Stenus

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Stenus is a group of semiaquatic rove beetles in the Steninae subfamily, with about 3,100 known species worldwide. They are predators that hunt tiny prey such as springtails (Collembola) and other small arthropods.

Adults have a special mouthpart called a labium that can shoot out quickly and has a sticky tip for catching prey. The labium is extended in 1–3 milliseconds and pulled back in 30–40 milliseconds, pulling the prey within reach of the jaws. The sticky tip doesn’t always work on prey with scales or hairs, or larger prey; in those cases Stenus comma may lunge forward and grab with its mandibles.

Stenus beetles are also known for skimming on water. They release secretions from the pygidial glands that act as a surfactant, creating a Marangoni effect that propels them forward. Stenus comma has been seen moving about 0.75 meters per second and can cover up to 15 meters if the secretion continues.