firefly vs lightning bug

firefly and lightning bug both are nouns.

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

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

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

firefly (noun)	lightning bug (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