COMPARATIVE ULTRASTRUCTURE OF LAYER-I RECEPTOR MOSAICS IN PRINCIPAL EYES OF JUMPING SPIDERS - THE EVOLUTION OF REGULAR ARRAYS OF LIGHT GUIDES

Previous work has shown that the mosaics of Layer I receptive segments in the tiered principal (AM) retinae of most jumping spiders (Salticidae) are organised as regular arrays of light guides which are competent to sustain fine visual discriminations. The retinae are narrow strips which arise in development by lateral compression of a primordial hemispherical monolayer of nascent receptive segments. Foveal Layer I receptive segments each contain a single rhabdomere in most species, but simple geometry suggests that the developmental route will generate a vertical 'suture line' of sampling ambiguity in which contiguous rhabdomeres of adjacent segments act as single light guides. In members of two primitive subfamilies, the Lyssomaninae and Spartaeinae, such suture lines are indeed present; their optical consequences are discussed in the context of the evolution of foveal rhabdomeres that are long light guides. In several notionally advanced subfamilies collectively termed the Salticinae here for convenience, suture lines have been eliminated by rotations of the positions of single rhabdomeres with respect to the longitudinal axes of their receptive segments. The resulting mosaic patterns of rhabdomere distribution are similar in genera distantly related within the Salticinae, and are not bilaterally symmetrical with respect to horizontal axes bisecting the boomerang-shaped receptor fields. The basic pattern is not disturbed in genera in which Layer I receptive segments are separated from neighbours by a structureless extracellular matrix. This separation of segments conserves the organisation found in juvenile jumping spiders designated as 2nd instar by Blest (1988). The present material confirms that the evolution of retinal tiering preceded that of a foveal Layer I mosaic of high acuity in the Lyssomaninae as well as Spartaeinae (Blest and Carter 1987). The evolutionary history of Layer I in the Salticinae remains obscure. © 1990 Springer-Verlag.