Water-beetles, beetles which live on or in the water, especially the somewhat similar Amphizoidæ, Halipidæ, Dytiscidæ, and Gyrinidæ, and also the quite different clavicorn Hydrophilidæ. The Dytiscus, common in stagnant water, is olive-green above, and oval in shape. The respiratory organs of the perfect insect are not adapted to obtaining air from the water; the creature must therefore come occasionally for air to the surface of the water, where it lies on its back, the openings of its air-tubes, which are in the last segment of the abdomen, being exposed.