sand fly vs gnat

sand fly and gnat both are nouns.

Word	Noun	Adjective	Verb	Adverb
sand fly	Yes	No	No	No
gnat	Yes	No	No	No

As nouns, gnat is a hypernym of sand fly; that is, gnat is a word with a broader meaning than sand fly:

sand fly: any of various small dipterous flies; bloodsucking females can transmit sandfly fever and leishmaniasis
gnat: any of various small biting flies: midges; biting midges; black flies; sand flies

sand fly (noun)	gnat (noun)
any of various small dipterous flies; bloodsucking females can transmit sandfly fever and leishmaniasis	any of various small biting flies: midges; biting midges; black flies; sand flies
	(British usage) mosquito