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]

WordNet (r) 3.0 (2006):

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, stemma, stock]
    3: an eye having a single lens [syn: simple eye, stemma,
       ocellus]