The Collaborative International Dictionary of English v.0.48:

Leptus \Lep"tus\ (l[e^]p"t[u^]s), n. [NL., from Gr. lepto`s
   thin, small.] (Zool.)
   The six-legged young, or larva, of certain mites; --
   sometimes used as a generic name. See Harvest mite, under
   Harvest.
   [1913 Webster]