648 RADIATION BIOLOGY 



1933). In eyes so limited there seems to be no possibility that an image 

 can be formed at receptor level, so that detection of intensity differences 

 seems to be all that the ommatidium can provide for. Compound eyes 

 used at light intensities of the order of daylight operate in this way, with 

 each ommatidium isolated from its neighbors, and any central picture of 

 the outside world must be built by the nervous system from intensity 

 information sent along the many individual optic nerve fibers. 



Compound eyes used in twilight and night intensities have the pigment 

 very differently distributed. Grenacher (1879), who discovered this dif- 

 ference, referred to the isolated ommatidia of a day-type eye as com- 

 posing an "apposition" type of compound eye, whereas the lack of iso- 

 lation in a night-type eye permitted light entering many ommatidial 

 lenses to be refracted and fall on the receptors of a central visual unit ; 

 hence the night-type eye was a "superposition" eye. The ray paths 

 were traced by Exner (1889a, 1891), and attention was drawn to optical 

 inhomogeneity within the corneal lens and crystalline cone, so that the 

 dioptric system functioned in a far more complex fashion than merely 

 as a long lens. In 1890 Szczawinska described migration of the masking 

 pigment in a single compound eye, permitting the same organ to operate 

 as an apposition type by day and a superposition type by night. The 

 same action was discovered in a wide variety of crustaceans and insects 

 (Stefanowska, 1890; Exner, 1889b, 1891; Herrick, 1891; Kiesel, 1894; 

 Parker, 1895). 



Not until Perkins (1928) learned that pigment migrations in crustacean 

 compound eyes were controlled by hormones released into the arthropod 

 blood, called forth by a reflex stimulation through the nervous system, 

 did the time course of events make sense. Later it was learned that 

 inherent diurnal rhythms in hormone production complicate the picture 

 still more (Bennitt, 1932a,b; Welsh, 1935, 1936; Jahn and Wulff, 1941). 

 Moreover in decapod crustaceans two different sets of pigment cells are 

 involved — a distal set and a basal set— and the receptors themselves con- 

 tain a migrating pigment (Welsh, 1930). The literature on this subject 

 has become ciuite extensive, but most of it centers on the hormonal 

 aspects rather than on the effect on vision and reactions to light stimu- 

 lation. In insects, moreover, there is evidence that local responses and 

 reactions mediated directly through the nervous system may explain pig- 

 ment migration without involvement with hormones (Demoll, 1911; Day, 



1941). 



In many arthropod compound eyes a white or yellowish pigment is 

 contained in special cells associated with the basement membrane through 

 which the optic nerve fibers pass from the ommatidia. This is a tapetum, 

 and pigment-cell migrations may expose it at low light intensities, per- 

 mitting radiant energy received by the eye to have a second — reflected — 

 chance to affect the receptor cells. As such it becomes an "occlusible" 



