Mechanics of the ptychoid defense mechanism in Ptyctima (Acari, Oribatida): One problem, two solutions

The most complex mechanical defense of oribatid mites is ptychoidy, in which the animals can retract their legs and gnathosoma into the idiosoma and encapsulate by deflecting the prodorsum. Since Acari lack most antagonistic musculature, extension of appendages is facilitated through hemolymph pressure that in mites mostly is generated by dorso-ventral compression of the opisthosoma. The hardened notogaster of box mites requires a different system of pressure generation that is also able to accommodate huge hemolymph movement accompanying ptychoidy. We compared the functional morphology of ptychoidy in one model species from each of the two ptyctime superfamilies, Euphthiracaroidea and Phthiracaroidea, using synchrotron X-ray microtomography and high-speed videography. We show that the two groups evolved very different functional modes of hydrostatic pressure control. While euphthiracaroids employ a lateral compression of the notogaster using all muscles of the opisthosomal compressor system, phthiracaroids employ a dorsoventral compression generated by only the notogaster lateral compressor and additionally the postanal muscle; these retract the temporarily unified ventral plates into the idiosoma, revealing the poam as an integral part of the opisthosomal compressor system in this group. The primitive mode of operation for generating hemolymph pressure in the Ptyctima probably was lateral compression, as molecular studies indicate that Phthiracaroidea evolved within Euphthiracaroidea. In this hypothesis, dorsoventral compression evolved secondarily in phthiracaroid mites, but whether the immediate ancestors of Ptyctima used lateral or dorsoventral compression remains to be determined.