The internal female genital organs of Chaetodiopsis and Diasemopsis (Diptera: Diopsidae) and their systematic relevance

The morphology of the internal female reproductive organs of Chaetodiopsis meigenii and four Diasemopsis species, all based on material from Natal (South Africa), is descriptionbed. Variation is evident, especially in the form and size of the ventral receptacle, the ventral sclerite, and the spermathecae. Phylogenetic analysis of these characters, in combination with results available from other diopsids, resolves generic but not species relationships. It confirms that this character system can be valuable for phylogenetic analysis, as suggested by previous authors. The vast enlargement of the multi-chambered ventral receptacle in Chaetodiopsis and Diasemopsis has various implications on the possibilities of sperm competition and spermatozoa selection, that deserve further investigation. The ventral receptacle possibly acquires part of the sperm storage function from the spermathecae, that are concurrently reduced in number and size. Furthermore, the parallel arrangement of several hundred chambers, housing individual spermatozoa, provides an arena for spermatozoa competition. The elongation of the chambers, coinciding with the elongation of the spermatozoa, constitutes a new example for the coevolution of spermatozoa and female sperm receptacles.