Flies can be a persistent nuisance, often seeming to appear from nowhere and gravitating toward humans. This attraction is not random; it stems from a complex interplay of biological cues that flies are expertly equipped to detect. Understanding the specific sensory mechanisms and chemical signals involved sheds light on why these insects are drawn to our presence.
How Flies Sense Their Surroundings
Flies possess a sophisticated array of sensory organs that allow them to navigate their environment and locate sources of attraction. Their compound eyes are highly adapted for detecting movement, which helps them track potential hosts or food sources.
Beyond vision, flies rely heavily on their antennae, which serve as primary organs for smell (olfaction) and taste (gustation). These antennae are covered with specialized hairs called sensilla, containing olfactory neurons with receptors tuned to specific chemical compounds. Flies also have internal thermosensors, located in their brains and antennae, which help them detect and respond to environmental warmth, enabling them to find optimal temperatures.
The Allure of Human Chemistry
Human bodies emit a variety of chemical signals that are highly attractive to flies. Carbon dioxide (CO2), exhaled during breathing, is a significant attractant for many insects, including flies and mosquitoes, which use it to locate hosts. Flies possess specialized gustatory receptors, such as Gr21a and Gr63a, located in their antennae, that are sensitive to CO2. While high concentrations of CO2 can sometimes be aversive, fluctuating pulses, like those from breath, often signal a potential food source.
Sweat also plays a substantial role in attracting flies, containing components like lactic acid and ammonia. Lactic acid, produced during perspiration, can be particularly appealing, while ammonia attracts flies at low concentrations.
The unique scent profiles produced by bacteria on human skin further enhance this chemical allure. These microbes metabolize compounds in sweat and skin oil, releasing volatile organic compounds (VOCs) that flies can detect. For example, certain Staphylococcus species found on skin are known to emit odorants that attract flies. These bacterial byproducts, combined with other human-emitted VOCs, create a complex chemical signature that guides flies to us.
Visual and Thermal Signals
Beyond chemical cues, visual and thermal signals also contribute to a fly’s attraction. Body heat serves as a thermal beacon, as flies possess internal thermosensors that detect warmth, allowing them to identify warm-blooded hosts and select preferred temperatures.
Human movement is another visual cue that draws attention. Flies have excellent motion detection capabilities, enabling them to perceive subtle movements. This ability helps them spot and approach moving targets, including people. Certain colors or contrasts in clothing can also play a role, with some biting flies being particularly attracted to blue objects, potentially mistaking them for animals.