Wa·ter bee·tle Zool. Any one of numerous species of aquatic beetles belonging to Dytiscus and allied genera of the family Dytiscidae, and to various genera of the family Hydrophilidae. These beetles swim with great agility, the fringed hind legs acting together like oars.
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water beetle
n : any of numerous aquatic beetles usually having a smooth oval
body and flattened hind legs for swimming