The Collaborative International Dictionary of English v.0.48:

Stemma \Stem"ma\, n.; pl. Stemmata. [NL., fr. Gr. ?, pl. ?, a
   garland or chaplet.] (Zool.)
   (a) One of the ocelli of an insect. See Ocellus.
   (b) One of the facets of a compound eye of any arthropod.
       [1913 Webster]