diving beetle

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

• noun Any of various predatory aquatic beetles of the family Dytiscidae, which store air under their wing covers when submerged.

from The Century Dictionary.

• noun A popular name for various aquatic beetles of the family Dytiscidæ. They swim freely in the water, and may often be seen diving rapidly to the bottom, whence their name. See cut under Dytiscus.

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

• noun Any beetle of the family Dytiscidae.