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Abstract

Practically no other invertebrate has been used so extensively for the investigation of the visual system as the fly, particularly the genera Musca, Calliphora and Drosophila. The reasons are manifold: The fly’s visual system represents an intermediate grade of complexity at which sophisticated neural analysis is performed, but the underlying hardware is relatively simple. Not only are there far fewer neurones than in, for example, the vertebrate CNS, but the neurones are also organized into very precise repeating units (retinotopic columns). Thanks to extensive anatomical studies the neuroanatomical pathways, down to the level of single cells, are probably better known than in any other neuropil of comparable complexity (reviews: Strausfeld 1976; Strausfeld and Nässel 1981). The fly is also very amenable to behavioural experiments, and extensive quantitative data are available for a variety of visually guided behaviour (Reichardt and Poggio 1976; Poggio and Reichardt 1976; Heisenberg and Buchner 1977; reviews: Buchner 1984; Wehrhahm 1984).