Phorid flies, often called humpbacked flies or coffin flies, are small insects belonging to the family Phoridae. These flies are globally distributed and are frequently encountered in human environments. Because of their common presence around homes and commercial facilities, many people wonder about their potential to inflict harm, specifically whether these tiny flies bite humans.
The Direct Answer: Phorid Flies and Biting
Phorid flies do not bite humans or animals. The adults of this fly species possess mouthparts that are designed for sponging or lapping up liquids, not for piercing skin. Their feeding apparatus is structured to consume fluids from decaying organic matter, nectar, or honeydew, which is a significant biological distinction from biting insects like mosquitoes or horse flies. The perception of a bite may sometimes occur when a phorid fly lands on a person and runs rapidly across the skin in its characteristic erratic movement. This quick, scuttling behavior can feel sudden or prickly, leading to the mistaken belief that the fly has attempted to bite or sting.
Identification and Preferred Habitats
Adult phorid flies are small, typically measuring only about 1/16 to 1/8 inch in length, and are generally tan to dark brown. Their most distinctive physical feature is the severely arched or “humpbacked” shape of the thorax when viewed from the side. They are also known as “scuttle flies” because of their unique behavior; when disturbed, they tend to run quickly and erratically across a surface rather than immediately taking flight.
These flies thrive by breeding in moist, decaying organic matter, which provides the necessary food source for their larval stage. This includes a wide variety of unsanitary materials such as sewage-contaminated soil, the sludge lining of drain pipes, rotting produce, and animal carcasses. Because they can develop in such diverse and often concealed environments, including under concrete slabs from broken sewer lines, they are frequently encountered indoors, often emerging near floor drains or sinks.
Why Phorid Flies Are Considered Pests
Phorid flies are categorized as “filth flies” because they spend their developmental and adult lives in highly unsanitary environments. They pose a public health concern due to their breeding and feeding habits. The primary danger they present is the mechanical transmission of pathogens.
As they move between contaminated materials, such as sewage or decomposing animal matter, and human food surfaces, their bodies pick up and carry bacteria and disease-causing organisms. These flies can physically transport microbes like those that cause salmonellosis, gastroenteritis, and other infectious diseases onto food preparation areas, utensils, or directly onto food intended for human consumption. This ability to shuttle dangerous contaminants from a breeding site to a kitchen counter is what makes the phorid fly a health hazard, particularly in food-handling facilities and hospitals.
Distinguishing Phorid Flies from Other Small Flies
Phorid flies are often confused with other small household insects like fruit flies or fungus gnats. A key visual difference is the phorid fly’s humpbacked thorax. Fruit flies, for instance, typically have bright red eyes and a rounder body shape, contrasting with the phorid fly’s generally darker eyes and more robust build.
Behaviorally, the scuttle fly’s erratic, stop-and-start running across surfaces is a definitive way to distinguish it. Fruit flies usually hover or fly directly around their food source, while fungus gnats are weaker fliers often found near potted plants. Furthermore, the wings of a phorid fly have thick, dark veins clustered near the leading edge, a unique pattern that differs from the less complex wing venation of a fruit fly.