Sperm ultrastructure in Chironomoidea (Insecta, Diptera).

The fine structure of spermatozoa from several species of chironomids, of Culicoides sp. (Ceratopogonidae) and of Odagmia pontina (Simulidae) was studied. A synapomorphic feature, consisting of nine kidney-shaped structures forming the centriole adjunct, was found in the chironomid species. All members of Chironomoidea share a mono-layered acrosome and a flagellar axoneme, provided with accessory tubules with 15 protofilaments in their tubular wall. The axoneme has a 9+9+2 pattern, but in an unidentified species of chironomid, a 9+9+0 model was observed where the central complex and the spokes are missing. Sperm motility is, however, maintained in all the examined species. The spermatozoa of this taxon have the tendency to complete maturation during their progression along the deferent ducts. Thus, in the proximal region of these ducts, they often show remnants of the spermatid cytoplasm.