ommatidium vs stemma

ommatidium and stemma both are nouns.

Word	Noun	Adjective	Verb	Adverb
ommatidium	Yes	No	No	No
stemma	Yes	No	No	No

As nouns, stemma is a hypernym of ommatidium; that is, stemma is a word with a broader meaning than ommatidium:

ommatidium: any of the numerous small cone-shaped eyes that make up the compound eyes of some arthropods
stemma: an eye having a single lens

Other hypernyms of ommatidium include ocellus, simple eye.

ommatidium (noun)	stemma (noun)
any of the numerous small cone-shaped eyes that make up the compound eyes of some arthropods	an eye having a single lens
	the hereditary derivation of an individual
	a tree diagram showing a reconstruction of the transmission of manuscripts of a literary work