It is common to have a fly persistently buzzing around you, following your every move. This behavior, while often irritating, stems from biological drivers and sensory capabilities that allow flies to detect and respond to their environment. Understanding these mechanisms reveals why humans can be such compelling targets for these small insects.
How Flies Sense Their Environment
Flies possess a highly developed sensory system that enables them to navigate and locate resources. Their most prominent sensory organs are their compound eyes, which are composed of thousands of individual visual units called ommatidia. Each ommatidium acts as a tiny, independent eye, collectively forming a mosaic-like view of the surroundings. This visual system excels at detecting motion and changes in light, rather than sharp, detailed images like human eyes. This motion sensitivity helps them quickly perceive movement, including that of larger objects like humans.
Beyond vision, flies rely heavily on their antennae, which are equipped with specialized sensory hairs. These antennae function as primary organs for olfaction, allowing flies to detect airborne chemical cues. They also contain chemoreceptors for taste, which can be found on their mouthparts and even their legs. Flies also possess thermoreceptors, which enable them to sense temperature changes and locate warm environments.
What Attracts Flies to Humans
Flies are drawn to humans by a combination of chemical and physical cues. A significant attractant is the carbon dioxide (CO2) we exhale with every breath. Flies are highly sensitive to CO2, using it as a long-range signal to locate potential food sources or hosts. This explains why flies often buzz around a person’s face, where CO2 is most concentrated.
Body odor also plays a substantial role in attracting flies. This includes various chemical compounds found in sweat, such as lactic acid and ammonia, as well as the byproducts of bacteria residing on human skin. These substances provide nutrients and signals that flies find appealing. Additionally, the warmth radiated by the human body serves as an attractant. Moisture from sweat and skin oils also provides a source of hydration and nutrients for flies.
The Persistence of Following Behavior
Once a fly is initially attracted to a human, its persistent following behavior is driven by several underlying motivations. Flies are constantly searching for suitable landing spots for various purposes, including resting, grooming, or even laying eggs. A human can represent a large, warm surface that might offer such an opportunity.
Flies are also opportunistic scavengers, and they may perceive humans as potential sources of food or moisture. They are drawn to the salt, dead skin cells, and oils present on our skin, which they can “lick up” with their sponging mouthparts. Even if shooed away, a fly’s strong instinctual drives mean it will often return to the detected attractants, perceiving the human as a valuable, albeit mobile, resource.