The ptychoid defensive mechanism in Phthiracarus longulus (Acari, Oribatida, Phthiracaroidea): Exoskeletal and muscular elements
The most complex defensive mechanism in oribatid mites is ptychoidy, a special body form allowing the animals to retract their legs and coxisternum into a secondary cavity in the idiosoma and to seal it off with the prodorsum. Many exoskeletal and muscular adaptations are required to enable the functionality of this mechanism, e.g. a soft and pliable podosoma. Its membranous part not only gives the coxisternum the ability to move independently from the rest of the hardened cuticular elements, but also builds up the ‘walls’ of the secondary cavity. Here, using scanning electron microscopy and synchrotron microtomography we present the first detailed study on ptychoidy in a phthiracaroid mite, Phthiracarus longulus, and compare it to the Euphthiracaroidea. Morphological differences regarding ptychoidy between these groups are already noticeable from the outside: the ventral plates of P. longulus are embedded into the soft anogenital membrane, whereas euphthiracaroid mites connect the ventral plates to the notogaster through the hardened plicature plates. Internally, we discovered a not yet described coxisternal protractor muscle, which presumably assists haemolymph pressure during the deployment of the coxisternum during ecptychosis.
Synchrotron X-ray microtomography, Phthiracarus longulus, ptychoidy, Phthiracaridae, box mite, convergent evolution, predator defence