Horse flies (family Tabanidae) are well-known for their painful bites and persistent pursuit of hosts. This biting behavior is limited exclusively to the females, as they require a blood meal to provide the necessary protein for producing eggs. Understanding how these insects locate a host is a fundamental step in mitigation and avoiding their attacks. Female horse flies employ a sophisticated, multi-stage hunting strategy that relies on a combination of sight, smell, and temperature detection.
Visual Cues
Horse flies are highly visually oriented predators that primarily hunt during the day, using their large compound eyes to locate potential targets. They are strongly attracted to large, moving objects, which they interpret as a mammal host such as cattle, deer, or horses. This visual recognition is triggered by movement, signaling a living food source.
Color plays a significant role in their initial selection, with dark colors being particularly appealing because they stand out against a lighter background. They show a strong preference for non-reflective hues like black, dark brown, and dark blue, which absorb more light and contrast sharply with the environment. Complex patterns, like the stripes on a zebra, appear to confuse the fly’s visual system, disrupting their ability to land accurately. The preference for dark surfaces is also linked to the heat they absorb, a factor the flies can sense from a distance. Horse flies are sensitive to polarized light patterns, often reflected off dark, shiny surfaces, which they use to distinguish a host from the surrounding landscape.
Chemical Signals
Once a horse fly’s vision has guided it into the vicinity of a host, chemical signals detected through olfaction become the primary means of homing in for the attack. The most significant chemical cue is carbon dioxide (\(\text{CO}_2\)), which is exhaled by all mammals and serves as a long-range signal of a host’s presence. The concentration gradient of this gas directs the fly closer to the source.
As the fly nears the host, it begins to detect more specific volatile organic compounds emitted from the skin and sweat. Lactic acid, a component of sweat, is a powerful attractant that signals a body is active and warm. Other odorous compounds found in sweat, such as fatty acids and alcohols like 1-octen-3-ol (octenol), further confirm the presence of a warm-blooded animal. The combination of \(\text{CO}_2\) and these sweat-derived chemicals creates an irresistible scent plume that guides the female fly to the target.
Environmental and Thermal Factors
Body heat acts as the final, close-range homing beacon for the female horse fly, directing her to the precise spot for feeding. Horse flies prefer warmer hosts and often attack sunlit, dark areas of an animal, which are typically the warmest surfaces. This preference for elevated temperatures is hypothesized to benefit the fly by increasing the function of their wing muscles, allowing for a quicker escape from a defensive host.
Environmental conditions significantly influence horse fly activity, which is highest during the warmest, sunniest parts of the day. The intensity of the sun maximizes the thermal signature of hosts, making them easier to detect. Horse flies tend to be found near water or moist ground, as their larvae develop in semi-aquatic habitats. Hosts encountering these areas during warmer months are more likely to be targeted by active, blood-seeking females.