Retinal mosaics of the principal eyes of two primitive jumping spiders, Yaginumanis and Lyssomanes : clues to the evolution of Salticid vision

Abstract

Retinae of the principal eyes of jumping spiders (Salticidae) have four tiers of photoreceptors. A previous paper described the receptor mosaics in all four layers at the retinal fovea for two forms, Lagnus and Portia , shown by behavioural experiments to possess high visual acuities. Only Layer I farthest from the dioptric apparatus has a mosaic quality that can sustain the demonstrated visual discriminations, leaving the roles of Layers II—III open to question. We now describe the retinal mosaics of two presumptively primitive species Yaginumanis is placed with Portia in a newly erected subfamily of Salticidae, the Spartaeinae, whose members possess functional posterior median eyes; those of Lyssomanes are vestigial as in advanced Salticids, but the genus is usually considered primitive and its ethology indicates restricted visual capabilities. Layer I receptors of Yaginumanis each bear two rhabdomeres on opposite faces of the cell which are contiguous with those of adjacent receptors, so that optical pooling substantially limits acuity. The same arrangement is present in receptors in the peripheral retina of Lyssomanes but at the fovea each cell bears only a single rhabdomere, as in Salticids with high visual acuities. In both species, Layer II receptors have two rhabdomeres throughout the retina and centre-to-centre spacings that approximately match those of the Layer I receptors that they overlie. Layers III and IV are essentially the same in all Salticids: Layer III amounts to a virtually continuous sheet of rhabdomeres, and Layer IV is divided into three regions with a complex organisation. Neither is suitable to sustain fine visual discriminations. The organization of the salticid principal retina emerges as anatomically conservative, and contrasts with that of the secondary eyes which varies greatly between primitive and advanced species. A working hypothesis is proposed to explain its evolution.