\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}.