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

Representatives of the staphylinid beetle Philonthus marginatus are charact erized by highly modified raptorial forelegs which are used to strike prey in a particularly fast manner. Beetles ready to capture prey remain in a ch aracteristic precapture ambush posture characterized by lifted and folded f orelegs. 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. T his movement is based on the presence of a coxo-trochanteral catch mechanis m 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 fle xor and extensor muscles suggesting a spring-loaded system. This phase of t he strike is immediately followed by fixation of the prey by the ventral ad hesive tarsal setae supported by a grasp of the flexing last tarsomere and the claws. After withdrawal of the forelegs together with the prey, the seq uence eventually results in the formation of a capture-basket formed fronta lly by the perpendicularly flexing head and laterally by the spiny inner si des of the coxae. (C) 2001 Elsevier Science Ltd. All rights reserved.