(•,50 RADIATION BIOLOGY 



mittiiig the eyes to follow flickering at much higher rates. Since the 

 criterion for visibility of flicker was irregularities in the electroretinogram, 

 the conclusion does not relate to the central nervous system but to events 

 within the compound eye itself — perhaps unlike rates of recovery after 

 photochemical bleaching of the primary photosensitive pigment. 



Flicker detection involves discrimination between two unlike intensi- 

 ties of stimulation presented in succession. Presumably the problem is 

 scarcely diff"erent from that of two unlike intensities presented side by 

 side simultaneously. For flicker the arthropod eye has been tested at 

 length in terms of the intensity differences required. For detection of 

 movement in the visual field a honeybee at its optimum intensity must 

 have one stimulus 25 per cent greater or less than the other for detection 

 of a difference (Wolf, 1933a,b). For the fruit fly Drosophila the differ- 

 ence must be of the order of 225 per cent (Wald and Hecht, 1933; Hecht 

 and Wald, 1934). For the human eye, for comparison, a difference of 

 1.5 per cent is entirely adequate in good illumination. Hence the visual 

 field of the arthropod eye contains a gray scale with far fewer steps than 

 are characteristic of the human eye. The range of sensitivity is seldom 

 so great, and only a fraction of the 500 stepwise increments between black 

 and white detected by the human eye can be distinguished by the 

 arthropod. 



The contention of van der Horst (1933) that each ommatidium of an 

 apposition eye must operate as a unit like a photometer, without detecting 

 any image, still leaves room for additional abilities. The receptor cells 

 in each ommatidium may be of two or more physiological types. If their 

 photosensitive pigments are unlike, the wave length at which maximum 

 sensitivity is reached in one receptor population could be different from 

 the wave length for maximum in another population. This would corre- 

 spond to the rod-and-cone system of the vertebrate eye, and a Purkinje 

 shift with major changes in intensity could be shown in the spectral- 

 sensitivity characteristics. So far this duality of receptor system has 

 been demonstrated only in the fruit fly Drosophila (Fingerman, 1952; 

 Fingerman and Brown, 1952). A neural basis for utilization of nerve 

 impulses from separate receptor populations was described in dipteran 

 eyes by Cajal (1909). Sanchez (1922, 1923) extended Cajal's findings 

 into a theory to explain hue discrimination among arthropods, using a 

 mechanism similar to that postulated by Young and Helmholtz for the 

 vertebrate eye. No evidence has been offered to substantiate Sanchez's 

 proposed differences among the receptor populations. But then no 

 physiological differences have been found to have anatomical or chemical 

 counterparts among human cone-cell populations in a color-sensitive eye. 



Many arthropods do show definite hue discrimination. A peripheral 

 basis for this ability was discovered by Graham and Hartline (1935) in 

 the xiphosuran Limulus, even though no hue discrimination seems present 



