Cluster flies are a common household insect often seen indoors, particularly as seasons change. These insects, also known as attic flies, typically measure between 8 to 10 millimeters in length, making them slightly larger than the familiar housefly. They are dark grey with a distinctive black and silver checkered pattern on their abdomen, and their thorax is covered in golden or yellowish hairs. Unlike houseflies, cluster flies move more sluggishly and rest with their wings overlapping over their back.
Adult Cluster Fly Feeding Habits
Adult cluster flies exhibit feeding habits that differ significantly from other common flies, as they are not drawn to decaying organic matter or food waste found indoors. Their diet primarily consists of natural substances found in their outdoor environment. Adult cluster flies feed on plant sap, the nectar from various flowers, and juices from ripe fruits. They may also consume honeydew, a sugary liquid secreted by aphids and other plant-sucking insects.
The presence of adult cluster flies inside a home is generally not for feeding purposes. Instead, their indoor activity is driven by entirely different biological needs. They do not seek out human food or garbage, nor do they reproduce while inside structures.
The Larval Diet: A Surprising Twist
The most distinctive aspect of the cluster fly’s life cycle is the diet of its larval stage. Unlike many other fly species, cluster fly larvae are endoparasites, meaning they develop and feed inside another organism. These larvae specifically target earthworms as their hosts.
The life cycle begins when an adult female cluster fly lays her eggs in cracks in the soil, often near earthworm burrows. After hatching, typically within about three days, the tiny, cream-colored larvae, or maggots, actively seek out an earthworm. Once a host is located, the larva burrows into the earthworm’s body and feeds internally, consuming its tissues for approximately two to three weeks. This parasitic development ultimately leads to the earthworm’s demise. After maturing, the larvae exit the earthworm and pupate in the soil for about 11 to 14 days before emerging as adult flies.
How Their Diet Influences Their Presence
The unique diet of cluster fly larvae profoundly influences why these insects are often found in and around homes. Since the larval stage relies on earthworms for development, cluster flies are commonly found in outdoor areas with healthy earthworm populations, such as lawns, gardens, and pastures.
As outdoor temperatures begin to drop in late summer and early fall, adult cluster flies, having completed their life cycle outdoors, seek sheltered locations for overwintering. Homes, particularly those with sun-warmed walls, offer ideal protective sites. They commonly enter structures through small cracks, crevices, gaps around windows and doors, and openings in attics or wall voids. Their indoor presence is purely for shelter from the cold, and they typically remain in a dormant state until warmer weather arrives in spring, when they attempt to return outdoors to lay eggs and continue their life cycle.