A black fly about 3/8 inch long (8 mm). Cluster flies resemble house flies but are slightly larger and narrower. At rest they overlap their wings at the tip whereas the house fly does not. The thorax is without distinct stripes, has many yellow hairs (visible in photo 3) and the dark gray abdomen has dark and light patches. Legs are black, eyes are large and dark brown.
There are six species of cluster flies in North America and they are widespread and common. They are commonly seen inside human habitation in fall when they seek locations for hibernation for winter.
Most cluster flies are known to be parasitoids of earthworms. Females lay eggs in the soil and the larvae seek out their earthworm hosts. The biology of these cluster flies is poorly known. Some cluster flies are recorded from other hosts including caterpillars and bees.
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