The Collaborative International Dictionary of English v.0.48:

Gallfly \Gall"fly`\, n.; pl. Gallflies. (Zool.)
   An insect that deposits its eggs in plants, and occasions
   galls, esp. any small hymenopteran of the genus Cynips and
   allied genera. See Illust. of Gall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gallfly
    n 1: small solitary wasp that produces galls on oaks and other
         plants [syn: gall wasp, gallfly, cynipid wasp,
         cynipid gall wasp]
    2: fragile mosquito-like flies that produce galls on plants
       [syn: gall midge, gallfly, gall gnat]
    3: any of various insects that deposit their eggs in plants
       causing galls in which the larvae feed