742 



THE EYE IN EVOLUTION 



her ; if she stops emitting light he wanders off aimlessly but if he reaches her in time 

 the two join together in the " mating dance," scattering sj^erm and eggs in a lunninous 

 spiral in the water (Galloway and Welch, 1911). The mating of the fire-fly, Photinus, 

 is equally pretty. The male fire-fly dances in the air in the evening intermittently 

 flashing a light ; in the grass the female glow-worm responds by an answering flash 

 exactly two seconds later, turning her abdomen with its luminous organs towards 

 him (Figs. 893-4), and immediately the male flies directly towards his mate.^ Within 

 a species the timing of the answering flash is the important recognition signal and the 

 eager male can be tricked by a flash-light on the ground provided the proper interval 

 is maintained (Buck, 1937). 



Luminous flashes also serve as social signals, particularly among schools 

 of fishes ; while a protective function is equally well established. They may 



Figs. 893 and 894. — The Luminous Organs of Lampyris ><plesdidula 



Fig. 893.— The ventral surface of the 

 female glow-worm. There are paired 

 lateral luminous organs on segments 

 2 to 6, a small median organ on seg- 

 ment 3, paired median organs on 6, and 

 a large unpaired organ on segment 7. 



t 



Fig. 894. — The ventral surface of the 

 male fire-flJ^ There are only 2 median 

 luminous organs on segments 5 to 6 

 (after Bongardt). 



scare a predator or even serve as a warning to other members of the species, 

 while they act as a means of concealment by dazzling an enemy. Thus, 

 when attacked, the bathypelagic shrimp, Acanthe])hyra, ejects from gland- 

 like luminous organs a luminescent cloud in which it escapes (Harvey, 1931) 

 (Fig. 895) ; two deep-sea prawns found in the Indian Ocean emit a substance 

 of the same nature from their antennary glands (Alcock, 1902) ; while the 

 deep-sea squid, Heteroteuthis, ejects a similar cloud, the counterpart of the 

 black ink of its shallow- water relative. A deep-sea fish, Malacocephalus 

 Icevis, uses a gland near the anus in the same way (Hickling, 1925-26). A 

 peculiar sacrificial protection is suggested by the behaviour of the scale- 

 worm, Acholoe ; if it is cut in two by a predator, the posterior portion 



1 p. 58 



