Flies are a diverse order of insects, and the act of biting is not a universal trait among them. The common house fly, for instance, is merely a nuisance with a sponging mouthpart, but other species within the order Diptera have evolved specialized structures to consume blood. Understanding why these specific flies bite requires examining their biological imperatives, their unique anatomy, and the consequences the bite has for both the insect and the host.
Identifying the Biting Species
The majority of flies encountered daily, such as house flies and fruit flies, do not possess the physical capability to bite a host, instead having mouthparts designed to soak up liquid food sources. True biting flies, conversely, are equipped with hardened, specialized mouthparts that allow them to penetrate the skin of mammals and birds. Common examples of flies known for this parasitic behavior include the Horse Fly and Deer Fly, which are large and aggressive biters often found near wooded areas and water. Stable Flies, which closely resemble house flies, have a piercing proboscis and frequently target the ankles and lower legs of humans and livestock. Smaller species like Black Flies, sometimes called buffalo gnats, inflict painful bites near running water, and tiny Sand Flies can transmit diseases in tropical regions.
The Biological Need for Blood
For most biting fly species, the motivation for a blood meal is tied directly to reproduction. Only the female fly requires blood, while the male typically sustains itself on nectar, plant sap, or other sugary liquids. The blood meal is a necessary component of the female’s gonotrophic cycle, the process of developing a batch of mature eggs. The blood provides a concentrated source of protein and lipids that the female fly cannot obtain from sugar alone, ensuring the viability and quantity of her offspring. This biological imperative drives the female’s persistent and sometimes aggressive search for a host.
The Mechanics of the Bite
The painful sensation of a fly bite is largely due to the mechanical nature of the feeding apparatus. Flies like the horse fly and deer fly possess specialized, scissor-like mandibles that they use to slice or tear into the host’s skin, creating a small pool of blood (telmophage feeding). The pain is immediate because these flies do not inject an anesthetic, unlike some other blood-feeding insects. Other biting flies, such as the stable fly, use a more needle-like, piercing-sucking proboscis, similar to a mosquito, to tap directly into a blood vessel. During feeding, the fly injects saliva containing powerful biochemical compounds, including anticoagulants and vasodilators, which prevent clotting and increase blood flow to the site.
Immediate Effects and Disease Risk
The host’s immediate reaction to a fly bite is typically sharp pain, followed by localized irritation, swelling, and redness, often resulting in an itchy welt or raised bump. In some cases, a severe allergic reaction can occur, leading to more pronounced swelling or other systemic symptoms. Beyond the localized discomfort, certain biting flies act as vectors for pathogens, posing a serious health risk. Deer flies, for instance, are known to mechanically transmit the bacterium Francisella tularensis, which causes tularemia, or rabbit fever, in humans, while Black flies are vectors for parasitic worms that cause onchocerciasis, or river blindness. This risk is compounded for livestock, where persistent biting from stable flies and horse flies can lead to significant blood loss, stress, and the transmission of various animal diseases.