CENTRAL-NERVOUS-SYSTEM OF BRACHYCERAN LARVAE (INSECTA, DIPTERA)

A comparative study of the external organization of the central nervou s system (CNS) in various brachyceran larvae has been undertaken, whic h revealed distinct steps of a progressive fusion and/or contraction o f ganglia and adherent structures. Closely apposed compound eye Anlage n and supraesophageal ganglia have been found in all larvae examined, and thus represent a ground plan feature of the Brachycera. The primit ive arrangement of the ventral ganglia appears to be a ''five-limbed c hain'', where the first five ganglia an closely apposed (Xylophagidae, Vermileonidae, Therevidae and Bombyliidae). In Stratiomyidae, Atheric idae, and Empididae, all ventral ganglia are fused to form a ''12-limb ed chain''. The most contracted CNS among Orthorrhapha is found in Tab anidae where external segmentation of the ventral chain is rudimentary . In Cyclorrhapha with primi tive characters, i.e. Lonchopteridae, Pho ridae and Platypezidae a single mass of completely fused ganglia is fo und as well as in groups with derived characters. Thus the apomorphous state of these features for the Cyclorrhapha is confirmed. According to the generally accepted traits of Brachyceran phylogeny, several con vergent lines of fusion of the CNS need to be suggested. Otherwise, a closer relationship between Stratiomyomorpha, Tabanomorpha, Empidoidea and Cyclorrhapha (Muscomorpha) had to be assumed.