sheath vs theca

sheath and theca both are nouns.

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

As nouns, theca is a hyponym of sheath; that is, theca is a word with a more specific, narrower meaning than sheath:

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

Other hyponyms of sheath include lorica, medullary sheath, myelin sheath, neurilemma, neurolemma, husk.

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