Source : Webster's Revised Unabridged Dictionary (1913)

Gallfly \Gall"fly`\, n.; pl. {Gallflies}. (Zo["o]l.)
   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}.

Source : WordNet®

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