The presence of flies buzzing around humans is a common occurrence. These small insects frequently appear to single out individuals, circling or landing on them. This behavior is not random; it stems from a fly’s fundamental biological needs and their highly developed sensory systems, which detect specific cues emitted by the human body and its immediate environment. Humans inadvertently offer resources flies seek for survival and reproduction.
Why Flies Are Drawn to Humans
Flies possess an acute sense of smell, detecting compounds from human skin. Our body odor, a complex mixture of volatile compounds from sweat, skin oils, and skin bacteria, provides appealing signals. These include substances like lactic acid, ammonia, and certain fatty acids, which flies find attractive. Individuals who perspire more or have a unique skin microbiome may become more appealing to flies.
The carbon dioxide (CO2) we exhale is another strong attractant for many insect species, including various flies. Flies are equipped with specialized receptors that detect CO2 from a distance, using it as a signal for a potential host or food source. The amount of CO2 expelled varies with body size and activity level, potentially making larger or more active individuals more noticeable.
Human body heat acts as a beacon for flies. As cold-blooded creatures, flies rely on external sources for temperature regulation. The warmth radiated by our bodies provides a resting spot, guiding them towards us, especially in cooler surroundings.
Flies require moisture to survive and are drawn to damp areas on human skin. Sweat, with its water content and dissolved salts, serves as a source of hydration and essential minerals. They are particularly drawn to areas where sweat glands are concentrated, such as the neck, armpits, and around the eyes and mouth.
Beyond moisture, flies are scavengers with a broad diet, and human bodies can offer food residues. They are attracted to food particles, sugary residues, natural oils, or dead skin cells present on our skin or clothing. Their ability to taste with their feet allows them to quickly identify potential nutrients upon landing.
Common Fly Actions When Near People
Once attracted to a human, flies engage in specific behaviors driven by their search for sustenance. They frequently land on skin, clothing, or nearby surfaces to investigate for food and moisture. House flies do not bite; they use soft, sponge-like mouthparts to suck up liquids. If the food source is solid, they may regurgitate digestive enzymes to liquefy it before consuming it.
Flies may seek warmth or shelter on a human body. Our bodies provide a stable, warm perch, especially if external environmental conditions are unfavorable. This offers a brief respite before they resume their search for resources.
While perched, flies engage in grooming behaviors, cleaning their legs and antennae. This action maintains the sensitivity of their sensory organs, vital for detecting scents and navigating their surroundings. They might perform these cleaning rituals while resting on a human.
Flies are naturally exploratory creatures, continuously investigating new scents they detect. Their keen olfactory sense leads them to circle or land on people as they try to pinpoint the source of appealing odors. This investigative behavior is part of their foraging strategy.
Different Flies, Different Reasons
The specific reasons for a fly’s attraction vary depending on the species, each having distinct needs and preferences. Common house flies (Musca domestica) are primarily drawn to decaying organic matter, including food waste, animal excrement, and other decomposing materials. When they land on humans, it is often because they detect residual traces of these substances or are exploring a warm surface for potential nutrients like salts and oils.
Fruit flies (Drosophila species) have a strong affinity for fermenting fruits, sugary liquids, and alcohol. They are attracted to the yeast and microbial volatiles produced during fermentation, which indicate a suitable food source and breeding ground. A person who has recently handled fruit, consumed sugary drinks, or used fruit-scented products might attract these smaller flies.
In contrast, some smaller flies, such as gnats or biting midges, have a different motivation for approaching humans. Species like biting midges actively seek blood meals, necessary for the female to produce eggs. These flies are drawn to the same cues as other insects, such as carbon dioxide and body heat, but their ultimate goal is to obtain blood, distinguishing their interaction from non-biting species.