The attraction of common flies, such as house flies and various biting species, to humans involves a sophisticated interplay of sensory cues. These insects utilize a combination of chemical, thermal, and visual signals to locate a host or a nutritional source. A human body constantly emits a complex mix of breath, heat, and skin odors, and understanding this multi-sensory mechanism reveals why some individuals are more attractive to flies than others.
The Role of Body Chemistry and Odor
The most compelling attractants for many fly species are the volatile organic compounds (VOCs) constantly released from the human body surface. These airborne chemical signals are often byproducts of the bacteria that colonize our skin, which break down components of sweat and sebum (skin oil). The resulting chemical cocktail acts as an olfactory beacon, signaling a rich source of nutrients, moisture, and salts.
A primary attractant is lactic acid, a component of human perspiration that signals an active, warm host. Certain flies are also strongly drawn to the nitrogenous compound ammonia, which is released as bacteria metabolize urea in sweat. These compounds often work synergistically, meaning the presence of both lactic acid and ammonia together is more attractive than either chemical alone. Carboxylic acids, found in high concentrations in human sebum, further contribute to the scent profile and are strong attractants to many insect species.
Detecting Breath and Heat
Flies use signals related to the body’s physiological processes, such as respiration and temperature, to find a target from a distance. The exhaled plume of carbon dioxide (\(\text{CO}_2\)) is a long-range cue that alerts flies to the presence of a breathing organism. Specialized receptors located on the fly’s antennae detect slight increases in \(\text{CO}_2\) concentration, guiding the insect toward the general area of the host.
Once the fly is in closer proximity, it begins to detect the body’s thermal signature. Biting flies, such as horse flies and deer flies, are particularly sensitive to body heat, which they perceive as infrared radiation. Their antennae contain sensitive thermal sensors that detect the difference between ambient air and the warmth of a mammal’s skin. This heat acts as a final close-range cue, signaling the location of a warm-blooded host and directing blood-feeding species to areas where blood vessels are closer to the surface.
Visual Triggers and Environmental Context
The fly’s visual system plays a significant role in attraction, particularly its sensitivity to movement and color. Flies are highly attuned to motion, and a moving human figure represents a clearer target than a stationary object. Their compound eyes excel at detecting rapid changes, which is why swatting at them often results in a quick escape.
Flies exhibit preferences for certain colors and contrasts, often favoring blue and white, which are utilized in commercial fly traps. Dark colors tend to absorb and radiate more heat, which can inadvertently make a person wearing dark clothing a more attractive thermal target for heat-seeking flies.
Flies are also attracted to humans because of the surrounding environment, making the attraction incidental rather than direct. Humans often gather near decaying organic matter, trash, animal waste, or open food sources, which are primary attractants for common house and filth flies. In these fly-rich environments, the human body becomes a convenient secondary target, offering the flies a source of moisture, salts, and a landing platform.