The biology of steninae

Abstract

Current knowledge of the biology of the megadiverse beetle subfamily Steninae is reviewed here with regard to its systematics, general morphology, life history, behaviour, (chemical) ecology and evolution into various ecomorphs. Comprising >3000 species worldwide, the staphylinid genus Stenus is one of the most speciose animal genera on Earth. Steninae are well characterized by a number of adult and larval autapomorphies. Adult Stenus beetles are diurnal, optically oriented, epigeic predators of springtails and other small arthropods. The most obvious autapomorphic character defining Stenus is its protrusible elongated labium with the paraglossae being modified into adhesive pads. This prey-capture apparatus can be rapidly ejected towards potential prey by increased haemolymph pressure. The paired anal glands of Steninae are described morphologically and with respect to their secretion chemistry. The alkaloid and terpenoid secretions significantly act as defensive compounds against both bacteria and various predators. The unique skimming behaviour of selected species on water surfaces is described in detail, and the chemotaxonomic value of all gland constituents is discussed. We describe the developmental stages of these beetles from egg to adult and provide details of the functional morphology of the prey-capture apparatus including its adhesion performance via viscous forces. Steninae beetles prefer moist habitats and inhabit waterside environments such as reeds or sparsely vegetated sites on river or lake margins or the litter and humus layer in tropical forests. More than 70 distinct behavioural patterns can be assigned to the functional categories of feeding, reproduction, grooming, resting and protection. The ecomorphology of Steninae is reviewed in terms of their compound eyes, the labial prey-capture apparatus including the sticky pads (paraglossae) at its tip, the legs including the tarsi and the abdominal tergites.