Anatomy and function of the ptychoid defensive mechanism in the mite Euphthiracarus cooki (Acari: Oribatida)

Ptychoidy is a defensive adaptation of several groups of oribatid mites in which legs and coxisternum can be fully retracted into the opisthosoma and protected by a ventrally deflected prodorsum, resulting in a seed-like appearance. Using Euphthiracarus cooki as a model, we examined details of exoskeletal and muscular anatomy in combination with studies of live individuals to provide the first functional analysis of ptychoidy. There are two main functional components: the first is a set of exoskeletal and muscular adaptations, mostly of the podosoma and prodorsum, that combine to effect leg withdrawal and prodorsal deflection; the second comprises adaptations of the opisthosoma that allow control of hydrostatic pressure during the large hemocoel volume adjustments associated with ptychoidy. Adaptations important in the closing process (enptychosis) are found in four body regions. Much of the podosomal exoskeleton (especially pleural) is unsclerotized, which facilitates leg retraction and prodorsal deflection during enptychosis. The coxisternum has several flexible furrows along which it folds in order to bring legs into a tightly parallel arrangement. The prodorsum has specialized attachment surfaces (manubrium and inferior retractor process) for retractor muscles and a paired bothridial scale that participates in prodorsal alignment during enptychosis. The subcapitulum has a prominent capitular apodeme on which important retractor muscles insert. The mineralized notogaster has an anterior "collar" that accommodates the retracted prodorsum; it includes paired notches and receptacles that accommodate the bothridial scales, thereby creating a temporary fixed axis for rotation of the prodorsum in a "lazy hinge" mechanism. Specialized muscles form the retractor system; most conspicuous are the large coxisternal retractors and prodorsal retractors, both of which originate on the notogaster. Other components have adjustor roles; among them are muscles of the endosternal system that control retraction of the subcapitulum and assist leg retraction, and the dorsoventral muscles which adjust the folded coxisternum. Hemolymph pressure control is a function of the opisthosoma, where the principle exoskeletal elements form a pleated venter, having a cross-sectional shape like an inverted "W." Paired holoventral plates (each representing fused genital, aggenital, anal, and anal plates) form the inner angle and are flanked by paired plicature plates. The holoventral plates are connected medially in two ways : 1) by three permanent bridges of sclerotized cuticle that include an anterior phragmatal bridge and two widely spaced, hollow apodemes (preanal, postanal); 2) by temporary zipper-like closures of two different types. Lateral compression of the pleats is effected by a series of transversely arranged, lateral compressor muscles that run from plicature to holoventral plate edges, and from holoventral plate edges to the medial apodemes. Compression increases hydrostatic pressure in the opisthosoma and stores energy in both the slightly deformed, mineralized notogaster and in the three holoventral bridges. During normal activity the compressor system is active and the inflated podosomal region provides support for the extended legs. When the mite is irritated, the prodorsum is hydraulically ejected from its active position in the notogastral collar, then relaxation of the compressors causes a fall in hemolymph pressure and return of the notogaster to an undeformed condition. Muscles of the retractor system then act in specific sequence to retract and adjust the coxisternum and prodorsum until they are precisely positioned at the completion of enptychosis. The process takes between 0.5 and 1 sec. When irritation ceases, partial opening allows sensory leg hairs to "test" the environment. Resumption of normal, active posture (ecptychosis) involves activation of the lateral compressor system and hydraulic inflation of the podosoma, through which legs are extended and the prodorsum is reflected.