Callophorid fly or bottle flies
The callophorid or bottle flies are members of a fairly large family of flies which are most often associated with the bodies of dead animals or other organic material in an active state of decay. However, if the flies are found in the home during the winter or early spring when insects are not active outside, they may have entered the home in the fall of last year seeking a protected site to spend the winter and incidentally found their way into the living space. One member of this family, the cluster fly, is notorious for invading homes in the fall. In nature, callophorids overwinter under tree bark and other nooks and crannies they find suitable.
Unfortunately, if these flies are the overwintering type, there is no easy treatment because they hide in wall voids, attics, under siding, and other cracks and crevices where it is almost impossible treat with an insecticide. Effective chemical treatment requires the introduction of insecticide into all of the wall voids and other hidden sites. This can be an expensive procedure and it may not be totally effective. Anything that can be done to deny these insects access into the structure, (e.g., caulking cracks around windows and doors, installing screen on vents), will help. Aerosol insecticide bombs can be used in attics if the flies are observed there in large numbers. In the fall, spraying outside the building, especially beneath eves and around windows, when flies congregate on warm walls will help can prevent cluster fly problems in some situations. To control flies inside, use any household insecticide containing synergized pyrethrins (like Raid) or a vacuum cleaner.
Flies that are found indoors and in large numbers during the summer months may be coming from an animal which has recently died somewhere in the house. In this case, it may be prudent to locate the remains and remove from the home or structure.