The abdomen is gray or yellowish with dark midline and irregular dark markings on the sides. The underside of the male is yellowish. The sexes can be readily separated by noting the space between the eyes, which in females is almost twice as broad as in males. The potential reproductive capacity of flies is tremendous, but fortunately can never be realized. It has been stated that a pair of flies beginning operations in April may be progenitors, if all were to live, of 191,010,000,000,000,000,000, flies by August.
Adults suck liquids containing sweet or decaying substances.
Larvae feed on moist food rich in organic matter. Although they are attracted to a variety of food material, house flies have mouth parts which allow them to ingest only liquid materials. Solid materials are liquified by means of regurgitated saliva. The flies are inactive at night, with ceilings, beams and overhead wires within buildings, trees, and shrubs, various kinds of outdoor wires, and grasses reported as overnight resting sites. In poultry ranches, the outdoor aggregations of flies at night are found mainly in the branches, and shrubs, whereas almost all of the indoor populations generally aggregated in the ceiling area of poultry houses.