The Collaborative International Dictionary of English v.0.48:

Dobson \Dob"son\, n. (Zool.)
   The aquatic larva of a large neuropterous insect (Corydalus
   cornutus), used as bait in angling. See Hellgamite.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dobson
    n 1: large brown aquatic larva of the dobsonfly; used as fishing
         bait [syn: hellgrammiate, dobson]
    2: large soft-bodied insect having long slender mandibles in the
       male; aquatic larvae often used as bait [syn: dobson,
       dobsonfly, dobson fly, Corydalus cornutus]