Source : Webster's Revised Unabridged Dictionary (1913)

Ephemeron \E*phem"e*ron\, n.; pl. {Ephemera}. [NL. See
   {Ephemera}.] (Zo["o]l.)
   One of the ephemeral flies.

Source : WordNet®

ephemeron
     n : an insect that lives only for a day in its winged form [syn:
          {ephemera}]
     [also: {ephemera} (pl)]