Flies are often drawn to human legs due to specific attractants on the skin and their sophisticated sensory capabilities. Understanding these interactions provides insight into why these insects seem drawn to us.
Specific Attractants on Human Legs
Human legs provide a unique combination of attractants. One primary factor is sweat, which contains various compounds flies find appealing. Components like lactic acid, ammonia, and other volatile organic compounds (VOCs) are released through perspiration and act as chemical signals. Flies are also drawn to the salts and sugars in sweat, which serve as a source of moisture and nutrients.
Body heat emitted from the skin is another attractant. Flies, being ectothermic, seek out warm surfaces, and human skin offers a comfortable landing spot, especially in cooler environments. Carbon dioxide, a byproduct of human respiration and metabolism, is also detected by flies, signaling a potential host or food source. CO2 is also released through the skin, contributing to the overall “scent signature”. The microenvironment of legs, with potentially less airflow and greater sweat accumulation, can concentrate these attractants, making them particularly appealing.
Fly Sensory Systems
Flies possess highly developed sensory systems to detect human attractants. Their primary organs for detecting chemical cues are the antennae and maxillary palps, which are covered with specialized olfactory receptors. These receptors are highly sensitive to the volatile organic compounds, ammonia, and lactic acid found in human sweat and body odor. Detecting these molecules allows flies to effectively “smell” potential food sources or hosts.
Flies also use thermoreceptors to sense body heat. These receptors, located in various parts of their bodies, allow them to perceive temperature changes and locate warm objects. Some flies, particularly those that feed on blood, have specialized sensory structures capable of detecting infrared emissions. This combination of olfactory and thermal sensing enables flies to navigate and land on human skin.
Types of Flies Attracted
Various fly species are attracted to human legs for different reasons. Non-biting flies, such as house flies (Musca domestica) and flesh flies, are drawn to moisture, salts, and organic nutrients on the skin. House flies are scavengers that seek warm surfaces and may land to consume secretions or dead skin cells. These flies do not bite but can be persistent in their pursuit of these resources.
Biting flies are attracted to human legs for a blood meal, necessary for reproduction. Stable flies preferentially feed on the lower legs of humans and other warm-blooded animals. Horse flies and deer flies are also known to bite humans, attracted by warmth, sweat, and carbon dioxide emissions. These flies actively seek out hosts, and their painful bites are a direct result of their need for blood.