the; function oi^ the light emitted by uving 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 ; 

 id 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 . f 

 '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 pennsyhanica. 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 Photimis 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. For 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 {Phcngodcs) is said to occur in the daytime ; 

 hence the light emitted can not be of great service in mating for that 

 purpose. 



*P(itter, Leuchtende Organismen, Zeitschr. f. AUg. Physiol., 5, 1905. 

 tMangold, "Die Produktion von Licht," Winterstein's Handb. der Vergleich. Physi- 

 ologic, Band 3, 191 1. 



