Biting flies belong to the order Diptera, the order that includes all true flies. Only a small portion of this vast group, known as hematophagous Diptera, feeds on blood. These insects require blood to survive or reproduce. The true culprits are distinct from nuisance flies because they possess specialized mouthparts adapted for penetrating skin to access a blood meal. Understanding which flies bite and why is important for mitigating their impact on humans and animals.
The Biological Imperative: Why Flies Bite
The fundamental drive for blood feeding, or hematophagy, is tied directly to reproduction for most biting fly species. In many groups, only the female fly seeks a blood meal because the proteins and nutrients obtained are necessary for egg maturation. Male flies typically subsist on plant nectar and other sugar sources, which provide energy but lack the components needed for egg production.
The method of obtaining blood differs depending on the fly’s specialized mouthparts. Some flies, such as stable flies, utilize a piercing-sucking mechanism. A rigid, needle-like proboscis is inserted directly into a blood vessel, and the fly injects saliva containing anti-clotting agents to maintain blood flow.
Other species, like horse flies, employ a cutting or slashing mechanism. Their mouthparts are equipped with blade-like components that slice into the host’s skin, creating an open wound. The fly then laps up the pooling blood, a process that is often immediately painful and results in a larger injury at the feeding site.
Identifying the Most Common Biting Species
The most frequently encountered biting flies fall into four distinct categories, each with unique characteristics and feeding behaviors.
Horse Flies and Deer Flies
These large flies belong to the family Tabanidae. Horse flies can measure up to an inch or more in length, while deer flies are slightly smaller and often possess distinct banded wings. They primarily inhabit moist environments near wetlands, forests, or livestock pastures, and are active during the warmer parts of the day. Their scissor-like mouthparts physically cut tissue to induce blood flow, causing immediate, sharp pain. Female horse and deer flies are highly persistent until they acquire their blood meal.
Stable Flies
Stable flies, or Stomoxys calcitrans, look similar to the common house fly but possess a distinct, pointed proboscis that juts forward. Unlike many other biting species, both male and female stable flies feed on blood, which they require for sexual maturation and survival. These flies are commonly found near livestock operations or where decaying organic matter, such as rotting hay or manure, is present. They frequently target the lower legs and ankles of humans and animals, delivering a painful, stabbing bite. They often bite in the early morning and late afternoon, and their presence can be disruptive to grazing animals.
Black Flies
Black flies, members of the family Simuliidae, are small, dark, and possess a characteristic humpbacked appearance. Their larvae develop exclusively in fast-running, well-oxygenated water, such as rivers and streams. Adults can travel significant distances from these breeding sites and are notorious for swarming around the head and face of their hosts. The bite is caused by minute, serrated mouthparts that lacerate the skin, resulting in a small, bleeding wound. Large numbers of feeding black flies can cause severe discomfort and irritation.
Biting Midges
Biting midges, sometimes called “no-see-ums” or “punkies,” are among the smallest biting flies, often measuring less than one-eighth of an inch. Their minute size allows them to easily pass through common window screening, making them bothersome in coastal or marshy environments where they breed. They are most prevalent near the edges of ponds, lakes, and seashores. The bite is often disproportionately irritating compared to the fly’s size. While the bite may not be immediately painful, the reaction is characterized by persistent and intense itchiness. They are known to be active both during the day and at night depending on the species and environmental conditions.
Immediate Reactions to Fly Bites
The immediate physical response to a fly bite is primarily an immune reaction triggered by the fly’s saliva. When a fly feeds, it injects a complex cocktail of compounds into the host’s skin to facilitate the blood meal. This saliva contains anticoagulants to prevent clotting and vasodilators to increase blood flow.
These foreign proteins prompt the host’s immune system to release chemicals like histamine, which causes localized inflammation. Common symptoms include pain, redness, and swelling at the bite site. The swelling often manifests as an itchy papule or a raised, reddened wheal.
The intensity of the reaction varies based on the fly species and the host’s sensitivity. Bites from cutting flies, such as horse flies, often cause more immediate pain due to physical trauma. Excessive scratching of the itchy lesions can break the skin, leading to potential secondary bacterial infections that may require medical attention.
In rare instances, an individual may experience a widespread or severe allergic reaction. Systemic reactions, including anaphylaxis, have been documented, particularly following bites from species like horse flies. These severe reactions are related to the host’s hypersensitivity to the salivary proteins injected during the feeding process.