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 ad hesion-capture apparatus in species of the genus Stenus have been carr ied out to achieve a better understanding of its mode of working and e volution. An interspecific morphological comparison of the sticky cush ions (paraglossae) of the labium of 47 Stenus species has established that, in most of the investigated species, they represent a general ty pe that has apparently been modified in different ways during the cour se of evolution. These changes primarily concern the number of adhesiv e 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 adhesi on and thus lead to enhanced capture success. In addition to the morph ological comparison of the paraglossae, some general aspects of the fu nctional anatomy and morphology of the labium have been examined. It h as been established that, during attack, the sticky cushions and the t ips of the sense-spines are directed frontally with respect to the pre y. This concurrent erection of both paraglossae and the glossae is con ditioned by an exocuticular groin that connects the glossae and paragl ossae on the left and on the right. The interior of the sticky cushion s is made up of a loose reticulum of endocuticular fibres. This cushio n-like endocuticle may absorb the thrust caused by the impact of the s ticky cushions on the prey. All tissue and chitinous components within the connecting tube of the labium are surrounded by an extra inner co vering that Separates the tissue within the labium from the exterior w ail of the labium. During the protrusion of the labium, this inner tub e is most probably required to direct the haemolymph, flowing into the interior of the labium, towards the front. The hypopharynx is situate d directly in front of the opening of the mouth and narrows the openin g into the pharynx. It may be well suited for retaining large food pie ces in front of the mouth in order to prevent their premature movement towards the mouth.