water beetle Meaning, Definition & Usage

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

WordNet

Wa"ter bee"tle
Definitions
  1. (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.

Webster 1913