The relevance of neural architecture to visual performance: phylogenetic conservation and variation in Dipteran visual systems.

In cyclorrhaphan flies, giant tangential neurons in the lobula plate are supplied by isomorphic arrays of evolutionarily conserved achromatic elementary motion detecting circuits originating in the retina. The arrangements among giant tangential neurons is characteristic of a taxon and can differ between taxa having different visual performances. Observations of 12 brachyceran and 4 nematoceran species have identified different behaviors associated with visually stabilized flight. Neuroanatomical comparisons between closely related species having different behaviors and phylogenetically distant species that have similar behaviors suggest that such differences relate to differences of giant tangential cell architecture in the lobula plate. These functionally related differences contrast to anatomical features that reflect phylogenetic affinities. For example, the lobula plates of robber flies, typified by ballistic flight behavior, all differ from other taxa in lacking cyclorrhaphan-type vertical motion-sensitive neurons; instead, they possess an extra complement of horizontal cells in their place. The results suggest that, although circuits that compute elementary motion are conserved across the Diptera, selective pressure has resulted in modifications of their target neurons, thus contributing to the wide variety of visual behaviors observed within this group of insects.