Flies landing on humans is a common occurrence, often perceived as merely an annoyance. This seemingly simple act is a complex interaction driven by their biological needs and sensory capabilities. Understanding these reasons reveals how flies perceive their environment and seek resources.
Key Attractants
Flies are drawn to humans through a combination of sensory cues, primarily focusing on chemical signals. Our bodies constantly release a “cloud of effervescence”—a unique blend of aerosolized molecules and skin oils flies can detect. These compositions vary among individuals, making some people more appealing to flies. Flies possess sensitive receptors on their antennae and bodies that identify specific compounds in human odor.
Exhaled breath, particularly carbon dioxide (CO2), acts as a long-range beacon for flies. Many species use CO2 to locate potential hosts from a distance. Individuals who exhale more CO2, such as during physical activity, become more noticeable targets.
Body heat also serves as an attractant, as flies are ectothermic and seek warmth. They detect subtle temperature differences, making a warm human body a favorable landing spot, especially in cooler conditions. Moisture from sweat is another strong draw, providing essential water and salts. Sweat contains carbohydrates, proteins, and other nutrients flies can consume. Beyond chemical and thermal cues, flies also rely on visual signals, possessing large, multifaceted eyes sensitive to movement. This visual acuity, combined with their ability to sense odors, helps them pinpoint and approach humans.
What Flies Do When They Land
Once a fly lands, its activities are driven by its need for sustenance. Many common flies seek moisture and nutrients on the skin’s surface. They often lap up sweat to replenish water and salt. Human skin also provides dead skin cells and oils certain fly species can consume.
Flies do not possess chewing mouthparts; instead, they regurgitate digestive enzymes onto a surface. This liquid breaks down solid matter, allowing the fly to suck up the liquefied meal using its sponge-like proboscis. Their feet are equipped with taste receptors, enabling them to quickly assess if a surface offers an edible substance. Sometimes, flies may land to rest or groom themselves, cleaning their legs and bodies. While many common flies do not bite, specific species like horse flies or stable flies land to feed on blood. These flies possess specialized mouthparts to cut into skin and draw blood.
Common Landing Flies
Several types of flies interact with humans, each with distinct reasons for landing. House flies (Musca domestica) are perhaps the most common, known for their generalist feeding habits. They are attracted to a wide range of organic matter, including decaying food, garbage, and animal waste. They readily land on humans to seek sweat, skin oils, and other residues. House flies do not bite.
Fruit flies (Drosophila melanogaster) are much smaller and primarily drawn to fermenting substances. They are attracted to yeast and sugars in overripe fruits, alcoholic beverages, and some cosmetic products. Blow flies (or bottle flies), recognizable by their metallic blue or green bodies, are attracted to decaying organic matter. While they don’t bite, they can be drawn to humans by faint odors of decay, even from skin.
Biting flies, such as stable flies, horse flies, deer flies, and black flies, land on humans to obtain a blood meal. Female horse flies and deer flies use scissor-like mouthparts to cut skin, while stable flies possess a pointed proboscis for blood feeding. These species often inflict painful bites, driven by the need for blood to develop their eggs.
Why This Matters
Flies landing on humans is not just an irritation but can also pose health considerations. Flies are mechanical vectors, meaning they pick up and transfer pathogens from one surface to another. Before landing on a person, flies often visit unsanitary environments like garbage, decaying matter, or feces.
Pathogens (including bacteria, viruses, fungi, and parasites) can adhere to a fly’s external body parts, such as their legs, mouthparts, and feet. They can also carry pathogens internally. When a fly lands on human skin or food, it can deposit these microorganisms, potentially spreading diseases like typhoid, dysentery, cholera, and bacterial infections. Flies regurgitating digestive fluids and defecating on surfaces further contributes to pathogen transmission.