The common house fly, Musca domestica, and its relatives frequently land on human skin, food, and surfaces. This habit is not arbitrary; it is driven by biological imperatives related to feeding, survival, and sensory perception. Flies view humans not as targets for aggression, but as large, mobile sources of the moisture, warmth, and vital nutrients they require. This article examines the biological mechanisms and attractants that explain why flies are drawn to land on us.
Sensory Tools Used by Flies
Flies employ a sophisticated sensory system to locate potential food sources, with their antennae acting as olfactory detectors. These antennae contain specialized receptors capable of detecting volatile chemical plumes over considerable distances, including carbon dioxide (CO2) and various body odors. The CO2 exhaled by humans is a primary long-range signal that indicates the presence of a large, warm-blooded animal.
The fly’s feet, known as tarsi, function as organs of taste. These appendages are covered in chemoreceptors that allow the insect to identify non-volatile substances upon contact. A fly must land to taste, using these receptors to determine if the surface—like human skin—offers salts, sugars, or other organic compounds. This contact-based tasting explains why a fly will often briefly land and walk across a surface before settling down.
Key Biological Attractants on Human Skin
The human body emits a blend of physical and chemical signals that flies find irresistible, starting with heat and carbon dioxide. Body heat acts as a physical beacon, signaling a warm, active source of potential moisture and sustenance. The plume of CO2 we exhale creates a concentrated trail that guides the fly closer, confirming the presence of a living organism.
Once a fly is close, chemical cues from the skin become the deciding factor for landing. Sweat is a rich source of salts and minerals necessary for the fly’s physiological functions, making a salty residue a potent attractant. The skin’s resident microbiome also plays a large role, as the breakdown of sweat and oil produces volatile organic compounds like lactic acid and carboxylic acids. These microbial breakdown products create an odor profile that flies perceive as a sign of available nutrients.
Why Landing is Necessary for Flies
Landing is a required step in the fly’s feeding and hydration process. House flies lack the mouthparts to bite or chew solid food, possessing instead a sponging proboscis designed for lapping up liquids. They land on the skin to ingest any available moisture, such as sweat, for hydration and salt replenishment.
If a fly encounters a solid food source on the skin, such as dried sweat residue or dead skin cells, it must first liquefy it. The fly regurgitates a small amount of digestive fluid onto the surface, which dissolves the solid matter into a digestible solution. It then sucks this liquid mixture back up through its proboscis. Landing also serves as a temporary rest stop or a place for the fly to perform necessary grooming activities before continuing its search for food or breeding sites.
Simple Prevention Measures
Since flies are drawn to humans by biological signals, prevention measures should focus on reducing these attractants. Regularly showering, particularly after physical activity, removes the accumulated sweat, salts, and the microbial byproducts that create attractive odors. This practice significantly reduces the chemical signature that draws flies in.
Using air movement is an effective strategy because it disrupts the concentrated plumes of CO2 and body odor. Placing a fan nearby or ensuring good ventilation disperses these scent trails, making it harder for flies to locate a target. Simple physical exclusion measures, such as ensuring windows and doors have tight-fitting screens, prevent the fly from gaining access to the attractive environment inside.