I was wondering why flies are called flies. They’re not the only things that can fly. Why aren’t birds called flies? For that matter, why aren’t fish called “swims” or certain land animals called “walks” or “runs” or “growls” for that matter?
Gallagher fan huh? They are called flies because they do indeed fly but it would be pointless to call everything that flies that. The name tends to be given just to insects and only certain ones of those. You will notice that nothing is truly just called a fly unless it is for short or to describe mixed flying insects in one place. There are house flies, dragon flies, horse flies, sand flies etc. Mosquitoes are not usually called flies except as a generic term if they are mixed in with other types.
The reason fly is a reasonable term for them and not, say, birds or bats, is that insects are a lot smaller and much harder to differentiate exactly if there a lot of them around so a generic term is useful. Fish and birds are useful generic terms on their own whereas insects isn’t a good term when you only mean the ones buzzing around and not the ones crawling on your food.
Originally, “fly” was generic, and referred to pretty much any flying arthropod. The “fly=housefly by default” apparently happened much later.
Because they make a sound that’s sort of like a zipper.
In some languages, the word for bird enpasses all “animals” that fly including bats and any number of gliding animals.
We don’t have much to learn from those cultures however. That is way too primitive not only in biological terms but in practical terms. English is good at differentiating different types of just about everything even if the word choice isn’t that great.
A firehose sprays water, not fire. Yet it is called a firehouse.
I got a million of these! 
Do you drive on a parkway and park on a driveway?
except etymologically: the word is from the Spanish diminutive for “fly” (mosca + ito). I don’t know why they switched genders.
The guy who first named it had exceptionally good eyesight?