The Collaborative International Dictionary of English v.0.48:

Flea \Flea\, n. [OE. fle, flee, AS. fle['a], fle['a]h; akin to
   D. vtoo, OHG. fl[=o]h, G. floh, Icel. fl[=o], Russ. blocha;
   prob. from the root of E. flee. [root]84. See Flee.]
   (Zool.)
   An insect belonging to the genus Pulex, of the order
   Aphaniptera. Fleas are destitute of wings, but have the
   power of leaping energetically. The bite is poisonous to most
   persons. The human flea (Pulex irritans), abundant in
   Europe, is rare in America, where the dog flea
   (Ctenocephalides canis, formerly Pulex canis) and the
   smaller cat flea (Ctenocephalides felis) take its place.
   See Aphaniptera, and Dog flea. See Illustration in
   Appendix.
   [1913 Webster]

   A flea in the ear, an unwelcome hint or unexpected reply,
      annoying like a flea; an irritating repulse; as, to put a
      flea in one's ear; to go away with a flea in one's ear.

   Beach flea, Black flea, etc. See under Beach, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Pulex irritans
    n 1: the most common flea attacking humans