Members of the genus Stenus (Coleoptera, Staphylinidae) possess a unique adhesive labial prey-capture apparatus as an adaptation to their predatory behaviour. In order to examine the relationships between the morphology of the prey-capture apparatus, its adhesive performance and the prey-capture success, we combined force measurements, morphological and behavioural investigations of representatives of 14 Central European species of this genus. The direct relationship between these traits was studied by using phylogenetic generalized least squares and a molecular phylogeny generated from mitochondrial cytochrome c oxidase I partial sequences. Force measurements revealed strong interspecific differences in the adhesive forces generated during the predatory attack; these differences entail significant differences in the prey-capture success. The interspecific differences in adhesive performance were functionally correlated with the pad morphology and divergences in the compressive forces generated during the predatory strike. Allometric analyses revealed that the pad area scaled with positive allometry with respect to body size, whereas the adhesive forces scaled with negative allometry with respect to body size, that is, the pads' efficiency decreased with body size. Our experimental approach demonstrates a direct functional relationship between the morphology of the prey-capture device of Stenus beetles and its performance in a natural behavioural context.

This study was partly financed by the German Science Foundation (project PAK 478: BE 2233/10–1). LZG was supported by funds from The Ministry of Economy and Competitiveness (Spain) (CGL2015-70639-P) and The National Research, Development and Innovation Office (NKFIH, K-115970).