ADAPTATIONS 435 



exceptionally brilliant light, which may, therefore, enable the 

 winged male to find her. In many cases the eyes of these 

 nocturnal insects are unusually well developed, as is also the 

 case with the deep sea fishes, a fact that renders probable the 

 view that the luminescent organs are a means by which the 

 sexes find each other. The luminescence of the eggs and larvae 

 are not understood. Photogenic organs are common in other 

 families of beetles, and also occur among flies. 



873. Oxygen is said to be necessary to the action of photo- 

 genic organs, but no appreciable heat is generated. 



874. Electrical Organs. — Organs for generating electricity 

 are developed in a number of Fishes. The electric eel (Gym- 

 notus) of the Amazon and Orinoco rivers, the electric catfish 

 (Malapterurus) of tropical Africa, and the electric rays (Tor- 

 pedo) of the warmer seas are all capable of producing an 

 electrical discharge sufficient to stun large animals. The 

 electric organ of the Gymnotus is a modified muscle of the 

 ventral side of the long tail. In Torpedo the organ is also of 

 modified muscles, but of the head region. In Malapterurus the 

 glands of the skin have been the starting point from which 

 the electric organ developed. In neither case, of course, 

 does the fully developed electric organ bear any resemblance 

 to muscle or gland. The nerves supplying the electric 

 organs are developed to an extraordinary degree and the 

 electric discharge is under voluntary control. These organs 

 are doubtless organs of offense and defense. 



875. Instinct. — When the Pronuba moth deposits her egg in the 

 pistil of the Yucca and then stuffs pollen in the stigma she is not 

 aware of the end secured by her acts. She does not know that 

 pollination will cause the ovules to develop. She does not know 

 that there are ovules. She cannot even know that she has de- 

 posited an egg. She never sees the eggs; she never sees her off- 

 spring. The whole performance is to her without meaning 

 and is enacted in obedience to an internal impulse originat- 



