The adult cluster fly with the approach of cool weather will enter into homes to over winter. In suburbs, older homes become more vulnerable. As the winter warms into spring, these large sluggish flies appear mostly around windows to look for a lighted escape route. They enter your living quarters through window rope pulley holes, overhead lighting, vents, a/c units and access points from your attic. Large gatherings (clusters) can be seen on the sunny side of your house during the warm days of spring. These flies are of no immediate threat to humans.
The eggs are laid in the soil and hatch in three days. The newly emerged larvae (maggot) will penetrate earthworms and feed for 13-22 days and then pupate. The pupae stage is 11-14 days before adulthood. Control of these insects begins with preventing entry by making sure screening is tight fitting and sealing cracks and crevices with caulking. Air conditioning units should be checked often to be sure there are no gaps that flies can use as an entry point. This is an annual pest that you may see inside the home when weather temperatures fluctuate.
In some cases, the amount of flies warrants professional treatment especially in sensitive buildings such as hospitals and nursing homes.