058 RADIATION BIOLOGY 



optic units, the compound eye might provide for appreciation of distance. 

 He saw no significant difference between the central difficulty of summing 

 information from 12,000 lens-equipped eyes and the difficulties for man's 

 brain to correlate impressions from two. Yet Plateau (1888) was unable 

 to find reactions to moving objects beyond a few centimeters and con- 

 cluded that all compound eyes were hopelessly myopic. Only when the 

 angular size of objects in the visual field and the angular divergence of 

 ommatidia were appreciated was this difficulty cleared up. Yet Notthaft 

 (1881), who provided the needed information, was not much more opti- 

 mistic than Plateau, since he assumed that each ommatidium followed 

 an all-or-none course in detecting targets. 



Evaluations of acuity and binocular fields do check well with behavior 

 (Zacharias, 1890; Luedtke, 1938, 1940), and the structure of the omma- 

 tidia is related to the amount of light available. For example, the tsetse 

 fly Glossina suhmorsitans, which forages in open country, has more omma- 

 tidia per unit solid angle than G. tachinoides, which haunts shady thickets 

 (Eltringham, 1936). 



No confirmation has been found for the claim of Vigier (1904) that 

 insects of rapid flight, such as dragonflies, have an accommodatory 

 mechanism consisting of elastic and extensible parts represented by tra- 

 cheae and myofibrils. According to Vigier, contraction of the myofibrils 

 diminishes the curvature of the eye, and elasticity of the tracheae pro- 

 vides force for recovery. What good these mild gross changes would do 

 for the compact ommatidia is hard to visualize. Any flexing would occur 

 at the boundaries of facets rather than change the curvature and focal 

 length of each cuticular lens. Vigier' s view seems to depend upon the 

 reality of image formation at receptor level, and this is lacking in the 

 apposition eyes characteristic of fast-flying insects. 



Controversy continues as to whether degeneration of compound eyes 

 precedes or follows adoption of a lightless habitat (Pike, 1943). Com- 

 monly cave-dwelling species belong to genera in which considerable vari- 

 ation is known in the eye development of species living in illuminated 

 habitats. Reed (1898) concluded that blind trilobites included both 

 adaptive and degenerate types. In blind shrimp investigated by Neher 

 (1901), the young had far greater eye development than the adults, and 

 neural connections never degenerated. Kapterew (1912) found no irre- 

 versible loss of eye pigmentation in generations of Daphnia reared in the 

 dark, but depigmentation of his initial light-adapted strain began after 

 12 days in the dark; in succeeding generations depigmentation (appar- 

 ently by phagocytosis) began 4 days after birth. Tschugunoff (1913) 

 noted stages of eye-pigment degeneration in the dark for the crustacean 

 Leptodora. Genetic lack of eye pigment was suspected in an amphipod 

 (Gegenbaur, 1858) and the daphnid Simocephalus (Banta, 1921). Defi- 

 nite differences in light reactions correspond to eye mutants in the moth 



