Flies often seem to have an uncanny attraction to humans, buzzing and landing on our skin with persistent regularity. This common, sometimes irritating, behavior prompts a question: why are flies so drawn to us? Their attraction stems from their biological needs and sophisticated sensory systems, as our bodies provide resources for their survival.
The Allure of Human Skin
Human skin offers a rich array of substances that attract various fly species. Sweat, a primary draw, contains valuable components like salts, amino acids, carbohydrates, and proteins. Natural skin oils, including fatty acids, also serve as a food source. Beyond these secretions, dead skin cells and moisture provide additional sustenance and hydration.
The warmth radiating from the human body is another significant attractant for cold-blooded insects like flies. Flies rely on external heat sources to regulate their body temperature; our body heat offers consistent warmth. Carbon dioxide, which humans exhale, also acts as a beacon, guiding flies to a potential host.
How Flies Sense Their Targets
Flies possess highly developed sensory mechanisms to detect and locate humans. Their primary olfactory organs are their antennae, covered with tiny hairs called sensilla containing specialized chemoreceptors. These chemoreceptors are sensitive to volatile organic compounds (VOCs) from human skin, including lactic acid, acetic acid, and ammonia in sweat. Flies also have taste receptors on their feet and mouthparts, allowing them to “taste” surfaces.
Complementing their chemical senses, flies use their compound eyes for visual detection. Each compound eye is composed of thousands of individual visual units called ommatidia, providing a wide, nearly 360-degree view. While their image resolution is not as sharp as human vision, flies are good at detecting movement, which helps them spot potential hosts and avoid threats. They can move their retinas to improve depth perception and track objects.
More Than Just a Resting Spot
Flies land on humans to fulfill several biological needs. Our skin provides opportunities for feeding on proteins, carbohydrates, salts, and sugars in sweat and skin oils. They also consume dead skin cells, which serve as a readily available food source. Houseflies lack biting mouthparts; they regurgitate digestive enzymes onto solid food, including skin particles, to liquefy it before sucking it up with their proboscis.
Humans are also a source of hydration. Moisture from sweat, tears, and exhaled breath provides essential liquids. Landing on humans also offers a temporary warm spot, helping flies maintain their body temperature. Ultimately, the persistent landing behavior of flies is driven by these survival instincts, as they seek resources for nourishment and hydration.