Egg -- The yellowish-white egg has two ridges on its dorsal side and is elongated at one end into a slender, grayish-black stalk. Each egg is about 3.0 mm long (including the stalk) and about 0.5 mm wide.
Larva -- This yellowish, maggot-like larva develops through three instars and attains a length of 13 mm. The anterior spiracles have 8 to 9 finger-like openings and the posterior spiracles are D-shaped.
Puparium -- Slightly smaller than the mature larva, the puparium is gray or dirty white.
Feeding Habits -- Female face flies feed primarily on the tears, saliva, mucous, blood and excrement of pasturing animals, especially cattle. The flies are sometimes annoying to man.
Damage -- Face flies do not bite their victims. Rather, they annoy livestock by constantly alighting around eyes, nose, mouth, and wounds. As a result, animals may hide in shady places, may not graze, may lose weight, and may produce less milk. Face flies sometimes carry the causative organisms of infectious bovine keratoconjunctivitis (pinkeye), an eye disease of cattle. In addition, face flies sometimes transmit Thelazia eyeworms to horses and cattle. Thelazia eyeworms cause symptoms similar to those of bovine keratoconjunctivitis in the U.S. when large numbers of eyeworms occur in the infected eye.
Life History -- Adult face flies overwinter in buildings and other sheltered areas and become active in early spring. In general, female flies congregate in bright, sunny areas, avoiding shade and never venturing into barns. At intervals of several days, eggs are deposited in batches of 6 to 26 in fresh bovine manure in pastures, and eggs hatch within 16 to 18 hours. When temperatures are warm, maggots feed and develop in the manure for 3 to 5 days. At temperatures of 11oC (52oF) or less, however, larval development may last as long as 3 weeks.
Mature larvae usually burrow into the upper layer of soil to pupate though pupae have been found in dry manure. After a pupal period of 8 to 10 days, a new generation of flies emerges. About 3 to 5 days later, the newly emerged females mate and begin laying eggs. New generations continue to be produced approximately every 2 weeks until late autumn. Populations increase during periods of relatively high humidity and decline in dry periods. In late August and early September, the adult face flies enter diapause and begin to seek overwintering sites. One preferred site for overwintering is white frame buildings, many times houses. During the warm days of January and February, large numbers of overwintering flies may become active, clustering near windows. During March and April, the adults leave the overwintering site and return to the fields and pastures.