How to deal with indoor flies
If the source is inside—usually the case with fruit flies; even with blow flies—clean it up. This may mean finding a dead bird in your chimney, taking your trash out more often, and making sure food is contained.
For cluster and house flies, the source is often outside. Keep them out. Well-fitting screens are essential! Repair holes in screens. Caulk cracks around windows. Fix doors so they close tightly. Use a screen door if you want the breeze. Keep a flyswatter handy and use it. If a bunch get in, hang sticky fly tape near where they hang out, or suck ‘em up with a vacuum.
Don’t bother with bug zappers—flies ignore them. Besides, they often kill beneficial insects.
What they look like
All flies have two wings. If you’ve got flies inside they’re most likely small fruit flies, shiny green or blue blow flies, window-loving cluster flies, or your standard-issue house flies.
Where they live
In your house, yes … but it’s tracking down the eggs and maggots (fly larva) that will solve your problem. Fruit fly babies feed on rotting fruit, blow flies on rotting animals, cluster flies in earthworms, and house flies on pretty much anything that’ll rot—manure, grass clippings, garbage, etc. Sounds yucky, yes, but what if we didn’t have critters to eat that stuff?
What they do
Spread germs around.