Fine Structure of Male Genital System and Sperm in Solifugae Does Not Support a Sister-group Relationship with Pseudoscorpiones (arachnida)

Comparative spermatology may provide characteristics that can be useful in systematics. Previous observations on sperm structure of the solifugid Eusimonia mirabilis revealed that the most similar arachnid sperm cells are found within the Actinotrichida (Acari). The general morphology of the testis and the tendency to form sperm aggregates are also similar in both taxa. Since knowledge of sperm in Solifugae until now came only from one species, in contrast to Acari in which all the higher taxa have been investigated, these characters were difficult to assess with regard to systematical implications. The present paper confirms the derived, simple-aflagellate structure of sperm in Solifugae and the similarity with sperm of Actinotrichida, presenting results for two further species of another family (Ammotrechidae) from Argentina. Sperm cells of representatives of both taxa are small, devoid of a flagellum, contain a chromatin body that is penetrated and surrounded by circles of the acrosomal filament, and have a tendency to form peripheral protuberances. Sperm morphology does not support the frequently suggested sistergroup relationship between Solifugae and Pseudoscorpiones.