Flies are often perceived as mere annoyances, but their tendency to land on us is driven by specific biological requirements. Understanding these motivations reveals how our bodies and surroundings provide resources these insects seek for survival and reproduction.
What Draws Flies to Humans
Flies possess highly sensitive sensory systems that detect various cues emitted by humans, drawing them into our personal space. A primary attractant is carbon dioxide (CO2), which humans constantly exhale. Flies can detect even minute changes in CO2 concentrations, guiding them toward a potential host. This explains why they often hover near faces, where CO2 is most concentrated.
Body warmth also serves as a significant attractant for flies, which are cold-blooded insects relying on external heat sources. Our body temperature provides a detectable thermal signal, particularly from exposed skin. Additionally, human body odor, including sweat, skin oils, and the byproducts of bacteria on our skin, provides a complex array of chemical signals that flies find appealing. These scents, often imperceptible to humans, act as beacons, drawing flies closer to us.
Why Flies Land and What They Seek
Once in close proximity, flies land on humans primarily to obtain resources. They seek moisture and salts, readily available in human sweat and tears, which are important for hydration and electrolyte balance. Flies also look for protein and nutrients in dead skin cells, skin oils, or any open wounds or residues on the skin.
Houseflies, for example, have sponging mouthparts, not biting ones. These mouthparts consist of a flexible labium with sponge-like lobes (labella) that absorb liquids through capillary action. When a fly encounters solid food, it may regurgitate digestive enzymes to liquefy it before sponging up the solution. While flies may land to rest, their primary motivation for landing on humans is to acquire essential nutrients and moisture.
Flies and Potential Health Concerns
Flies can pose health concerns due to their habits of visiting unsanitary environments before landing on humans. They frequently land on decaying organic matter, feces, and other contaminated surfaces, where they can pick up bacteria, viruses, and other pathogens on their bodies and legs. A single fly can carry millions of bacteria.
When these flies subsequently land on human skin, food, or surfaces, they can transfer these microorganisms. This mechanical transmission of pathogens can contribute to the spread of various illnesses. The mechanism involves flies carrying infectious agents externally on their fuzzy feet and bodies, or internally in their digestive tracts, which can then be deposited through physical contact, regurgitation, or defecation.