Flies, from common house flies to biting gnats and deer flies, are not drawn to humans randomly. These insects interpret the human body as a mobile, resource-rich environment. Their attraction is an instinctual response to a cascade of stimuli we constantly emit into the surrounding air. Locating and landing on a person involves a complex, multi-stage sensory hunt, beginning with airborne chemicals and culminating in close-range physical cues.
How Chemical Signals Guide Flies
The initial detection of a human host occurs over long distances, primarily through the sense of smell (olfaction). Flies use specialized receptors on their antennae to detect specific volatile organic compounds (VOCs) that form an invisible chemical plume around us. This long-range tracking often begins with carbon dioxide (CO2), which is expelled with every breath and signals the presence of a large, breathing mammal.
Flies follow the rising concentration gradient of this CO2 plume, which can guide them from many meters away toward a potential host. As the insect gets closer, it begins to detect the more complex, low-level odors emanating directly from the skin.
These odors are a blend of chemicals produced when the skin’s natural bacteria break down compounds found in sweat and oils. Specific compounds like lactic acid, ammonia, and short-chain carboxylic acids are powerful attractants to many fly species. Lactic acid is a byproduct of human metabolism found in sweat, while ammonia is a breakdown product of aged sweat.
Detecting Visual and Thermal Signatures
Once the chemical plume has brought the fly into close proximity, non-olfactory cues take over to guide the final approach and landing. Fly vision is attuned to motion and contrast, helping them lock onto a large, moving object like a person. The complex, compound eyes of flies detect sudden changes in light and shadow that accompany movement, offering a roughly 360-degree field of view.
Flies are particularly drawn to areas of sharp visual contrast, such as a human silhouette against a bright sky or dark clothing against a light background. This visual information is often integrated with the initial CO2 signal. The detection of breath can trigger a stronger attraction to visual features, helping the fly orient toward a landing site.
For blood-feeding species, the final stage of detection involves thermal signatures, which are sensed at very short range. They use heat sensors to detect skin that is elevated above ambient temperature, signifying a warm-blooded target. This heat cue helps them pinpoint the location of the host’s skin before attempting to locate a blood vessel close to the surface.
Biological Reasons for Landing on Humans
The primary motivation for landing on human skin is the search for essential resources necessary for the fly’s survival and reproduction. Many generalist flies, such as the common house fly, are attracted to the thin film of residue on our skin that contains protein, carbohydrates, and salts. They land to lap up these secretions, using their sponging mouthparts to ingest the nutrients found in sweat and skin oils.
Salt is a strong attractant, as it is a difficult-to-obtain mineral required for nerve and muscle function in the insect’s body. Flies possess taste receptors on their feet, allowing them to instantly assess the edibility and nutrient composition of a surface upon landing.
House flies may also land to exploit minor wounds or areas of bodily secretion, such as tears or mucus. These provide readily available moisture and nutrients.
For species like horse flies and deer flies, the attraction is driven by the need for a blood meal to facilitate egg development. They use the integrated system of CO2, visual cues, and heat to locate a host and obtain the necessary proteins and lipids for reproduction. The human body represents a source of sustenance, warmth, and moisture in the fly’s environment.