Where are the freaking house flies coming from?

[arachadillo]

6. More on Flies

I had a similar problem, however the flies were not coming in from the outside when the door opened. They just popped up in large numbers. When I tried to figure out what was going on, I realized that there are so many different fly species that pinpointing the specific problem to solve was hard.

They could have been drain flies, if there was a drainage problem somewhere in the immediate vicinity. They could have been cluster flies, they are like house files, but a little bit larger. Sometimes they gain access to houses through any opening to the outdoors such as cracks in exterior etc, or the door opening. They lay eggs in baseboards, window areas, etc, and they keep emerging multiple times per season, so it's not a matter of taking out the garbage consistently, they are already in their development stage.

Best thing to do to remedy the situation is first figure out which particular species you're dealing with and go from there.
http://greennature.com/article499.html