Stemmata

from 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]