Life forms and hunting behaviour of some Central European Stenus species (Coleoptera, Staphylinidae)

Abstract

Stenus species exhibit their greatest diversity in moist environments such as reeds. As seen in habitat choice experiments, they forage in three different zones: they are either (1) inhabitants of moist humus or plant debris near the ground, (2) plant climbers or (3) surface runners on bare ground. A thorough comparison of 18 Stenus species has revealed different complexes of adaptations concerning predatory behaviour and linked morphological characters. Surface runners (3), such as S. comma, have laterally protruding eyes with a large number of ommatidia, long legs and slender tarsi. Accordingly, standardized prey-capture experiments with springtails have revealed that these predators are highly agile and are capable of pursuing prey which move quickly and unpredictably. Their searching behaviour is of the ambush-searching type. Moreover, they are less reliant on their specialized labium for prey-capture but have refined the mandible-attack mechanism. However, such life forms are an exception and may have evolved from (1) detriticolous or (2) planticolous predator types that make up most recent Stenus species (e.g. S. pubescens). These beetles are not agile and stalk in plant debris or in the vegetation in order to hunt stationary or slow moving prey. Correspondingly, they have flat eyes, wide tarsi and depend to a higher degree on the labium, since it permits these predators, in spite of their limited agility, to catch prey in a sudden manner.