370 



The Journal of Heredity 



insect. The light might serve to at- 

 tract prey; it might illuminate the path 

 of the insect ; its prey might be blinded 

 by the brilliant flash, and thus its escape 

 be prevented ; enemies might be blinded 

 similarly, so that they would be unable 

 to attack the torch bearer successfully; 

 some think it may serve only as an 

 adornment; but the last and most 

 plausible view of the matter is that it 

 serves as a signal between the male and 

 female of the same s])ecies. 



"At night the males leave their 

 concealment and fly about. A little 

 later the females ascend to the tops of 

 blades of grass and remain there without 

 glowing. A male passes by and flashes 

 his light; the female flashes back. 

 Instantly he turns in his course to the 

 spot whence the signal came and alights. 

 He signals again. She replies. He 

 ascends the blade, and if he cannot find 

 her, he signals again and she responds. 

 The signals continue until the female is 

 found, and the drama of sex is finished. 



"Mast has recently shown that the 

 female firefly docs more than simply 

 respond to the signal of the male. If a 

 male flies above and to the right of the 

 female, she bends her abdomen so that 

 its ventral surface is turned u]jward 

 and to the right. If the male is above 

 and to the left, the light is turned in 

 this direction. If the male is directly 

 above, the abdomen of the female is 

 twisted almost upward. But if the 

 male is below her, she emits her hght 

 without turning her body." 



If a number of females are caught and 

 placed in a bottle out of doors on a 

 suitable night, mem1)ers of the opposite 

 sex will often fly u]) and light on the 

 coat of the person holding the bottle. 

 It might ajjijcar that this is due to an 

 odor emitted by the females which 

 could remain on the person handling 

 them, but the following ex])eriment of 

 S. O. Mast demonstrates conclusively 

 that the light is the determining factor. 

 If a female is ])laced where the reflec- 

 tion of her flash may he seen in a 

 mirror by a male, he will start for the 

 mirror every time she flashes, even if 

 some color screen be interposed. This 

 brings up another question, how the 

 male can determine whetlier the flash 



is that of a female or not, if its color be 

 thus changed. It seems probable that 

 the duration of the flash is in this case 

 important, and many observers have 

 deduced that the flash of the female 

 differs slightly in duration from that 

 of the male, although often the differ- 

 ence is imperceptible. In some species 

 the male emits two short flashes con- 

 secutively, while the female emits one 

 long fulmination of light. In such a 

 case, the duration of the flash would 

 explain the matter satisfactorily, and 

 it seems probable that differences de- 

 tected by the insects might be too mi- 

 nute for the human eye to observe. 



CONTR.\DICTORY CASES 



In one species of Lampyridae, the 

 larva possesses photogenic power while 

 the adult does not. In such a case, 

 the light can hardly be a secondary 

 sexual character. Furthermore, the 

 glow-worm frequently mates in the day 

 time, when the light would be of 

 practically no use. But evidence points 

 strongly to the fact that the flash is of 

 assistance to the male in locating his 

 mate in many instances. 



But whatever its use, the mechanism 

 which produces the light is perhajjs more 

 of a mystery than the use itself. The 

 structure of the photogenic ajjparatus 

 is apparently the same in all species. 

 As heretofore mentioned, in Photinus. 

 the male has more fully developed 

 photogenic organs than has the female, 

 while in Photuris. both sexes are 

 equally provided. When the flash is 

 emitted, the entire abdomen generally 

 glows, but this is due to i)art of the 

 photogenic a])])aratus, and not to any 

 fluorescence of any ])art excci)t the 

 definitely marked i)hol()genic arcrs of 

 the abdomen. 



Throughout all of the light organs, 

 fine tubes are found, connected with 

 the main air system and evidently the 

 pathway of oxygen. This would seem 

 to indicate that oxidation is the source 

 of the light, and that air is forced 

 through the line jiassages in the ])hoto- 

 genic tissue, wlurc tlu' oxidation might 

 take place. If one part of the light 

 organs is paralyzed by ])ressure, it will 

 not flash but merelv glows, while the 



