to the abundance of the fluorescent light added to the
light produced in the photogenic cells. But the amount of fluorescent mate-
rial present in a single specimen of Photuris is exceedingly small, and but
little of it is situated in the abdominal segments; hence, this is not a satis-
factory explanation of the bluish color of the light emitted by this insect.
‘Turning to the Photinus pyralis we find the fluorescent material present in
great abundance, so that a single specimen suffices to produce a strong fluor-
escence in 5 to 10 c.c. of alcohol and water. The fluorescent material is most
abundant in the wings and in the thorax, with but a small proportion in the
photogenic segments. The amount of fluorescent material present in the
photogenic segments is not sufficient to impart, by fluorescence, a bluish
tint to the light generated in the photogenic cells, and it does not appear to
be sufficient to explain the production of the observed yellowish light by
absorption in passing through the fluorescent material. Indeed, it would be
poor economy to generate frequencies other than those observed and then
absorb them by the fluorescent material. No doubt occasionally a speci-
men may be found in which the light emitted is modified by absorption, as,
for example, in the sample of consanguineus mentioned on another page.
This, however, was an exceptional case, in which the photogenic material
extended farther than usual under the adjoining dark segment. Conse-
quently, in passing out through the outer brown integument, the shorter
frequencies (the violet) were absorbed, thus imparting a deep reddish hue
to the light emanating from the junction of the bright and dark segments.
If the insect had been observed in flight, or by a casual inspection, it might
easily have been said that this specimen emitted a far more reddish light
than the orange-red commonly observed. Close examination was required
to show that this departure from the ordinary orange-red did not extend
over the entire luminous segments.

40 A PHYSICAL STUDY OF THE FIREFLY.

XI. INFRA-RED ABSORPTION MEASUREMENTS.

In view of the fact that it has been impossible to detect infra-red radiation
emanating from the firefly, the question arose as to whether there is no infra-
red radiation, or whether the radiation is absorbed after it leaves the photo-
genic cells. Obviously there can be but little infra-red radiation absorbed
without special provision to keep the temperature normal, otherwise the
insect would become hot. ‘The solution of this question was undertaken
from two points of view. Ives* searched for radiation lying between 0.74
and 1.5u by noting the extinguishing action upon the phosphorescence of
Sidot blende when exposed directly to the light of the firefly, and when a
plate of red glass was interposed. Exposing the phosphorescing plate
directly to the light of the firefly, the intensity of the phosphorescence was
markedly reduced in half an hour, but exposing the plate through red glass
for 34 hours had no reducing effect upon the intensity of the phosphores-
cence of the Sidot blende. Since the reducing effect was practically the

T T T T “T T T T
50
3 Wave lengths in =.001 mm.
2 40b 4
MY
ca)
a
30 “I
g
fe}
a
“a '20- 4
5 b
a
g
BH IOP (a 4
4 1 Nilo x «© ix = O x} xX»

I 2 3 4 5 6 7 8 ou
Fic. 14.--Transmissivity of the integument of fireflies.

same for the infra-red to 1.5u as for the visible, it was concluded that no
radiations lying between 0.7 and 1.54 were emitted. Using a quartz spec-
trograph, no emission could be detected in the ultra-violet. ‘The writer
attacked the problem} by examining the opacity of the outer integument
(the chitinous layer) of various fireflies for different regions of the infra-red.
For, assuming that appreciable radiation is generated in the photogenic
cells, it is important to learn what frequencies can pass out into space.

In Fig. 14, curve a gives the transmission through one of the horny abdom-
inalsegments of a dried specimen of the Cuban firefly, Pyrophorus noctilucus.
This integument is dark brown (in reflected light), which causes heavy
absorption through the visible spectrum to the red, at which point the trans-
mission increases rapidly. In this firefly,it may be recalled, the abdominal
light is emitted through a slit between two of the segments, of which the

*Ives, Phys. Rev., 31, p. 637, 1910; Electrical World, 56, Oct. 13, 1910.
Electrical World, 56, p. 1012, 1910; also 54, p. 1184, 1909.

INFRA-RED ABSORPTION MEASUREMENTS. 41

above is a sample. The size of the specimen examined was about 1.5 X
7mm. In this same illustration, Fig. 14, curve 6 gives the transmission
through one of the eye-like fenestrate membranes on the thorax of the Cuban
firefly, and through which is emitted the so-called ‘thoracic light.”” The
specimen was only 1.52 mm., and hence very difficult toexamine. After
removing all the photogenic cell ligaments which adhered to it this specimen
was fairly translucent. In the region from 1p to 2u the transmissivity is
probably higher than observed, the low values being due to scattering of the
radiation in passing through the specimen.

