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 which it flashed with the 

 same rapidity for another hour, then suddenly ceased flashing. In the 

 meantime it 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 ioo 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, 

 0.54/i), 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 



