Being outdoors often brings the unwelcome experience of a fly bite, prompting questions about why these insects draw blood. While the common housefly is generally harmless, many other fly species do bite for specific biological reasons. Not all flies bite, but certain species are equipped with specialized mouthparts for feeding on blood.
Which Flies Bite and Why
Not all flies bite; many species, such as the common housefly, possess sponging mouthparts designed for liquid food sources like nectar or decaying matter. However, several fly species are equipped to bite humans and other animals, primarily for a blood meal necessary for reproduction. Female flies require the protein and nutrients in blood to develop their eggs, a biological need that drives their biting behavior.
Among the most common biting flies are horse flies and deer flies, both belonging to the Tabanidae family. Horse flies are large, often exceeding an inch in length, while deer flies are slightly smaller with distinctive patterned wings. Stable flies, resembling house flies but with a noticeable piercing mouthpart, also bite and are commonly found near livestock. Black flies, also known as “buffalo gnats,” are small, humpbacked flies typically found near flowing water. Sand flies are tiny, pale, and hairy, often active at dawn and dusk.
How Flies Bite
Biting flies employ specialized mouthparts to obtain their blood meal, a process distinct from the piercing action of mosquitoes. Horse flies and deer flies, for instance, have scissor-like mouthparts that cut into the skin, creating an open wound. These “blades” lacerate the skin and blood vessels, causing blood to flow to the surface. Other mouthpart components then sponge or lap up the exposed blood.
Stable flies, despite their housefly-like appearance, possess a rigid, bayonet-like proboscis that extends forward from their head, which they use to pierce the skin. Black flies create a small puncture wound, sometimes injecting a numbing substance, allowing them to feed on the blood that pools. As the fly feeds, it injects saliva containing anticoagulants into the wound. This prevents the blood from clotting, ensuring a continuous flow for the fly to consume.
Factors Attracting Biting Flies
Biting flies locate hosts by sensing various cues, making some individuals more appealing targets than others. A primary attractant is carbon dioxide (CO2) exhaled from breathing, which acts as a long-range signal guiding flies to potential hosts. Body heat and moisture, particularly from perspiration, also draw biting flies closer. The lactic acid and other compounds present in sweat can further enhance their attraction.
Movement serves as another visual cue, with flies often detecting and pursuing moving targets. Dark colors are also known to attract biting flies, making individuals wearing dark clothing potentially more susceptible. These factors combine to help flies efficiently find and feed on humans and animals.
Common Reactions to Bites
Reactions to fly bites vary, but typically involve an immediate immune response to the fly’s saliva. The injected saliva, containing anticoagulants and other proteins, can trigger local inflammation. Common symptoms include pain, redness, and swelling at the bite site. The area often develops an itchy, raised bump or welt, which can persist for several days.
Horse fly bites are often painful, resulting in significant welts and itchiness. Stable fly bites are described as a sharp, needle-like prick, commonly occurring on the legs and ankles. Black fly bites can lead to considerable swelling, sometimes the size of a golf ball, along with itching and a small puncture wound. In some cases, systemic symptoms like headache, nausea, or swollen lymph nodes, sometimes called “black fly fever,” can occur with black fly bites. While most reactions are localized and resolve on their own, persistent itching can lead to secondary bacterial infections if the bite is scratched excessively.