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. 
