Most homeowners will see cluster flies migrating to the sunniest portion of a home. As the name would imply, cluster flies love traveling in large groups, and they can be found both inside and outside of homes. Unlike other types of insects, cluster flies love flying during winter. It's common for homeowners to witness these flies crawling out of attics and wall voids. Experts believe cluster flies originate from Europe, but through different types of ships, cluster flies have been able to migrate to North America. The earthworm is known to be the preferred host for cluster flies.