PHOTORECEPTION 189 



dorsal part of the eyes were found to have curves with a maximum at 

 489 ma and one at 345 mix. Three receptor types were found in the 

 ventral region of the eye: a green receptor (maxima at 491 m[x and 

 345 m[x); a blue receptor (maxima at 468 m[x and 345 mix); a yellow- 

 green receptor (maxima at 524 mix and 345 mix). Some elements show a 

 peak at 616 m^x. The numerical relationships among the green, blue, 

 and yellow-green receptors is 18:4:3. This is almost the ratio to be 

 expected if one assumed that of the seven retinula cells in an ommati- 

 dium five are green receptors, one blue, and the remaining one 

 yellow-green. Since all have a peak at 345 m[x, corresponding to a 

 stable position of the p-absorption in derivatives of vertebrate visual 

 pigments, it is possible that the most common visual pigment in 

 Calliphora is retinene with a maximum absorption at 490 mjx (Autrum 

 and Burkhardt, 1961). 



Some workers have been tempted to think of the insect eye as 

 possessing analogues of rods and cones, especially since Hanstrom 

 (1927) demonstrated retinal cells with short and long axons and 

 suggested the analogy. But Wolf and Zerrahn-Wolf (1935 a), in dis- 

 cussing the relation between threshold intensities and time of dark 

 adaptation for the honeybee, called attention to the fact that the curve 

 describing this relation was, unlike that for the human eye, a smooth 

 one. Nor do curves relating visual acuity and flicker fusion frequency 

 to intensity exhibit any changes in slope that could be attributable to 

 two populations of receptors of different sensitivities analogous to 

 rods and cones (Hecht and Wolf, 1929; Wolf, 1933e;HechtandWald, 

 1934; Wolf and Zerrahn-Wolf, 1935 b). In Drosophila, however, 

 Fingerman and Brown (1952, 1953) reported a 'Purkinje shift', that is, 

 a shift of sensitivity of the eye towards shorter wavelengths as the light 

 intensity is decreased. They interpreted this as indicating a shift from a 

 photopic to a scotopic mechanism. According to their conclusions, 

 Drosophila loses its ability to discriminate among wavelengths as 

 intensity is lowered. No such change could be demonstrated in the 

 cockroach (Walther, 1958 a, 1958 b), nor in the honeybee (Goldsmith, 

 1960, 1961). The data of Fingerman and Brown (1952, 1953) have 

 been criticized by Goldsmith (1961). On the other hand, Weiss et al. 

 (1942) reported that the order of attractiveness of 365 my. and 470- 

 528 mjx at low intensities was reversed at high intensities. With 

 Calliphora Autrum (1955 a) found that the wavelength of maximum 

 sensitivity was 540 mpi at high intensities but shifted to shorter wave 

 lengths (480 m^x) as the intensity was dropped. Schneider (1955, 1956) 

 reinvestigated this problem by comparing spectral sensitivity curves 



