The predatory legs of Philonthus marginatus (Coleoptera, Staphylinidae): functional morphology and tarsal ultrastructure.

Representatives of the staphylinid beetle Philonthus marginatus are characterized by highly modified raptorial forelegs which are used to strike prey in a particularly fast manner. Beetles ready to capture prey remain in a characteristic precapture ambush posture characterized by lifted and folded forelegs. Triggered by sensory input from the antenna or other parts of the fore body, the actual strike is released, essentially taking the form of a rapid (about 9 ms) depression of the unfolding forelegs towards the prey. This movement is based on the presence of a coxo-trochanteral catch mechanism and a particularly wide angle of rotation in the coxo-trochanteral joint. It is made possible by the specific mechanics of this joint which probably also involves a co-contraction of the antagonistic trochantero-femoral flexor and extensor muscles suggesting a spring-loaded system. This phase of the strike is immediately followed by fixation of the prey by the ventral adhesive tarsal setae supported by a grasp of the flexing last tarsomere and the claws. After withdrawal of the forelegs together with the prey, the sequence eventually results in the formation of a capture-basket formed frontally by the perpendicularly flexing head and laterally by the spiny inner sides of the coxae.