Structure of the coxa and homeosis of legs in Nematocera (Insecta: Diptera)

Construction of the middle and hind coxae was investigated in 95 species of 30 nematoceran families. As a rule, the middle coxa contains a separate coxite, the mediocoxite, articulated to the sternal process. In most families, this coxite is movably articulated to the eucoxite and to the distocoxite area; the coxa is radially split twice. Some groups are characterized by a single split. The coxa in flies is restricted in its rotation owing to a partial junction either between the meron and the pleurite or between the eucoxite and the meropleurite. Hence the coxa is fastened to the thorax not only by two pivots (to the pleural ridge and the sternal process), but at the junction named above. Rotation is impossible without deformations; the role of hinges between coxites is to absorb deformations. This adaptive principle is confirmed by physical modelling. Middle coxae of limoniid tribes Eriopterini and Molophilini are compact, constructed by the template of hind coxae. On the contrary, hind coxae in all families of Mycetophiloidea and in Psychodidae s.l. are constructed like middle ones, with the separate mediocoxite, centrally suspended at the sternal process. These cases are considered as homeotic mutations, substituting one structure with a no less efficient one.