The photoreceptor axon projection and its evolution in the neural superposition eyes of some primitive brachyceran diptera.

All known brachyceran flies have inherited the same trapezoidal pattern of rhabdomeres in the ommatidia of the compound eye, different from that of their presumed nematoceran ancestors. The projection pattern of axons to lamina cartridges needs to match this in order to generate efficient neural superposition vision. This pattern has been traced in serial sections or via stimulated dye uptake in two families that diverged from the dipteran stem line more than 200 million years ago (Rhagionidae, Stratiomyidae) and in one family of intermediate age (Dolichopodidae). All show the same asymmetric projection as modern muscoid families and a concordantly 180 degrees-twisted axon bundle, suggesting that evolution to the 'modern' versions of both optics and projection coincided closely in evolutionary time. The basic developmental plan of the cartridge has also been retained since the origin of the Brachycera, but the synaptic participation of at least one axon (that of photoreceptor R7) varies markedly between species.