Fly infestation

[in-parentheses.livejournal.com]

I came home from a 12-hour day at work to find that the 4 or 5 flies that were in my house when I left this morning had multiplied and freaking taken over. There are easily 50 flies in my kitchen and dining room; possibly more like 100. Not fruit flies, flies.

So I turn to the wisdom of you, internet. Has this happened to you? What did you do? Is there any way to figure out where the hell they all came from? My roommate and I are very clean, I swear -- we take the garbage out, we keep all our food wrapped, the compost lives outside...

I bought hanging sticky traps, but I don't think they're going to cut it. UV traps are $50 a pop, so I want to know they're going to be awesome before I spend the money. (Or maybe you have one I could borrow?) I don't want to spray insecticide in my kitchen if there's any other way. Is there any non-toxic spray I could use?

Please, please help. I'm so grossed out right now I can't even tell you.