Cluster flies are common household pests, particularly noticeable during cooler months when they seek shelter indoors. Understanding their unique appearance is important for accurate identification, helping to distinguish them from other more common flies.
Overall Physical Characteristics
Cluster flies typically measure between 8 to 10 millimeters in length, making them slightly larger than a common house fly. Their bodies are generally dark gray, appearing dull and non-metallic, unlike some other fly species. A notable feature is a subtle, checkered pattern on their abdomen, which consists of light and dark gray areas.
Distinctive Markings and Structures
Closer inspection reveals more specific identifying features of cluster flies. Their thorax, the upper body segment, is covered with numerous short, crinkly, golden-yellow or brownish hairs, which can sometimes give them a golden sheen. These hairs are a key differentiator, though they may be less apparent on older flies.
When at rest, cluster flies hold their wings flat over their back, with the tips often overlapping in a scissor-like fashion, a characteristic posture that differs from many other flies. Their eyes are typically reddish-brown. Their antennae are short and segmented.
How They Differ from Other Flies
Cluster flies are often confused with other common household flies, such as house flies and blow flies, but several distinctions exist. Unlike house flies, which typically have four distinct dark stripes on their thorax, cluster flies lack these prominent stripes and instead possess the golden thoracic hairs. House flies also tend to have a more pronounced checkerboard pattern on their abdomen and hold their wings separately at rest, whereas cluster fly wings overlap. Blow flies, in contrast, are generally characterized by their metallic blue or green bodies, a stark difference from the dull, non-metallic gray of cluster flies. Compared to much smaller flies like fruit flies or gnats, cluster flies are significantly larger and possess a distinctly different body shape, making misidentification unlikely due to size disparity alone.
Behavioral Indicators
Beyond their physical traits, certain behaviors can further help identify cluster flies. They are often described as sluggish and slow-moving, particularly when indoors, exhibiting less erratic flight than house flies. This slower movement can make them seem confused as they crawl along walls or windowsills.
True to their name, cluster flies tend to gather in large numbers, or “clusters,” in undisturbed areas such as attics, wall voids, or window frames, especially as temperatures drop in autumn and winter. This clustering behavior is a significant indicator of their presence. They seek shelter indoors to overwinter, often becoming active and noticeable when indoor temperatures rise or during sunny periods, drawing them towards windows and other light sources.