Curve c, Fig. 14, gives the transmission through the outer integument
covering the photogenic cells of Photinus pyralis. ‘The specimen examined
was a single dried abdominal segment, 1.54 mm., which had been moist-
ened in water and freed from all ligaments and photogenic material. The
specimen was not very smooth and it was only semi-translucent. No doubt
a better sample could have been prepared by dissecting a fresh undried
insect. In both curves, 6 and c, the opacity at 1 to 2u and at 5u is greatly
increased by scattering of the transmitted radiation which did not reach
the bolometer. However, as was anticipated in previous communications,
these curves have the characteristic absorption spectrum of complex carbo-
hydrates, in which there is great (and usually complete) opacity from 2.8y
to 3.8u, and beyond 6, with a fairly transparent region at 4.54 to 5u. It
may be noticed that these specimens were dry. If they had been fresh,
then the water present would have absorbed considerable of the radiation at
4 to Su.

During the present summer an examination was made of the infra-red
absorption of the fresh photogenic substance of Photinus pyralis, excised and
placed between fluorite plates. The material was an inhomogeneous white
mass, which in the preliminary examination showed complete opacity from
3u to 3.8u, a more transparent region at 4.5 to 5.5u, followed by complete
opacity beyond 5.8u. This is the same sort of absorption as found in the
chitinous layer, Fig. 14. A thin layer of the white fluorescent fluid which
exudes freely from the wings of the pyralis was also examined between fluo-
rite plates. Complete opacity was observed from 2.9 to 3.24 and beyond
5.84; and an appreciable absorption band was observed at 4.74. These
bands were considered to be due to water, and since no others were observed
a more thorough examination seemed unnecessary. Apparently the fluor-
escent material has no appreciable absorption bands in this region of the
spectrum.

The experiments of Ives have bridged the gap from the visible to 1.5y.
The data then published by the writer showed that but little if any radiation
of wave-lengths greater than 2.5 would pass outinto spaceeven if generated
inthe photogenic cells. Thatwhichcan beemitted at 4.5u to 5.5u will beso
weak in intensity that it will hardly be distinguished from the emission spec-
trum due to the ‘‘animal heat’? (maximum emission at 8 to 1ou) radiated
from the surface of the body. By exclusion, we have, therefore, reduced the
unexplored region to that part of the spectrum lying between the wave-
lengths 1.5uand 2.5u. There are no types of radiators known which would
lead one to expect emission bands in this region of the spectrum of the light
of the firefly, and while it is important to be on the alert for the unexpected,

42 A PHYSICAL STUDY OF THE FIREFLY.

it seems just as probable that the explanation will be found in biochemical
phenomena not requiring the production of radiations of low frequency in
order to produce a given amount of light. Set as we are in our ways of
thinking, we insist that the methods employed by the firefly to produce its
light must be similar to our own. We might as legitimately suppose that
the combustion of food in our bodies, with the resultant formation of COs
and H:0O, or that the combustion in a growing leaf, must be accompanied by
a rise in temperature and emission of radiant energy (other than “animal
heat”) simply because we observe all these phenomena when we light a
candle or heave coal into the fire.

Tn recent papers* the writer has published numerous illustrations showing
that in incandescent solids and in gases excited in vacuum tubes the produc-
tion of light is accompanied by a very considerable amount of infra-red
radiation; that in the arc light, especially the “flaming arc’”’ vapors, there
is little infra-red radiation; and that in the high-frequency spark nearly all
the radiation is in the ultra-violet. The light of the firefly is not unlike that
of an ideal radiator at a temperature of 5,000°, differing from it only in the
density of the radiation. If the specific emissivity of the sun were as low as
that of the firefly, it is quite possible that no infra-red radiation could be
detected. ‘The solar corona is another example of this type. Perhaps the
question may become clearer when we can formulate some ideas on the
emissivity from a selectively absorbing substance in the region of a band of
anomalous dispersion. The ideal radiator is one having no reflectivity.
Only in the region of anomalous dispersion, where the refractive index
becomes practically nil, is this condition fulfilled, as far as the reflecting
power is concerned.

*Jahrb. der Radioaktivitét und Electronik, vu, p. 123, 1910; Illuminating Engineer,
London, 3, p. 1, 1910.

NATURE OF THE LIGHT EMITTED BY FIREFLIES. 43

XII. NATURE OF THE LIGHT EMITTED BY FIREFLIES.

It is beyond the scope of the present paper to discuss the various experi-
ments which have been made in the endeavor to learn how the firefly pro-
ducesits light. The older publications usually contain bibliographies which
may be consulted,* and on the following pages only such data are quoted
as have a very direct bearing on the subject.

