Horse flies are common biting insects found globally, often encountered in warm, humid environments. It is the female horse fly that seeks a blood meal, which is necessary for her to produce eggs and reproduce. These insects are known for their painful bites, which can disrupt outdoor activities for humans and animals. Understanding the specific factors that draw these persistent insects to humans can help in avoiding their unwelcome attention.
Visual Signals
Horse flies possess specialized vision that significantly influences their host-seeking behavior. They are particularly drawn to dark colors, with black and blue being especially attractive. These darker shades are more visible to their compound eyes and tend to absorb more heat, making a dark-colored object a more apparent target. Wearing lighter colors, such as white or yellow, can therefore make individuals less appealing to these insects.
The size and movement of an object also play a significant role in attracting horse flies. They are attracted to large, moving objects, interpreting them as potential hosts. This is why active individuals or large animals like horses are frequently targeted. Their ability to track movement allows them to effectively pursue their intended targets.
Furthermore, horse flies are drawn to shiny or reflective surfaces. Their vision is sensitive to polarized light, and surfaces that reflect this light, such as water or wet skin, can capture their attention. This attraction to reflective qualities can explain why they may seem to target individuals who are wet from sweat or swimming.
Chemical Signals
Chemical cues emitted by humans are powerful attractants for horse flies, allowing them to detect potential hosts from a distance. Carbon dioxide (CO2), a gas exhaled during respiration, is a primary signal. Horse flies possess sensory organs capable of detecting CO2 plumes, guiding them towards a source. The amount of carbon dioxide released increases with physical activity, potentially making active individuals more noticeable to these insects.
Components found in human sweat also serve as strong attractants. Lactic acid, a byproduct of muscle activity, is a notable chemical that horse flies find appealing. Ammonia, another compound present in sweat and body odor, further enhances this chemical allure. Beyond these specific compounds, general body odors and skin oils contribute to the overall chemical signature that makes humans appealing targets.
Thermal Signals
Horse flies also rely on thermal signals to pinpoint their hosts. They can sense the infrared radiation, or heat, emitted by warm-blooded animals, including humans. Their thermoreceptors, located on their legs, antennae, and mouthparts, enable them to perceive temperature gradients.
Horse flies are attracted to warmer areas of the body or the general warmth of a host. This preference for warmth is why sunlit and warmer targets are often more attractive. The detection of heat directly links to their need for a blood meal, helping them efficiently locate and approach suitable hosts.