Flies often buzz around people’s faces. This behavior stems from specific biological reasons related to how flies sense their environment and what attracts them to humans.
How Flies Sense Their World
Flies possess sophisticated sensory capabilities. Their vision relies on compound eyes, which consist of thousands of tiny lenses, providing a nearly 360-degree field of view. This allows flies to detect movement with exceptional speed, perceiving over 200 frames per second compared to human vision at about 60 frames per second. This rapid motion detection helps them quickly react to changes in their surroundings.
Flies also use their antennae as tools for sensing odors and air currents. These antennae are covered with sensory receptors that detect minute concentrations of chemicals, guiding them to food sources, mates, or suitable habitats. Their antennae constantly wave, effectively scanning the air for chemical signals. Beyond vision and smell, flies have chemoreceptors on their legs and mouthparts, allowing them to taste surfaces they land on. These receptors help them assess the edibility of a substance, detecting sugars, salts, and other compounds.
What Draws Flies to Humans
Flies are drawn to humans by cues signaling potential sources of food, moisture, or warmth. Carbon dioxide (CO2) exhaled during breathing is a significant attractant, which flies detect using specialized receptors on their antennae. This CO2 plume acts as a beacon, guiding flies towards potential hosts.
Body heat is another attractant for flies, as they are cold-blooded and seek external warmth to regulate their body temperature. Our bodies radiate heat, making us appealing landing spots, especially in cooler environments.
Moisture from sweat and tears also draws flies, providing essential hydration and salts. Sweat contains proteins, carbohydrates, and salts that flies can feed on, while tears offer water and electrolytes.
Human body odors, a complex mix of volatile organic compounds, further attract flies. Specific compounds like lactic acid, ammonia, and various fatty acids found in sweat and skin oils are particularly appealing. These chemical signals, along with bacterial byproducts on the skin, guide flies in their search for nutrients. Flies can also detect sebaceous oils on skin.
Why the Face is a Frequent Target
The face is a concentrated hub of several attractants, making it an especially appealing target for flies. It is the primary source of exhaled carbon dioxide, creating a strong, localized plume that flies can easily detect. The warmth radiating from the face, with its rich blood supply, also provides an inviting thermal target.
The facial area is also abundant in moisture, from breath, sweat, and tear secretions around the eyes. This moisture, combined with salts and organic residues like skin oils and dead skin cells, offers flies a readily available source of hydration and nutrients. The face provides these elements for survival and reproduction. The movement of facial muscles during speech or expression might also visually attract flies, prompting them to investigate this dynamic part of the body.