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Abstract

The function of the facet-separating pigments in the compound eyes of the fruitfly Drosophila melanogaster with hypernormal (se), normal (+), subnormal (wa), and missing (w) pigmentation was studied by investigation of: (1) the in-flight optomotor responses to movement of striped patterns with a mean brightness of 300 cd/m2, and (2) the retinal action potentials evoked by flashes in a program of .0003 cd/m2 average brightness. The pigment deficient mutants (w a, w) are less sensitive to the pattern contrast in the bright adapted state, and more sensitive to the flash intensity in the dark adapted state than either the wild-type (+) or the overpigmented mutant(se). These differences are complementary and can be explained by the increased translucency of the pigment cells. Thus the photoreceptors in the equally illuminated eyes of the normal and mutant animals +, se, w a, and w are expected to receive light in a ratio of about 1∶1∶7∶19. However the sensitivity of the receptors as well as the half-peak widths and the density of their visual fields are apparently independent of the eye pigmentation and seem to be equal at common levels of adaptation. The effects of omnidirectional excess light reaching the receptors of the pigment deficient mutants can be simulated in less translucent eyes: when certain amounts of background illumination were combined with the optomotor stimulus in the visual fields of the wild-type receptors it was possible to elicit the predicted “mutant behavior”.