COMPARATIVE-STUDIES ON THE PREDATORY BEHAVIOR OF STENUS SPP. (COLEOPTERA, STAPHYLINIDAE) - THE SIGNIFICANCE OF ITS SPECIALIZED LABIAL APPARATUS

Among insects, the modified labium of Stenus species is one of the mos t specialized prey-capture structures. However, the question of its ad aptive value has remained unanswered, since many species make use of t heir mandibles as an alternative capture technique (Bauer & Pfeiffer, 1991). In order to elucidate the biological significance of the specia lized labium, within-and between-species comparisons of the performanc e abilities (i.e. velocity, mode of prey seizure, range, rate of occur rence and capture success) of both capture techniques were carried out . These investigations were performed in standardized prey-capture exp eriments, primarily by means of video-recordings and prey-capture cine matography. In addition, direct observations under semi-field conditio ns concerning habitat choice and searching behaviour were used to asse ss possible selection factors arising from the environment. In standar d experiments, most investigated species catch springtails much more s uccessfully with the labium than with the mandibles, thereby stressing the selective value of the labium. The marked capture success of the latter is attributable to its large range, its high capture velocity a nd its capability of fixing prey at the moment of contact. The prey-ca pture experiments revealed interspecific differences in agility and re action ability, which, to some extent, can be explained by differences in eye morphology. Some agile species, which usually forage on bare g round (e.g. S. comma), depend on the labium capture mechanism to a les ser degree. However, most species are not agile and forage in densely structured habitats such as plant debris or vegetation. The specialize d labial apparatus of Stenus is thought to provide a biological advant age in that it permits these predators, in spite of their limited reac tion ability and agility, to catch prey that are capable of rapid esca pe responses.