Flies’ attraction to humans is not random but stems from a combination of sensory cues that signal the presence of resources important for their survival and reproduction.
Chemical Signals We Emit
Flies possess a highly developed sense of smell, relying on chemical signals emitted by humans to locate them from a distance. A primary attractant is carbon dioxide (CO2), which we exhale with every breath. Flies are drawn to this CO2, which indicates a potential host or food source. The amount of CO2 exhaled can vary based on an individual’s size, activity level, and metabolic rate, potentially making some people more attractive to flies.
Body odor also plays a significant role in attracting flies. This odor results from bacteria on our skin breaking down sweat and oils, producing volatile organic compounds (VOCs). Specific compounds like lactic acid, carboxylic acids, and ammonia, found in human sweat and skin secretions, are particularly appealing to flies. Flies have specialized olfactory receptors on their antennae that can detect these compounds even at low concentrations, guiding them toward us.
Visual and Thermal Cues
Flies also use their vision and ability to detect heat to find humans. Their compound eyes, made of thousands of individual lenses, provide a wide, almost 360-degree field of view. This visual system is highly attuned to detecting movement, which makes a large, moving object like a human body readily apparent against a stationary background. While flies may struggle to focus on stationary objects, their exceptional motion detection helps them track us.
Beyond visual cues, humans emit body heat, which acts as a thermal signal for flies. Flies are cold-blooded and rely on external sources for temperature regulation, making warm bodies attractive. Their antennae contain neurons sensitive to temperature changes, allowing them to detect radiant heat, especially at closer ranges.
Moisture and Nutrient Sources
Humans provide flies with essential moisture and nutrients, which are vital for their survival. Sweat, for instance, is a source of water, salts, carbohydrates, and proteins that flies can consume. Flies have sponging mouthparts that allow them to lap up these liquid substances from our skin.
Other bodily secretions and conditions also serve as attractants. Tears, for example, contain water, salts, and proteins that flies may seek out. Saliva, along with food residues on skin or clothing, offers additional liquid and nutrient sources. Open wounds are particularly attractive as they provide a warm, moist environment rich in blood and decaying tissue, ideal for feeding and egg-laying for some fly species.