I've noted that flies in my house have no problem landing on even perfectly clean mirrors. Why don't they fly straight into them? Can they sense that there is a surface there, even though they can't see it?
This of course leads to the question of why flies can't see window glass. Maybe the large amount of light passing through at visible frequencies makes it look like a tinted fog to the housefly? Maybe they're trying to fly through mirrors too, but land on their feet and just roll with it?