water beetle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various aquatic beetles, especially the diving beetles of the family Dytiscidae, having a smooth oval body and flattened and fringed hind legs used for swimming.

from The Century Dictionary.

• noun A beetle which lives in the water.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) Any one of numerous species of aquatic beetles belonging to Dytiscus and allied genera of the family Dytiscidæ, and to various genera of the family Hydrophilidæ. These beetles swim with great agility, the fringed hind legs acting together like oars.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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