why do we have so many flies in our house

Sound like cluster flies to me, too-- Buzzy and also not so quick describes them perfectly. They're usually associated with the autumn, when adult flies creep into the sunny side of the house to find a place to spend the winter, but in my house it was the beginning of the summer: young flies emerged disgustingly through my kitchen wall where that room is dug into the hillside behind my house. Their larval stage is underground, since they're parasitic on earthworms; must have been a big batch of pupae in the soil on the other side of the wall. (The fact that they were coming through the wall shows that closing windows won't keep them out. ) It was really gross, and complicated by the fact that at the same time there
was a dead rat in the ceiling cavity above the kitchen, which they had absolutely nothing to do with--took me a while to realize that.

Anyway: I batted them out of the windows they clustered near, ate out (! ), cleaned the kitchen obsessively, etc. The plague died down within a few days.

I investigated the Cluster Buster and if they'd stayed around any longer I'd have ordered one on 24-hour delivery. I also took comfort from the fact that they aren't interested in human food--though I didn't leave any lying about. That was last year, a little earlier in the summer than this, and I'm glad to say that this year they haven't reappeared (touch wood). No dead rats either. I am now itching all over. posted by at on June 10, 2012 Like others have said, there's probably a dead thing somewhere.

However, depending on the fly type, they could be breeding in very dirty water or in pretty much any organic matter. For example, some will breed in rotting vegetation, some will eat wool or fleece if they have too. So looking for pretty much anything out of the way that could be rotting. You may not find it though - attics, basements, even in the walls if a rat died in there or something. The life cycle of flies is often very temperature dependent - it slows down when it is cold and speeds up when warm.

Some of the common flies found in houses can go from egg to adult in about a day in ideal conditions, so even if you kill them or let them out, this is probably how they are replenishing - new ones are hatching. I looked up the breeding time and so on a while back, when this happened to my Mum - all these flies showed up in the lobby to the house one morning, on the ceiling. It was kind of creepy. We cleaned out the room, and couldn't find any obvious points of entry from the ceiling or floor, so you're not the only one!

What do your flies look like? Different ones eat different things and have different habits. This interesting looking chart describes them and gives their "favourite foods" as well. This site gives many many pictures of flies. If you click on the headers above each row of pictures, you get more pictures of flies in that subgroup, so you can keep getting more and more specific pictures and ID. Not recommended if you are grossed out by fly pictures.