Peripheral visual function in the fly

Abstract

Receptors and peripheral neurons of the dipteran visual system have been studied in the flies, Calliphora stygia (wild type), Musca domestica (white-eyed mutant) and Calliphora erythrocephala (chalky mutant), principally by means of intracellular recording. Recordings were made from: the large peripheral retinula cells, Rl-6; the small, central, retinula cells, R7 and R8; and the large monopolar cells (LMC' s), Ll, L2 and L4. Representatives of each cell type were identified by intracellular dye injection with Procion yellow. The angular sensitivity functions of the receptors are shown to be at, or near the limit set by lens diffraction and rhabdomere diameter. Receptors R7 and R8, whose rhabdomeres are narrower than those of Rl-6,also have narrower angular sensitivity functions. Many of the basic response properties of photoreceptors are described in detail for the first time. For R7 and R8, spectral sensitivities, polarisation sensitivities, absolute sensitivities, and dark and light-adapted intensityjresponse functions have been determined. In Rl-6 cells, lightadaptation shifts the peak of spectral sensitivity, and narrows the angular sensitivity function, but had no effect upon the polarisation sensitivity. These new results from identified receptors lead to a reappraisal of receptor involvement in optomotor behaviour. A new functional class of photoreceptor (Rl-6) is described in which the spectral and polarisation sensitivity are both dependent upon the intensity of the test flash-used to measure them. In addition, the polarisation sensitivity is strongly dependent upon the stimulus wavelength. The experimental analysis suggests that these anomalous properties derive from the events occurring in a single cell, and this finding may have important implications with respect to the process of transduction in photoreceptors. A detailed study is made of the processes of light-adaptation 2 in receptors Rl-6 and their postsynaptic elements, the LMC's. Consecutive stages of light-adaptation are isolated by comparing the performance of receptors and LMC's when subjected to identical regimes of dark- and light-adaptation over a 5 log unit range of intensities, and possible mechanisms of lightadaptation are considered. A comparison with the performance of higher order visual interneurons indicates that the majority of adaptation is completed at the level of the LMC. A comparison of the results with those of similar studies in other arthropods and, also the lower vertebrates reveals striking analogies, which, it is argued, represent an optimal strategy for light-adaptation. The receptors R7 and R8 are intensively investigated to elucidate their function in terms of the pigments in their rhabdome res. There are two spectral classes of R7 which appear to have completely different pigments in their rhabdomeres. The properties of RB are profoundly influenced by the absorption of light in the overlying rhabdomere of R7.