Ultrastructure and function of the adhesion-capture apparatus of Stenus species (Coleoptera, Staphylinidae)

Abstract

The elongated labium of rove beetles of the genus Stenus forms an adhesion–capture apparatus that enables the animal to catch fast fleeing prey, for example, collemboles. Structural details of this labium have been reinvestigated by using transmission electron microscopy, and the functional model of the capture mechanism has been refined. The openings of glandular ductules have been found to be located at the outer margin of the sticky cushions formed by the paraglossae. These cushions can be expanded by hemolymph pressure and are compressed when the tip of the protrusible labium hits the prey. Endocuticular fibers stabilize the cushions internally and determine the shape of the cushions in both the expanded and the compressed state. Within the membranous connecting tube that connects the prementum with the head capsule, the existence of an extra inner membrane has been confirmed. It is formed by a portion of the epidermis that has become detached from the cuticle. The most important part of the functional model of the protrusion of the labium is that the membranous connecting tube turns itself inside out distally, but the extra inner membrane does so proximally. During protrusion of the labium by hemolymph pressure, the prementum is accelerated during the initial phase of the process, and the nerves, muscles, tracheae, and glandular ductules that are attached to it are passively drawn into the labium. The mechanoreceptive function of setae at the distal end of the prementum has been confirmed on the basis of their ultrastructural characteristics.