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. 
