'Water beetle' definitions:
Definition of 'water beetle'
(from WordNet)
noun
Any of numerous aquatic beetles usually having a smooth oval body and flattened hind legs for swimming
Definition of 'Water beetle'
From: GCIDE
- Water beetle \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. [1913 Webster]