Flies, often seen as nuisances, exhibit a distinct attraction to humans. This behavior stems from a complex interplay of sensory cues, including chemical signals, physical emanations, and resource availability. Understanding these biological drivers reveals why humans are such compelling targets for various fly species.
The Scents That Attract
Flies possess highly sensitive chemosensory organs, primarily on their antennae, which allow them to detect a range of volatile organic compounds (VOCs) emitted by the human body. These compounds act as chemical signals, guiding flies toward potential food sources and hosts. Our skin, breath, and sweat contain a rich mixture of these attractants.
Lactic acid, a byproduct of human sweat, serves as a significant attractant for many fly species. Similarly, ammonia, found in human sweat and urine, also draws flies and can act synergistically with carbon dioxide to enhance attraction. Additionally, fatty acids and carboxylic acids present in skin oils and secretions are appealing to flies. These diverse olfactory cues create a detectable “cloud” around humans.
Invisible Signals: Heat and Carbon Dioxide
Beyond chemical scents, flies are also drawn to invisible physical and gaseous signals emanating from humans. Body heat is a strong attractant, with flies possessing specialized thermoreceptors that detect temperature changes. These sensory neurons, located in areas like the antennae, guide flies toward warm-blooded hosts.
Carbon dioxide (CO2) exhaled during breathing is another potent signal. Flies detect CO2 using specific receptors located on their antennae. While high concentrations of CO2 can sometimes be aversive, ambient levels indicate the presence of a living organism and attract flies. This gaseous cue is a universal signal for many insects searching for hosts or food sources.
Resource Exploitation: Food and Breeding Grounds
Food Sources
Flies are attracted to humans primarily because our bodies and surrounding environments offer diverse food sources. Many species, such as houseflies, do not bite but instead feed on the readily available nutrients on our skin. This includes sweat, tears, mucus, and dead skin cells, which provide carbohydrates, proteins, and salts. Flies lap up these secretions using their sponging mouthparts.
Breeding Grounds
Beyond direct consumption from the human body, the human environment often provides ideal breeding grounds. Female flies lay eggs on decaying organic matter of animal or vegetable origin, which is abundant in human habitats. Garbage, food waste, compost heaps, and animal feces found near human dwellings serve as prime locations for egg deposition and larval development. These sites ensure a continuous food supply for their larvae, making human surroundings highly attractive for fly reproduction.