Stenus is a genus of semiaquatic rove beetles in the subfamily Steninae, and one of the largest genera in the kingdom Animalia, with some 2700 known species worldwide (only the beetle genus Agrilus is comparable in size).[2] They are predators of Collembola and other small arthropods. Adults have a protrusible labium with a sticky tip used in prey capture. To overcome the rapid escape of Collembola, the labium is protruded at high speed (1-3 ms in Stenus comma) by hemolymph pressure, and immediately withdrawn (withdrawn in 30-40 ms in Stenus comma), pulling the prey within the range of the mandibles. However, the labium tip does not easily stick to prey covered in scales or setae or that have a large body size. Stenus comma is more likely to catch such prey by lunging forward and grabbing them directly with its mandibles rather than using its labium.[3] Stenus species are also known for "skimming" on the water surface using their pygidial gland secretions that act as a surfactant and rapidly propel the beetle fast forward, a phenomenon known as the Marangoni effect. Stenus comma has been seen to achieve a velocity of 0.75 m/s, and to cover a distance of up to 15 m if the secretion is continuous.[4][5]