..If it's HOUSE-flies you're thinking of: well, no. They actually subsist on polysaccharides, whcih they break down into sugars by vomiting an enzymatic cocktail onto their prey-items. Like the film........
Those nasty little things that get into your eyes are midgey things, not true flies. And they mostly feed on blood-meals.
And to REALLY turn your stomach: the marvellously-talented Billie Holliday eventually ran out of places to inject her daily doses of H: so she resorted to injection into her vaginal lips.
Ouch.