walking stick vs stick insect

walking stick and stick insect both are nouns.

Word	Noun	Adjective	Verb	Adverb
walking stick	Yes	No	No	No
stick insect	Yes	No	No	No

As nouns, walking stick and stick insect are synonyms defined as:

walking stick and stick insect: any of various mostly tropical insects having long twiglike bodies

Other synonyms of walking stick include walkingstick.

walking stick (noun)	stick insect (noun)
any of various mostly tropical insects having long twiglike bodies	any of various mostly tropical insects having long twiglike bodies
a stick carried in the hand for support in walking