The face fly has a slate-gray thorax. Life Cycle and Habits Female cluster flies lay eggs singly in soil cracks and crevices in the vicinity of the earthworm, Allolobophthora spp. The life cycle is completed in 27 to 39 days. There are about four generations during the summer. Populations vary from year to year, sometimes worse after wet summers.
Flies cluster on the warm sides of buildings in late summer during the day.
As the sun goes down and temperatures cool, flies crawl into the building through cracks, especially under eaves, gaps in siding, etc. They are attracted to light, light-colored siding and structures on lawns and pastures inhabited by earthworms. They become active whenever temperatures rise above 54 degrees F indoors from early autumn to mid-spring, especially around windows with sunlight. Just as they become a nuisance in the fall while seeking hibernating quarters, they are also bothersome in the spring, trying to escape. Face flies are serious pests of cattle and horses.