A typical cluster fly is a tiny bit bigger than a common housefly and moves very slowly. The golden, short hairs on a cluster fly's thorax make it easy to identify. Put simply, the thorax is what the cluster fly's wings and legs are attached to. The larvae that grow into cluster flies develop inside of earthworms. The biggest difference between common house flies and cluster flies is the fact that cluster flies always fly in the direction of household windows that contain the most sunlight. The northern portion of the United States is where the highest concentration of cluster flies can be found.