The Collaborative International Dictionary of English v.0.48:

Hornbug \Horn"bug`\, n. (Zool.)
   A large nocturnal beetle of the genus Lucanus (as Lucanus
   capreolus, and Lucanus dama), having long, curved upper
   jaws, resembling a sickle. The grubs are found in the trunks
   of old trees.
   [1913 Webster]