The old idea that the light in animals is due to phosphorus has long been
refuted. The idea has also prevailed that the light is a species of phosphor-
escence. During the past fifty years various experiments have been made
which could have been used to refute this idea; but, owing no doubt to the
lack of a thorough knowledge of the behavior of phosphorescent materials,
not until within the past few years has this erroneous notion been losing
adherents. The phosphorescence idea no doubt originated from the fact
that some fireflies appear to be most active at dusk. ‘This observation of
the time of greatest activity in fireflies depends of course upon the extent
of the experimenter’s knowledge of the habits of the various species, some of
which donot appear until after it is dark, and are very active for hours there-
after, although it would not appear so to the casual observer, for the reason
that most of their activity (e. g., Photuris) is among the tops of tall trees.

The most important characteristics of phosphorescent material are (1) the
production of a ‘‘ phosphorescent light’’ which continues to be emitted for
some time after exposure to white light and especially to violet and ultra-
violet light; and (2) the extinction of this ‘‘ phosphorescent light”’ on expos-
ing the glowing material to infra-red rays. Some substances (e. g., fluorite)
emit a phosphorescent light, called thermo-luminescence, when heated to
from 50° to 200° C. Although the application of heat to insects has been
made from the beginning, it can not be a test for thermo-luminescence in the
living insect, because of the question of the heat acting as an irritant.

One of the first experiments tried by the writer, four summers ago, was
the effect of heat and cold (liquid air), of light and of darkness, upon living
and upon dead insects. The dry specimens showed no thermo-luminescent
light on heating; neither was there a phosphorescence when exposed to light.
The luminous organs of freshly killed, undried specimens did not show phos-
phorescence when exposed to white light or to the ultra-violet spark. Both
last year and this year the effect of X-rays was tried on fireflies, but neither
phosphorescence nor fluorescence could be detected. In the same manner
the precipitate from the fluorescent material found in the body fluid of fire-
flies, dried on filter paper, was not excited to luminescence by the X-rays or
violet light. ‘These experiments are of interest in connection with the data
showing that photogenic material may be kept in a dry state for a long time
when it can be made to glow by treating it chemically. Evidently the
photogenic material does not possess phosphorescent properties.

*Langley, ‘‘Cheapest form of light,” Amer. Jour. Sci. (3) 40, No. 236, 1890.
Seaman, The luminous organs of insects, Proc. Amer. Soc. Micros., 13-14, Pp. 133,
1891-2.
Watase, A treatise on the luminosity of insects. : 3
Mangold, ‘Die Produktion von Licht,’ Winterstein’s Handb. der Vergleich. Physi-
ologie, Band 3, 1911.

44 A PHYSICAL STUDY OF THE FIREFLY.

In parenthesis it may be added that the light of the firefly did not produce
fluorescence on the platinum barium cyanide screen, which is sensitive to
X-rays. According to Mangold (1. c.) the older experiments, which indi-
cated that fireflies emit X-rays, have long since been shown to be erroneous.
In his recent experiments, Ives* also has investigated the question of phos-
phorescence and the conclusion arrived at is that the light of the firefly has
none of the properties of true phosphorescence.

The oxidation theory has called forth more experiments than the theories
just mentioned. ‘To this end the firefly has been subjected to all sorts of
tests. For example, mechanical stimulation, such as percussion, pressure,
etc., causes the quiescent, dark photogenic organs to burst into a glow.
The application of heat or electric current causes the organs to glow. Ina
vacuum the freshly excised quiescent luminous organs have been found by
the writer to remain so, but on admitting air they glowed with great bril-
liancy. The color of the light was the same as the ordinary glow in the
living specimen.

The experiments on the action of chemical stimuli are very numerous, no
less than 68 different substances having been tried. ‘These substances
include gases and vapors; also liquids and solutions of acids, alkalies, salts,
and alkaloids. The most recent work along this line is by Kastle and
McDermott,} whose paper should be consulted, not only for their own con-
tributions, but also for their references (see appended Bibliography) to
similar tests by earlier experimenters. These experiments show that cer-
tain substances distinctly stimulate the light production; that others dis-
tinctly inhibit the luminescence; and that still others have little or no effect
on the light production. No definite relationship was found between chem-
ical composition and the power to excite the living photogenic material to
luminescence. The main point of interest is that anesthetics and related
compounds act as a powerful stimulus to the light production.

Probably the most important observation is the effect of water on the
production of light by the photogenic material. As early as 1798 Carradori
observed that the light emission is suspended by drying the photogenic
organs and is revived again by softening in water. This phenomenon has
been repeatedly verified by subsequent observers. Kastle and McDermott
have kept the dry material in sealed glass tubes for 13 months (to Sept.
9, I910; at the present writing almost two years have elapsed and the
tests show that the photogenic material is still active), at the expiration of
which time the specimens glowed actively for 15 minutes when moistened
with water in the presence of oxygen. This moistening and redrying was
repeated four times before the material ceased glowing.. Using hydrogen
peroxide solution in place of water, the glow was more intense.

