Primary and secondary sexual characters of Mesostigmata are often used in species descriptions and phylogenetic analyses. The use of these characters has been focused almost exclusively on external structures such as the male chelicera and genital plates, while internal structures have only been utilized for lightly sclerotized species.
Michael, A.D. (1890) IV. On the variations of the female reproductive organs, especially the vestibule, in different species of Uropoda. Journal of the Royal Microscopical Society, 10, 142–152. https://doi.org/10.1111/j.1365-2818.1890.tb00789.x