Chap. 17 



RESPONSIVENESS THE SENSE ORGANS 



323 



Twilight eye 

 Daylight eye 



Twilight -^ 

 eye 



Daylight 

 eye 



Ganglion 



Optic nerve 

 Esophagus - 



A. FRONT FACE 







■■■■■■IS . \im 



:>:■■•■• i-. 



^ 



B. SECTION OF COMPOUND EYE 



Fig. 17.12. Top, face of adult male mayfly, Callibaetis. In the compound eyes of 

 mayflies there are hundreds of lenses each one set so deep in a tube that no rays 

 can reach it except those coming from directly in front of it. The segment at the 

 top, "the daylight eye," provides detailed vision; the other segment, "the twilight 

 eye" provides images or general vision. The daylight eye of this mayfly is twice the 

 size of a period on this page. The majority of mayflies are twilight fliers. Bottom, 

 section of the eye of an adult male Callibaetis, highly magnified. {Bottom, after 

 Shafer: "Divided Eyes of Certain Insects," Proc. Wash. Acad, of Sciences, March, 

 1907.) 



it fits into the bird's whole pattern of behavior (Fig. 17.13). Even if a jellyfish 

 had the eyes of a hawk, it would still lack the plunge of a hawk. 



Chemical Reactions of the Light Receptors. Although eyes have developed 

 in different epochs of evolution and in widely different kinds of animals, 

 they almost universally contain lenses and carotenoid pigments (Figs. 17.14 

 and 17.16). The lens guides the light to the receptors; the carotenoid pigments 

 in the receptors take part in the chemical reactions that create the nerve im- 

 pulses passing to the centers of vision in the brain. 



