Function and evolution of the adhesion capture apparatus of Stenus species (Coleoptera, Staphylinidae)

Further to basic investigations, additional histological and scanning electron microscopical studies of the anatomy and morphology of the adhesion-capture apparatus in species of the genus Stenus have been carried out to achieve a better understanding of its mode of working and evolution. An interspecific morphological comparison of the sticky cushions (paraglossae) of the labium of 47 Stenus species has established that, in most of the investigated species, they represent a general type that has apparently been modified in different ways during the course of evolution. These changes primarily concern the number of adhesive setae on a cushion and the degree of branching of a single adhesive seta. It has been shown experimentally that larger sticky cushions and higher numbers of adhesive setae or adhesive con tacts improve adhesion and thus lead to enhanced capture success. In addition to the morphological comparison of the paraglossae, some general aspects of the functional anatomy and morphology of the labium have been examined. It has been established that, during attack, the sticky cushions and the tips of the sense-spines are directed frontally with respect to the prey. This concurrent erection of both paraglossae and the glossae is conditioned by an exocuticular groin that connects the glossae and paraglossae on the left and on the right. The interior of the sticky cushions is made up of a loose reticulum of endocuticular fibres. This cushion-like endocuticle may absorb the thrust caused by the impact of the sticky cushions on the prey. All tissue and chitinous components within the connecting tube of the labium are surrounded by an extra inner covering that Separates the tissue within the labium from the exterior wail of the labium. During the protrusion of the labium, this inner tube is most probably required to direct the haemolymph, flowing into the interior of the labium, towards the front. The hypopharynx is situated directly in front of the opening of the mouth and narrows the opening into the pharynx. It may be well suited for retaining large food pieces in front of the mouth in order to prevent their premature movement towards the mouth.