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Dytiscidae(true water beetles, diving beetles; class Insecta, order Coleoptera) Family of large, shiny beetles up to 38 mm long, in which the dorsal and ventral surfaces are equally convex. Air is trapped between the elytra and abdomen for breathing under water. The hind legs are flattened and paddle-like, and specialized for swimming. Segments of the male fore tarsi are expanded to form suckers which hold the female during mating. The antennae are thread-like. Larvae are aquatic, with an elongate body, well-developed legs, and large, sickle-shaped mandibles. Adults and larvae are predatory on small fish, snails, and tadpoles. There are 4000 species.