Here is a close up of a cluster fly.

In order to get inside homes, cluster flies search for any small openings outside that will get them into the house. Also when windows or doors are open, that becomes a great entry opportunity for the cluster flies. Cluster flies will start entering homes in the fall, as the weather begins to cool down. By the winter most cluster flies will now be hibernating within a warm home. While hibernating the cluster fly is not very active, so a homeowner might not even know they are there. Although there are occasions where the cluster flies will come out in the day to sun themselves. Even though the cluster flies are in a warm house, they still enjoy sunning themselves in actual sunlight. If homeowners were to spot them it would be near the windows of the home; otherwise they pretty much stay in their hibernation spots. The areas of a home that the cluster flies like to hibernate in are the attics and wall voids. The cluster fly is also known as an attic fly because that is their most popular spot to be found. Cluster flies also like to be higher up, hence the attic as a popular hibernation spot. Many times people may spot them in their attics because they all clustered together once they are in there. Seeing a big blackish looking spot in your attic might be frightening, but it is probably just cluster flies once you look up close. Once spring starts to turn the cluster flies will leave the house and venture back outside. Once outside again they will mate and eat, neither of which they do during hibernation.Now that spring is here and the cluster flies are back outside mating will begin. The cluster flies reproduce very frequently. The females do not have to do much; all they do is mate then they lay the eggs. The eggs are laid in soil near earthworms. About three days later the eggs will hatch and the larvae will migrate towards the earthworms, and then burrow inside of the earthworms. The rest of the development is done inside the earthworm using it as a food source. Once fully developed the cluster fly will be on its own. This cycle will continue from the spring to the summer. During this time up to four generations or more can be made. As summer comes to an end and fall approaches the cluster flies will have to look for another home, or the same home, to hibernate in.