The ptychoid defensive mechanism in Euphthiracaroidea (Acari: Oribatida): A comparison of exoskeletal elements

Abstract

Ptychoidy is a mechanical defensive mechanism of some groups of oribatid mites, in which the legs and coxisternum can be completely retracted into the idiosoma and the prodorsum acts as a seal to the encapsulated animal. Here, we use two microscopical techniques, scanning electron microscopy and synchrotron X-ray microtomography, to compare exoskeletal features of two species of ptychoid oribatid mites. Oribotritia banksi and Rhysotritia ardua both belong to the superfamily Euphthiracaroidea and are analysed here in direct comparison to Euphthiracarus cooki, for which the functional morphology has already been described. Rhysotritia ardua and E. cooki – both members of Euphthiracaridae – are similar in most skeletal features that relate to ptychoidy, but differ in the size of their postanal apodemes. Oribotritia banksi – a member of Oribotritiidae – differs from the former two in some well-known features, including the retention of articulations between components of the ventral plates (compared to fused, holoventral plates in Euphthiracaridae), and the absence of interlocking triangles that are associated with the pre- and postanal apodemes in Euphthiracaridae. Our study uncovered two internal skeletal differences in the prodorsum that relate to muscle attachment surfaces: compared to E. cooki and R. ardua, O. banksi lacks the sagittal apodeme and has a distinctly smaller manubrium, but only the latter functions in ptychoidy.