theca vs sheath

theca and sheath both are nouns.

Word	Noun	Adjective	Verb	Adverb
theca	Yes	No	No	No
sheath	Yes	No	No	No

As nouns, sheath is a hypernym of theca; that is, sheath is a word with a broader meaning than theca:

theca: outer sheath of the pupa of certain insects
sheath: an enveloping structure or covering enclosing an animal or plant organ or part

Other hypernyms of theca include case.

theca (noun)	sheath (noun)
outer sheath of the pupa of certain insects	a protective covering (as for a knife or sword)
a case or sheath especially a pollen sac or moss capsule	a dress suitable for formal occasions
	an enveloping structure or covering enclosing an animal or plant organ or part