The Collaborative International Dictionary of English v.0.48:

Musca \Mus"ca\, prop. n.; pl. Muscae. [L., a fly.]
   1. (Zool.) A genus of dipterous insects, including the common
      house fly, and numerous allied species.
      [1913 Webster]

   Note: Formerly, a large part of the Diptera were included
         under the genus Musca.
         [1913 Webster]

   2. (Astron.) A small constellation situated between the
      Southern Cross and the Pole.
      [1913 Webster]

   Muscae volitantes. [L., flying flies.] (Med.) Specks or
      filaments apparently seen moving or gliding about in the
      field of vision. Their appearance is often a symptom of
      disease of the eye, or of disorder of the nervous system.
      [1913 Webster] Muscadel

WordNet (r) 3.0 (2006):

muscae volitantes
    n 1: spots before the eyes caused by opaque cell fragments in
         the vitreous humor and lens; "floaters seem to drift
         through the field of vision" [syn: musca volitans,
         muscae volitantes, floater, spots]