The Collaborative International Dictionary of English v.0.48:

Nepa \Ne"pa\, n. [L. nepa scorpion.] (Zool.)
   A genus of aquatic hemipterous insects. The species feed upon
   other insects and are noted for their voracity; -- called
   also scorpion bug and water scorpion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Nepa
    n 1: type genus of the Nepidae: typical elongate-oval water
         scorpions [syn: Nepa, genus Nepa]