The sensitivity of receptors in the posterior median eye of the nocturnal spider,Dinopis

Summary1.Intracellular and extracellular (ERG) recordings were made of receptor responses in the posterior median eye of the nocturnal spiderDinopis subrufus. These are among the largest single lens eyes described among the arthropods and are used during prey capture at night.2.The receptor potential is a depolarising wave, typical of the responses recorded from other spiders, and from rhabdomeric photoreceptors in general. By comparison with other spiders, the receptor responses are slow in time course. Large (4–12 mV) discrete potentials are seen at very low intensities. The evidence suggests that bumps are responses to single photons. If this is the case the quantum capture efficiency for receptors shown to be in their diurnal state of photoreceptive membrane depletion is 7%, measured relative to photons incident on the cornea.3.Large responses to single photons and the known light gathering capacity of the lens (Blest and Land, 1977) make individual dark adapted photoreceptors very sensitive. A corneal flux of 5×105 photons·cm−2·s−1 produces an initial responses of half maximal amplitude. This corresponds to an intensity midway between starlight and moonlight under clear sky conditions. 2×105 more light is required to produce the same response in the diurnal jumping spiderPlexippus (Hardie and Duelli, 1978).4.The spectral sensitivities of single photoreceptors are almost identical to that of the massed receptor response (ERG), and this indicates a retina dominated by photoreceptors containing a single 517 nm rhodopsin. UV sensitivity is depressed by selective attenuation in the lens matrix.5.The mean receptor angular sensitivity function has a half-width of 2.3°, larger than expected from the optical data, but still sufficient for the resolution of spatial detail whose fineness matches the receptor mosaic.6.ERG recordings fail to detect the sensitivity shifts that should be associated with the daily cycle of photoreceptive membrane (Blest, 1978). Perhaps such shifts may still exist because we find that our test lights block the synthetic phase of photoreceptive membrane turnover.7.By comparison with vertebrate rods and cones,Dinopis photoreceptors exhibit the following properties of a scotopic system: large discrete responses to single photons, a slow time course of response, the possibility of electrical coupling between receptors and a spectral sensitivity in the blue/green. The resolving power ofDinopis retina is inferior to human rod vision at the same intensities, as expected of a smaller eye.