Source : Webster's Revised Unabridged Dictionary (1913)

Alder fly \Al"der fly\
   1. Any of numerous neuropterous insects of the genus {Sialis}
      or allied genera. They have aquatic larv[ae], which are
      used for bait.

   2. (Angling) An artificial fly with brown mottled wings, body
      of peacock harl, and black legs.

Source : WordNet®

alder fly
     n : dark-colored insect having predaceous aquatic larvae [syn: {alderfly},
          {Sialis lutaria}]