In the conclusions to be drawn from the chemical aspect of the processes
involved in the production of light by living organisms, three factors are
required, viz, water, oxygen, and a substance to be oxidized. Just what the
substance is that is oxidized is not known. Some have assumed it to be a
fat, or an albuminous body. The histological study shows that the photo-

*Ives, Phys. Rev., 31, p. 637, 1910.
{Kastle and McDermott, Amer. Jour. Physiology, 27, p. 122, 1910.

NATURE OF THE LIGHT EMITTED BY FIREFLIES. 45

genic layer and the so-called “reflecting layer” are penetrated by innumer-
able trache, which are supposed to be filled with air. During the experi-
ments recorded herewith, it was found that the fireflies must have access to
plenty of fresh air to keep them healthy and active. In fact, to the writer it
appeared that they required more fresh air (oxygen) than would be required
in the ordinary respiratory processes. Other insects (e. g., several non-
luminous Elaters) did not seem so sensitive in this respect.

While it is generally accepted that the light production is connected in
some manner with an oxidation process, there is no data showing how the
light emission is brought about. Optically the so-called ‘“‘reflecting layer’’
has little if any meaning. Whether it is the storehouse for waste products
or (what is more plausible) a reservoir containing an oxidizable substance is
a subject deserving further consideration. As mentioned elsewhere, it
would be desirable to learn the condition of this layer with the age of the
insect, and especially after it has flashed for a long time. Since samples of
Photuris can be obtained which will flash in rapid succession for at least 1 to
13 hours, it would be interesting to compare its ‘‘reflecting layer” with that
of a specimen which has not flashed in this manner. But while this may
give some information in regard to the light production in fireflies, it will
hardly apply to all organisms. In some organisms the light is developed in
a less highly organized cell; and, indeed, in the case of some marine animals
a liquid is ejected which becomes luminous on diffusing in the water. In
this connection Dubois has already been quoted that the light is produced
when the ejected liquid passes from a colloidal into a crystalline state.
Ultimately it may perhaps be shown that the light production is the result
of energy transformations in passing from the colloidal to the crystalline
state. In this connection the optical properties of thermo-luminescent fluo-
rites has been the subject of some study by the writer, but no definite
results have yet been obtained. Just what happens when fluorite is warmed
and the thermo-luminescence destroyed is not known. Whether this has
any bearing on the light emission in animals is difficult to determine.

WASHINGTON, D. C., September 1, rgrt.

lose

EXPLANATION OF PLATE.

. Photograph of the spectrum of a carbon glow-lamp, for different exposures.
. Spectra of: (1) helium vacuum tube; (2) ‘4-watt” carbon glow-lamp; (3) firefly

Photinus pyralis, taken in 1909; (4) and (7) firefly Photuris pennsylvanica;
(5) and (6) firefly Photinus pyralis, taken in 1911.

. Spectra of: (1) helium vacuum tube; (2) from 0.3889u to 0.49224; fluorescence

spectrum of a substance found in the blood of fireflies, e. g., in Photinus pyralis;
(2) from o.5015u to 0.66784, spectrum of light emitted by Photinus pyralis;
(3) same as (2) from 0.50154 to 0.6678, using a shorter exposure.

. Photographs obtained with short-focus spectrograph:

Photuris pennsylvanica, 3: 8, 9, 12, 13. II: 9, 10. TII: 310, 11, 12. IV: 6, 7.
V: 7, 8,9. VI: 2, 5, 6. VIT: 2, 3, 4.

Photinus pyralis, I: 2 to 7. II: 2 to 8 and 14. III: 2 tog and 13, 14. IV:
2to5and8to1r4. V: 2to6and roto 14. VI: 7 to 14. VIT: roto 14.

Photinus consanguineus, 1: 11. Tis 1t,-12., VIT: 5, 6, 7, 8,9.

Photinus scintillans, 1: 14.

Glow-worm, IT: 13.

Helium comparison spectrum, VI: 3, wave-lengths from left to right 0.471, 0.492,
a 0.587, and 0.667p. I: 10, yellow helium line, \=0.5876y. (0.501p is
faint.

In D, column I is made up of two plates. The succeeding columns, II to VII inclusive,
are reproductions of single plates containing a dozen or more exposures to firefly
light, with the helium comparison spectrum at the beginning and the end of
each series. The original negatives are of course much stronger in the red than
here reproduced,

PLATE 1

COBLENTZ

° 666 -860086-
SOSSSGeees —

2.2 0 2 oe 2a a | 6-
a

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47