woodlouse vs sow bug

woodlouse and sow bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
woodlouse	Yes	No	No	No
sow bug	Yes	No	No	No

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

woodlouse: 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 woodlouse include pill bug.

woodlouse (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)