The other is a parasite of earthworms. The face fly is a non-biting pest of pastured cattle and horses. Adult face flies are commonly seen sopping up protein rich secretions around the eyes, mouth and nostrils of their hosts. They closely resemble a house fly, but are slightly larger and often somewhat darker in color. Face flies go pretty much unnoticed during the summer by everyone except cattle ranchers and horse owners.
Cluster flies are also slightly larger than the house fly, but are dark gray in color.
Their thorax is also covered with golden hairs or setae. Interestingly, the larvae of the cluster fly are parasitic on earthworms. Adult flies lay eggs in small cracks in the surface of the soil where they hatch in about three days. The larvae then burrow into earthworms, completing their development in about 30 days. There are at least four generations per year.