Definitions for dayflyˈdeɪˌflaɪ
Random House Webster's College Dictionary
an adult mayfly.
Origin of dayfly:
mayfly, dayfly, shadfly(noun)
slender insect with delicate membranous wings having an aquatic larval stage and terrestrial adult stage usually lasting less than two days
Origin: since the adults typically live only for a day.
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