glowworm vs lightning bug

glowworm and lightning bug both are nouns.

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

As nouns, lightning bug is a hypernym of glowworm; that is, lightning bug is a word with a broader meaning than glowworm:

glowworm: the luminous larva or wingless grub-like female of a firefly
lightning bug: nocturnal beetle common in warm regions having luminescent abdominal organs

Other hypernyms of glowworm include firefly.

glowworm (noun)	lightning bug (noun)
the luminous larva or wingless grub-like female of a firefly	nocturnal beetle common in warm regions having luminescent abdominal organs