A fly is pretty much any insect in the order Diptera. That means a population of flies could become a new species and still be flies. That's what evolution predicts, since the change happens gradually across many insect generations. It's a branching process in which small differences enlarge with time.
First a new species emerges. With further divergence, the population may eventually become a distinct genus and its members would still be flies. "Order" is two taxonomic ranks above "genus."
The group could eventually become so different from other fly populations that its members would be considered a distinct family, with several genera and dozens of species--all that--and they would still be flies so long as they apparently belonged in the order Diptera. They don't stop being flies until they're so different they would be placed in a whole new order.
Thus, "Why do you think another species was formed? One fly turned into another fly" is a very uninformed objection.
You mean like rabbits that change but are still rabbits - so you give them a new name so that with labels instead of facts, you can tenatively point to something and claim by staging the argument that they're no longer rabbits; but, a new species so evolution has occurred. Never mind the fluffy tail, long ears, general rabbit look and characteristics.. lol. My rabbit has a black patch, it evolved. ROFLMAO. I have to laugh because when you guys are resorting to things this desperate, it tells us where things stand.