Source : Webster's Revised Unabridged Dictionary (1913)

Acarus \Ac"a*rus\, n.; pl. {Acari}. [NL., from Gr. ? the cheese
   mite, tick.] (Zo["o]l.)
   A genus including many species of small mites.

Source : WordNet®

acarus
     n : a mite of the genus Acarus
     [also: {acari} (pl)]

acari
     See {acarus}