The adult flies emerge in late summer and early fall and seek protected places to spend the winter. In many cases, this is within the walls, attics and basements of homes. Once the late winter and early spring comes around the flies don't all find the same way out that they got in and end up inside the home entering through window pulley holes, around the baseboards and through other small openings in walls.
These flies are not breeding in the home and constitute no health hazard. The females lay their eggs near earthworm burrows, and the larvae then develop as parasites in the bodies of earthworms. Not really where we want the guests of our homes to come from.
They are difficult to eradicate because they prefer to live in inaccessible spaces such as attics and wall cavities. They are often seen on windows of little-used rooms, especially in early spring as the sun warms the south west facing walls of houses. An early spring and late summer treatment by your Barrier Pest Control professional will help greatly to reduce the number of flies in and around your home.