Their eyes are not going to have evolved past the point of providing any incremental benefit, and a fly is after all a simple critter who's main concern is find

Their eyes are not going to have evolved past the point of providing any incremental benefit, and a fly is after all a simple critter who's main concern is finding a pile of shit to eat, and avoid being swatted by a cow's tail. I'd guess their eyes (& visual system) are perhaps better regarded as crude visual sensor rather than anything much like the visual system of an animal that benefits from recognizing whether that shitty ass belongs to a cow vs a tiger.