188 THE PHYSIOLOGY OF INSECT SENSES 



Stumpf concluded that there need be only two receptor systems to 

 account for the data, a blue-green receptor and a red receptor. The 

 maximum for the blue-green receptor lies between 500 and 550 my.. Its 

 sensitivity extends from the region of the near ultra-violet up to about 

 660 mil. The maximum for the red receptor lies in the vicinity of 

 625 my.. Its sensitivity extends from about 480 up to about 700 my.. 

 Several flies were found which were colour-blind when tested with 

 flicker and whose spectral sensitivity curves were abnormal. The 

 spectral sensitivity curve of one possessed the usual peak in the red but 

 none in the blue-green. This fly possessed no colour discrimination in 

 the region 400-500 my. as judged by flicker. Another fly lacked the 

 peak in the red (630 m^y.) and was totally colour blind. This fly possessed 

 the usual red eye-pigments in the pigment cells of the ommatidium. 

 Later Antrum (1955 a) described the spectral sensitivity of a white- 

 eyed mutant which lacked these pigments. Since this mutant also 

 lacked a sensitivity peak in the red (630 my.). Antrum concluded that 

 the usual 630-m[x peak resulted from transmission of long wave- 

 lengths by the red shielding pigments. 



Behavioural experiments conducted with a revolving drum and 

 light of low intensities did not reveal a marked sensitivity in the red 

 (Schneider, 1956). To reconcile the behavioural and electrophysio- 

 logical results Schneider proposed that since the enveloping pigments 

 of the ommatidia are partially transparent to red light, the ERG would 

 be higher at red than at other wavelengths because the red would be 

 stimulating a greater number of ommatidia. On the other hand, the 

 failure of the screening pigments to isolate the ommatidia when the 

 eye was illuminated with red light would lead to a loss in visual acuity 

 and since the behaviour in a revolving drum depends upon visual 

 acuity, it would fail to reveal a sensitivity to red. Antrum (1961) has 

 reported, however, that even in a white-eyed mutant, which completely 

 lacks screening pigment, the visual acuity equals that of wild-type flies. 



Recently Burkhardt and Antrum (1960) have been able to record 

 electrical events directly from single visual cells. The spectral sensi- 

 tivity was determined for thirty-nine single retinal cells of the wild type 

 of Calliphora and from the white-apricot mutant which lacks screen- 

 ing pigments (Antrum and Burkhardt, 1961). The red screening pig- 

 ment does not absorb Hght in the region 616 m.y., and in weakly 

 illuminated elements the amount of scattered red light is much greater 

 than that amount of light coming through the lenses. Except in these 

 weakly illuminated elements and in the far red, the screening pigments 

 are not relevant to the shape of the sensitivity curves. All cells in the 



