Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

stemma
     n 1: a tree diagram showing a reconstruction of the transmission
          of manuscripts of a literary work
     2: the descendants of one individual; "his entire lineage has
        been warriors" [syn: {lineage}, {line}, {line of descent},
         {descent}, {bloodline}, {blood line}, {blood}, {pedigree},
         {ancestry}, {origin}, {parentage}, {stock}]
     3: an eye having a single lens [syn: {simple eye}, {ocellus}]
     [also: {stemmata} (pl)]

stemmata
     See {stemma}