Definitely not flies, those things are nasty. And they come from maggots. Ants or spiders, I'm not sure. Ants seem to be more of a nuisance...they get into all your snacks, and when you have ants, you tend to have a lot of them. And the red ones bite. Spiders, on the other hand, can be venomous and can even kill you if they're the wrong species. So idk. I guess spiders (most of them are harmless).