Structure and function of the gnathosoma of the mange mite, Psoroptes ovis

Mites of the genus Psoroptes (Acari: Psoroptidae) are obligate, non-burrowing, astigmatid ectoparasites of mammals. A detailed understanding of the morphology of the gnathosoma is an important step towards elucidation of the feeding behaviour of this mite and, hence, the pathology of psoroptic mange. Scanning and transmission electron microscopy were used to examine Psoroptes ovis (Hering) (syn. P. cuniculi). The gnathosoma is composed of an infracapitulum (hypostome), which forms the floor of an extended U-shaped pre-oral trough, bounded by laterally compressed palps and enclosing paired chelicerae. Distally, each palp terminates in a flap-like process and four tapered terminal sensillae. The floor of the infracapitulum is interrupted along its length by a dorsally projecting ridge. At the distal end of the infracapitulum are paired, grooved pseudorutellae and, between them, paired, ungrooved lateral lips. Between the lateral lips is a grooved, elongate labium. Medially, a pair of finger-like projections emanates from the paraxial walls of the palps and project anteriorly. Each chelicera is made up of fixed and mobile digits, with a cheliceral membranous fold articulating the two. The distal ends of both digits are chelated. For much of its length, the mobile cheliceral digit appears to lie within a fold formed by the infracapitular ridge, thereby creating a central channel between the chelicerae. It is suggested that this arrangement of elements may facilitate a two-way flow of liquid, where saliva flows down a central salivary canal and spills out onto the host's skin over the labium at the point where the chelated tips of the chelicerae meet and abrade the skin. Liquid food may move along the grooves of the pseudorutellae and then be sucked up a food canal formed by the paraxial walls of the palps and the outer wall of the chelicerae.