Source : Webster's Revised Unabridged Dictionary (1913)

Dayfly \Day"fly`\ (d[=a]"fl[imac]`), n. (Zo["o]l.)
   A neuropterous insect of the genus {Ephemera} and related
   genera, of many species, and inhabiting fresh water in the
   larval state; the ephemeral fly; -- so called because it
   commonly lives but one day in the winged or adult state. See
   {Ephemeral fly}, under {Ephemeral}.

Source : WordNet®

dayfly
     n : slender insect with delicate membranous wings having an
         aquatic larval stage and terrestrial adult stage usually
         lasting less than two days [syn: {mayfly}, {shadfly}]