Structure and variability in the female genital atrium of Uropodina (Acari: Parasitiformes).

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. Digital 3D reconstruction based on synchrotron X-ray microtomography (SR-μCT) data allowed a comparative investigation of the structure of an internal system, the female genital atrium, in the mite lineage Uropodina (Parasitiformes: Mesostigmata). Despite substantial variability in observed structures, a general model for the endogynium, vagina, and muscle structure has been generated using a combination of SR-μCT and light microscopy. Most of the variations are hypothesized as related to species recognition and/or manipulation of the endospermatophore. The recorded variability may have substantial phylogenetic value, as a previously unreported modification of the vagina appears to diagnose a substantial lineage of "higher" Uropodina. This set of observations also support the hypothesis that the large family Urodinychidae is polyphyletic. Overall, SR-μCT and 3D reconstruction turned out to be very helpful for studies on internal organ systems in these very small organisms, lessening the need for laborious dissections or extensive Transmission electron microscopy-based investigations.