Efferent innervation of photoreceptors in spiders

The anterior median (AM) eye of the nocturnal spider Araneus ventricosus showed a marked circadian oscillation of sensitivity, but that of the diurnal spider Menemerus confusus showed no circadian oscillation. The AM eyes of the noct/diurnal spiders Argiope amoena and A. bruennichii have two types of photoreceptor cells with different sensitivities. The more sensitive cells showed a circadian oscillation of sensitivity, but the less sensitive cells did not. The circadian sensitivity change of the eyes was controlled by efferent neurosecretory fibers in the optic nerve. Illuminating the brain increased the frequency of efferent impulses in the optic nerve of Argiope, showing that certain photosensitive neurons are present in the brain. However, it seemed that the cerebral photosensitive neurons may be different from the efferent neurosecretory cells. The response of the cerebral photosensitive neurons increased transiently following diminution of the light intensity striking the eyes. The interaction between the cerebral photosensitive neurons and the eyes seemed to play a role in increasing this response.