wood louse vs sow bug

wood louse and sow bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
wood louse	Yes	No	No	No
sow bug	Yes	No	No	No

As nouns, sow bug is a hyponym of wood louse; that is, sow bug is a word with a more specific, narrower meaning than wood louse:

wood louse: any of various small terrestrial isopods having a flat elliptical segmented body; found in damp habitats
sow bug: terrestrial isopod having an oval segmented body (a shape like a sow)

Other hyponyms of wood louse include pill bug.

wood louse (noun)	sow bug (noun)
any of various small terrestrial isopods having a flat elliptical segmented body; found in damp habitats	terrestrial isopod having an oval segmented body (a shape like a sow)