MODIFICATION OF DISPERSAL PATTERNS OF BRANCHED PHOTORECEPTOR AXONS AND THE EVOLUTION OF NEURAL SUPERPOSITION

Using 3D reconstructions of serial ultrathin sections and extended-foc us pictures of Golyi-impregnated and cobalt-stained visual fibres. we studied the branched short photoreceptor axons found in ancestral dipt erans 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 Is neighbouring and next-bu t-one ommatidia, ill a regular pattern of asymmetrical neural pooling. Comparison of axon morphologies in different groups (Tipulidae, Chiro nomidae, Cullicidae) indicates that this pattern must represent an anc estral condition of the dipteran visual system and is thus a precursor of neural superposition found in flips (Brachycera) in which only one set of photoreceptors R1-R6 converges unto each cartridge instead of the four sets found in Chaoborus. It can be concluded that specific ax onal input channels from the large array of innervating photoreceptors found in midges have been retained during the evolution of neural sup erposition.