What is "water beetle"

Water beetle \Wa"ter bee"tle\ (Zo["o]l.) Any one of numerous species of aquatic beetles belonging to Dytiscus and allied genera of the family Dytiscid[ae], and to various genera of the family Hydrophilid[ae]. These beetles swim with great agility, the fringed hind legs acting together like oars.

n. Any of various freshwater aquatic beetles that have a smooth oval body and flattened hind legs adapted for swimming, and that carry an air bubble underneath their abdomens.

n. any of numerous aquatic beetles usually having a smooth oval body and flattened hind legs for swimming

A water beetle is a generalized name for any beetle that is adapted to living in water at any point in its life cycle. Most water beetles can only live in fresh water, with a few marine species that live in the intertidal zone or littoral zone. There are approximately 2000 species of true water beetles native to lands throughout the world.

Many water beetles carry an air bubble, called the elytra cavity, underneath their abdomens, which provides an air supply, and prevents water from getting into the spiracles. Others have the surface of their exoskeleton modified to form a plastron, or "physical gill", which permits direct gas exchange with the water. Some families of water beetles have fringed hind legs adapted for swimming, but most do not. Most families of water beetles have larvae that are also aquatic; many have aquatic larvae and terrestrial adults.