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(predacious diving beetles), a family of water beetles. The body is elongate-oval and flattened; less frequently it is convex. The beetles range in length from 1.5 to 50 mm. The oar-shaped hind legs are for swimming; the forelegs are prehensile. The elongated larvae have legs fitted for swimming, a large head, and sickle-shaped mandibles. The mandibles have ducts for sucking in prey.
Predacious diving beetles are widely distributed. Of the approximately 2,500 species, more than 270 are found in the USSR. The beetles live in fresh or, less frequently, brackish waters; they breathe air, which is stored under the elytra. At night the beetles often emerge from the water and fly. They pupate in soil near the water. The beetles and larvae are active predators and eat various aquatic invertebrates, including the larvae of mosquitoes. Large species, such as Dystiscus marginalis, even prey on tadpoles and young fish, thus sometimes adversely affecting the fishing industry.