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Hydraenidae (subclass Pterygota, order Coleoptera) Family of tiny beetles, about 1.5 mm long, found in stagnant water. The ventral surface is flat, the upper surface very convex. The elytra are generally black to metallic. The maxillary palps are long, the antennae short, with five-segmented clubs. The ocelli are well developed, possibly indicating an affinity with Staphylinidae. Some larvae are aquatic, others live only partly in water. Adults and larvae are phytophagous, often feeding on dead material. There are 300 species.