fruit fly vs dipterous insect

fruit fly and dipterous insect both are nouns.

Word	Noun	Adjective	Verb	Adverb
fruit fly	Yes	No	No	No
dipterous insect	Yes	No	No	No

As nouns, dipterous insect is a hypernym of fruit fly; that is, dipterous insect is a word with a broader meaning than fruit fly:

fruit fly: any of numerous small insects whose larvae feed on fruits
dipterous insect: insects having usually a single pair of functional wings (anterior pair) with the posterior pair reduced to small knobbed structures and mouth parts adapted for sucking or lapping or piercing

Other hypernyms of fruit fly include dipteran, dipteron, two-winged insects.

fruit fly (noun)	dipterous insect (noun)
any of numerous small insects whose larvae feed on fruits	insects having usually a single pair of functional wings (anterior pair) with the posterior pair reduced to small knobbed structures and mouth parts adapted for sucking or lapping or piercing