Face flies deposit eggs in cattle manure. The eggs are equipped with a breathing stock, the end of which sticks above the manure surface until the eggs hatch. The larvae pass through three larval stages in the manure then disperse to the edge or out of the manure to pupate. Face fly larvae (maggots) are yellowish and the puparium is white. All other manure-inhabiting flies have white larvae and brown pupal cases. When the flies emerge, they mate and females seek a protein source necessary for egg maturation. Female face flies feed on various secretions from cattle or other animals; tears, saliva, nasal mucus, blood serum exuding from wounds, perspiration, and excrement adhering to the hair of animals. Male face flies generally feed only on pollen.
The life cycle of face flies from egg to adult takes about three weeks during summer months. In mid-September, face flies cease the reproductive cycle and seek overwintering sites. The overwintering sites include barns, houses and other buildings and probably sheltered areas outdoors as well. The flies have a tendency to overwinter in the same sites each year. Winter survival is very low if these sites are unheated. Face flies and two or three species of cluster flies are often found in large clusters in the same overwintering site.
Source: John B. Campbell, Extension Entomologist