Retinal mosaics of the principal eyes of some jumping spiders (Salticidae: Araneae): Adaptations for high visual acuity

Summary Retinal mosaics of the four tiers of receptors (layers I–IV) in the principal eyes of two species of salticid spider known to exhibit high visual acuity are described at the ultrastructural level. The four tiers of receptors have been previously classified byLand (1969 a) andBlestet al. (1981). Only the tier (layer I) farthest from the dioptric system has a mosaic quality capable of sustaining the visual discriminations that have been demonstrated by behavioural experiments. More distal layers are of poor to negligible mosaic organization.It is shown that rhabdoms of layer I are disposed with respect to their surrounds in such a way as to enable them to act as light-guides: the ultrastructure of layer I receptors appears to be designed to minimize optical cross-talk between receptors. The cytoplasm of the more distal layers is packed with mitochondria which have been shown to equilibrate the refractive indices of the rhabdoms and the material that surrounds them. It is argued that the mitochondria show no evidence of metabolic activity, and that an optical role may be their only one. Their effect will be to minimize the degradation of images received by layer I.