Cluster flies, scientifically known as Pollenia rudis, commonly become a household nuisance, particularly as temperatures drop. These insects are often mistaken for the common house fly, but they possess distinct physical differences and behaviors that set them apart. This article details the size and specific characteristics necessary for accurate identification.
Physical Dimensions and Comparison
The size of an adult cluster fly is larger than that of a typical house fly. An average specimen measures approximately 7 to 8 millimeters in length, or about 3/8 of an inch. This makes them significantly chunkier than the common house fly, which typically ranges from 3 to 6 millimeters.
Their body shape contributes to a more sluggish and deliberate flight pattern, a stark contrast to the quick, darting movements of other flies. When observed, the cluster fly appears darker and duller than many other flies, lacking the vibrant, shiny coloration found on blow flies.
Key Identification Features
The most telling feature is the presence of numerous short, crinkly golden or yellowish hairs that cover the sides and top of their thorax, giving them a fuzzy appearance. This feature is entirely absent on the common house fly, which instead has four distinct black stripes running down its thorax.
Another reliable feature is their unique resting posture. When at rest, the cluster fly tends to overlap its wings completely over its abdomen, similar to a pair of scissors. Unlike blow flies, the cluster fly’s body is a dark gray or brown-black color, lacking any metallic blue or green sheen.
Why Cluster Flies Enter Homes
As temperatures begin to decrease in the late summer and early fall, adult cluster flies search for a sheltered, warm place to hibernate for the winter, a process known as overwintering. They are often seen clustering on the sunny, south or west-facing exterior walls of buildings before crawling into small cracks and voids.
The larval stage is unique because the maggots are subterranean parasites of earthworms, meaning the flies do not breed in filth or decaying matter like many other nuisance flies. Once they enter a home, they typically aggregate in secluded spaces like attics and wall voids, which is how they acquired their name.
They are considered a nuisance pest because they do not reproduce indoors or transmit diseases. However, their sheer numbers and tendency to emerge on warm winter days can be bothersome to residents.