'Dayfly' definitions:

Definition of 'dayfly'

From: WordNet
noun
Slender insect with delicate membranous wings having an aquatic larval stage and terrestrial adult stage usually lasting less than two days [syn: mayfly, dayfly, shadfly]

Definition of 'Dayfly'

From: GCIDE
  • Dayfly \Day"fly`\ (d[=a]"fl[imac]`), n. (Zool.) 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. [1913 Webster]