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Elmidae (subclass Pterygota, order Coleoptera) Family of small beetles, 2–5 mm long, which have punctured elytra and raised lines on the thorax. The antennae are filiform. The legs are long and slender, with an elongate terminal tarsal segment. They live in running water. Some breathe under water, using an air film trapped by hairs as a physical gill. Larvae are woodlouse-shaped, with terminal external gills, and are found clinging to stones or weeds in water. There are 300 species.