Cluster Flies closely resemble houseflies but are larger, slower, clumsier and their wings overlap when at rest. Cluster Flies are noted in Spring/Fall most times on the sunnier side of structure. Cluster Flies move to structures in late summer early fall to overwinter in attics, soffit/facia areas and under siding/sill plates… Once weather warms enough all Flies will leave structure for summer again breeding out in open areas. (May/June depending on weather) Cluster Flies DO NOT BREED IN STRUCTURE all breeding is done outside laying eggs in soil. The larval stage of Cluster Fly is parasitic to the earth worm actually living within the earth worm for a period of time. Most times country or large fielded areas are where Cluster Flies are most prevalent.