PHOTOGENIC ORGANS OF LAMPYRIDS 171 



The spectrum of the firefly has been studied by a number of 

 men, among whom are Young (70) , Langley and Very ('90), and 

 Dubois ('98). To quote Young: 



The spectrum given by the light of the common firefly of New 

 Hampshire (Photinus) is perfectly continuous, without trace of either 

 bright or dark. It extends from a little above Fraunhofer's line C, in 

 the scarlet, to about F, in the blue, gradually fading out at the ex- 

 tremities .... precisely this portion of the spectrum is com- 

 posed of rays, which, while they more powerfully than any others 

 affect the organs of vision, produce hardly any thermal or actinic 

 effect. 



By dissecting out the light-organs in living specimens and then 

 observing the same through the spectroscope a weak spectrum 

 was obtained; by adding hydrogen peroxide to the photogenic 

 organs the light, apparently unchanged in color, became more 

 intense and steadier. 



The purpose of the photogenic organs in insects has been 

 much studied. McDermott and Mast, among others, consider 

 the luminescence in the Lampyridae to be a secondary sexual 

 character. Others offer the explanation that they furnish pro- 

 tection to the insect in their warning or intermittent light emis- 

 sions. Olivier has made some interesting observations upon the 

 secondary sexual characters of these insects and shows that there 

 is a definite relation between the development of the photogenic 

 organs, the eyes, and the antennae. It is the combination of 

 these, and perhaps other organs which constitute a composite 

 sexual character. 



Inasmuch as the larvae .of many Lampyridae at least, are 

 gregarious, in that they inhabit defined areas, it is possible that 

 in this case, luminescence may assist in the preservation of the 

 colony. 



The adults of luminous Lampyridae are known to have a rather 

 well-marked periodicity in exercising their photogenic powers, 

 shining normally in the evening and not being long deceived 

 when placed during the day, in darkness. But Photinus pyralis 

 has been known to fly and to scintillate in the darkness caused 

 by the approach of a heavy thunder shower (see Laurent, Ent. 

 News, XXV, p. 334, 1914). But the larva of Photuris seem to 



