Among insects, the modified labium of Stenus species is one of the most specialized prey-capture structures. However, the question of its adaptive value has remained unanswered, since many species make use of their mandibles as an alternative capture technique (Bauer & Pfeiffer, 1991). In order to elucidate the biological significance of the specialized labium, within- and between-species comparisons of the performance abilities (i.e. velocity, mode of prey seizure, range, rate of occurrence and capture success) of both capture techniques were carried out. These investigations were performed in standardized prey-capture experiments, primarily by means of video-recordings and prey-capture cinematography. In addition, direct observations under semi-field conditions concerning habitat choice and searching behaviour were used to assess possible selection factors arising from the environment.
In standard experiments, most investigated species catch springtails much more successfully 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 and its capability of fixing prey at the moment of contact. The prey-capture experiments revealed interspecific differences in agility and reaction ability, which, to some extent, can be explained by differences in eye morphology. Some agile species, which usually forage on bare ground (e.g. S. comma), depend on the labium capture mechanism to a lesser degree. However, most species are not agile and forage in densely structured habitats such as plant debris or vegetation. The specialized labial apparatus of Stenus is thought to provide a biological advantage in that it permits these predators, in spite of their limited reaction ability and agility, to catch prey that are capable of rapid escape responses.