The Cluster Fly is commonly seen in the fall, winter and spring, especially on warm days. During the late summer, female cluster flies lay their eggs in the soil. Within a few days, the eggs hatch and the larvae emerge to feed on earthworms. In late summer and early fall, when the nights start to turn cold, adult cluster flies seek shelter inside homes, where they remain for the winter. Sometimes, called attic flies, cluster flies often enter a home by squeezing through tiny cracks and weatherstripping.