Ultrastructural characterization of Hexisopus psammophilus (Arachnida: Solifugae: Hexisopodidae) spermatozoa in comparison to other solifuge spermatozoal traits

The family Hexisopodidae is endemic to southern Africa. Hexisopodids represent a very peculiar group of Solifugae. They differ from all other solifuge families through various autapomorpic adaptations to a subterranean mode of life, most notably the presence of fossorial legs. The phylogeny of the Solifugae is widely unresolved. The ultrastructure of spermatozoa has successfully been used for phylogenetic analyses in other animal taxa. Therefore, the question arose whether the morphological peculiarity of the family Hexisopodidae might also be reflected in the ultrastructure of their spermatozoa. This was investigated for Hexisopus psammophilus Wharton 1981 (Hexisopodidae). Spermatozoa do not seem to aggregate in the testes, nor in the vasa deferentia. Sperm cells are aflagellate, roundish, and with irregularly shaped chromatin bodies. Each sperm is surrounded by a secretion sheath, thus representing a typical cleistosperm, the first record of this form of sperm transfer in solifuges. The sperm cells form finger-like protuberances and contain putative granules of glycogen, features shared with the Ammotrechidae, Daesiidae and Solpugidae. The acrosomal complex shows additional similarity with the Solpugidae. Overall, the spermatozoa of H. psammophilus share some morphological features with the Ammotrechidae and Daesiidae, but mostly resemble that of the family Solpugidae.