Betz, O.*; Mumm, R.: The predatory legs of Philonthus marginatus (Coleoptera,Staphylinidae): Functional morphology and tarsal ultrastructure
Within the Coleoptera, no predatory legs have become known thus far. However, within the staphylinid genus Philonthus representatives of the subgenus Onychophilonthus are distinct from other subgenera by the structure of their prolegs: both sexes have elongated coxae and enlarged tarsomeres I-IV. The claws can be bent downward to a large extent, thus almost setting against the ventral side of tarsomere V. We present the results of (a) behavioural observations concerning predatory performance, (b) motion analysis of the predatory strike, and (c) morphological examinations of the prolegs by light, scanning and transmission electron microscopy. These examinations were aimed to develop a functional model of this advanced prey capture mechanism. The predatory behavior of these beetles can be subdivided in three parts: (a) attempting an ambush, (b) predatory strike, and (c) formation of a catching basket. The mean total duration of the strike from the onset of the proleg till the first prey contact amounts to 31.6 ms. The movements of the prolegs during the strike can be subdivided in three components: (1) upward swing, (2) downward strike, (3) withdrawal. The structure of the coxo-trochanteral articulation and the high speed of the downward strike (7 ms) suggest a prestress mechanism, consisting of the co-contraction of the trochanterofemoral flexor and levator muscles. In addition, a coxal projection that fits into a trochanteral groove when the leg is flexed, serves as a cuticular supporting structure. The ultrastructure of the protarsi I-III demonstrates the rich development of epidermal glands, which probably produce an adhesive secretion. By this means, under mediation of tarsal adhesive setae, the prey can be fixed at the moment of contact, thus enabling the beetles to capture even fast-fleeing prey such as springtails.