Modification of dispersal patterns of branched photoreceptor axons and the evolution of neural superposition

Abstract

Using 3D reconstructions of serial ultrathin sections and extended-focus pictures of Golgi-impregnated and cobalt-stained visual fibres, we studied the branched short photoreceptor axons found in ancestral dipterans and in the scorpionfly Panorpa. In the 'phantom' midge Chaoborus (Nematocera), each cartridge of the lamina neuropil is innervated by collaterals of 24 photoreceptor axons from 18 neighbouring and next-but-one ommatidia, in a regular pattern of asymmetrical neural pooling. Comparison of axon morphologies in different groups (Tipulidae, Chironomidae, Culicidae) indicates that this pattern must represent an ancestral condition of the dipteran visual system and is thus a precursor of neural superposition found in flies (Brachycera) in which only one set of photoreceptors R1–R6 converges onto each cartridge instead of the four sets found in Chaoborus. It can be concluded that specific axonal input channels from the large array of innervating photoreceptors found in midges have been retained during the evolution of neural superposition.