The Structure of the Female Genital System of the Diving Beetle Scarodytes halensis (Fabricius, 1787) (Hydroporinae, Dytiscidae), and the Organization of the Spermatheca and the Spermathecal Gland Complex

SIMPLE SUMMARY: The large group of Dytiscidae, with more than 4300 species, is characterized by an extreme variation in male and female genital apparatus, with Hydroporinae exhibiting the greatest variation. Important structures, such as the spermatheca and the spermathecal gland, show a variable configuration in the several taxa. In Scarodytes halensis, different from Stictonectes optatus, the two structures are fused in a single complex organ; each region of this complex, however, exhibits an epithelium with a quite different ultrastructure. The epithelium of the region corresponding to the spermathecal gland consists of secretory and duct-forming cells, while the region homologous to the spermatheca has a simple epithelium. Sc. halensis is also provided with a very long spermathecal duct connecting the bursa copulatrix to the complex organ. This duct shows an epithelium rich of microtubules and junctional structures to support the thick layer of muscles surrounding the duct. A short fertilization duct is present between the complex organ and the common oviduct. The different organization of the genital apparatus in the two species suggests the presence of a different reproductive strategy. ABSTRACT: The fine structure of the female reproductive organs of the diving beetle Scarodytes halensis has been described, with particular attention to the complex organization of the spermatheca and the spermathecal gland. These organs are fused in a single structure whose epithelium is involved in a quite different activity. The secretory cells of the spermathecal gland have a large extracellular cistern with secretions; duct-forming cells, by their efferent duct, transport the secretions up to the apical cell region where they are discharged into the gland lumen. On the contrary, the spermatheca, filled with sperm, has a quite simple epithelium, apparently not involved in secretory activity. The ultrastructure of the spermatheca is almost identical to that described in a closely related species Stictonectes optatus. Sc. halensis has a long spermathecal duct connecting the bursa copulatrix to the spermatheca–spermathecal gland complex. This duct has a thick outer layer of muscle cells. Through muscle contractions, sperm can be pushed forwarding up to the complex of the two organs. A short fertilization duct allows sperm to reach the common oviduct where eggs will be fertilized. The different organization of the genital systems of Sc. halensis and S. optatus might be related to a different reproductive strategy of the two species.