lightning bug vs firefly

lightning bug and firefly both are nouns.

Word	Noun	Adjective	Verb	Adverb
lightning bug	Yes	No	No	No
firefly	Yes	No	No	No

As nouns, lightning bug and firefly are synonyms defined as:

lightning bug and firefly: nocturnal beetle common in warm regions having luminescent abdominal organs

lightning bug (noun)	firefly (noun)
nocturnal beetle common in warm regions having luminescent abdominal organs	nocturnal beetle common in warm regions having luminescent abdominal organs
	tropical American click beetle having bright luminous spots