hydrachna

from The Century Dictionary.

• noun A genus of acarids founded by Müller in 1781, at present restricted to those fresh-water mites in which the third joint of the palpi is the longest, the beak is as long as the palpi, and the mandibles have sharp blades.
• noun A genus of water-beetles, of the family Dytiscidœ, containing such as the European H. tarda.