Flies often appear to single out humans, but this behavior is a biological search for resources, not personal harassment. These insects are driven by an innate necessity to find sustenance, moisture, and minerals to survive and reproduce. The human body is an abundant, mobile source of these requirements, making us an attractive target. Understanding this interaction requires examining the substances we emit, the sensory tools flies use, and the reasons for their landing.
The Chemical Attractants on Human Skin
The primary reason flies gravitate toward human skin is the rich chemical cocktail exuded by our sweat glands and skin microbiome. Sweat contains diluted but readily available nutrients, including carbohydrates, proteins, and amino acids, serving as an easily accessible food source. Sweat is also rich in sodium chloride (salt), a mineral flies actively seek to replenish their bodily reserves.
The appeal is intensified by our natural skin oils (sebum) and the constant shedding of dead skin cells. These organic materials provide additional nutrition. The warmth of the human body helps volatilize these compounds, making them easier for the fly to detect from a distance.
The skin’s resident bacteria break down components in sweat and sebum, releasing volatile organic compounds like lactic acid and various carboxylic acids. These compounds are intensely attractive to the insects.
Flies also seek out the moisture and protein found in other bodily secretions, which is why they frequently target the face. Tears and saliva contain salts and proteins that are highly desirable for nutrient collection and hydration. If the skin has a minor cut or abrasion, the resulting blood proteins and moisture become a concentrated and appealing target for feeding.
How Flies Use Their Senses to Locate Humans
Flies initiate their search for a host using a powerful sensory system capable of detection across various distances. For long-range tracking, the insect relies heavily on the carbon dioxide (\(\text{CO}_2\)) we exhale. This gas diffuses from the body in a plume, signaling the presence of a warm-blooded animal that may be a food or moisture source.
As the fly gets closer, it switches to detecting specific odors using highly sensitive olfactory receptors located on its antennae. These receptors are tuned to the volatile organic compounds produced on the skin, such as lactic acid and short-chain carboxylic acids. This chemical mapping allows the fly to confirm that the \(\text{CO}_2\) source is a desirable target.
Flies possess large, complex compound eyes, which are excellent at detecting motion, serving as a final navigational tool. The warmth radiating from the human body also acts as a thermal beacon, helping the fly home in on the source of the chemical plume. The combination of \(\text{CO}_2\), body odor, and heat guides the fly directly to its landing site.
Landing Behavior and the Need for Moisture
The persistent landing behavior is driven by the fly’s need to “taste” the surface and acquire moisture. Flies do not have taste buds on their mouthparts; instead, they possess gustatory receptors on the tarsi (feet). By landing or walking on a surface, the fly instantaneously samples the chemical composition of the skin’s film.
If the tarsal receptors detect desirable compounds, such as sugars or salts, the fly initiates a proboscis extension response. The proboscis is a fleshy, sponging mouthpart used to soak up liquids. Since flies can only ingest liquid food, they must first liquefy any solid nutrients, such as dried salts or dead skin particles.
To accomplish this, the fly regurgitates a small amount of its stomach contents, including digestive enzymes, onto the chosen spot. This liquid droplet dissolves the solid material, creating a nutrient-rich “soup” that the fly then sucks back up. This constant need to sample, moisten, and feed explains why flies repeatedly land, particularly on moist areas like the corners of the eyes or mouth.