Flies are a common presence in many environments, and while their buzzing can be an annoyance, many wonder if they bite humans. While most fly species are harmless, some possess the mouthparts and behaviors to deliver bites. This article explains which flies bite, why they do so, and how to manage their bites.
Not All Flies Bite
Not all flies bite. A fly’s ability to bite depends on its mouthpart structure and feeding habits. Many common flies, like the house fly (Musca domestica), have sponging or lapping mouthparts designed to soak up liquids, not pierce skin. These flies feed on decaying matter, sugary substances, or liquefied food sources and cannot break human skin.
In contrast, biting flies have specialized mouthparts, often described as piercing-sucking or scissor-like, that enable them to penetrate skin and access blood vessels. When a biting fly feeds, it uses multiple blade-like structures to cut or tear the skin, allowing blood to flow. They also inject saliva containing anticoagulants to prevent the blood from clotting as they feed. In most biting fly species, it is the female that bites, as a blood meal provides the protein necessary for egg production and reproduction.
Common Biting Flies
Several types of flies are known for their painful bites to humans. These include stable flies, horse flies, deer flies, black flies, and sand flies, each with distinct characteristics.
Stable Flies (Stomoxys calcitrans)
Stable flies (Stomoxys calcitrans) closely resemble common house flies but are slightly smaller, typically measuring 5 to 7 millimeters, and have a visible piercing proboscis extending forward. They are often found around livestock and feed on the legs, ankles, and feet of humans, delivering a sharp, needle-like bite. Unlike many other biting flies, both male and female stable flies feed on blood.
Horse Flies (Tabanus spp.)
Horse flies (Tabanus spp.) are large, robust flies, sometimes growing up to 1.25 inches long, often recognized by their powerful wings and large, sometimes iridescent green or black eyes. They use scissor-like mouthparts to cut into the skin, leading to a painful bite that can bleed. Horse flies are common near water bodies and wooded areas.
Deer Flies (Chrysops spp.)
Deer flies (Chrysops spp.) are slightly smaller than horse flies, ranging from 0.25 to 0.5 inches, and are distinguished by their patterned or banded wings and brightly colored eyes, often gold or green. Their bites are painful and are caused by their cutting and lapping mouthparts. Deer flies are frequently encountered in wooded areas near water.
Black Flies (Simulium spp.)
Black flies (Simulium spp.), also known as buffalo gnats, are small, dark, humpbacked flies, typically 1/8 to 1/4 inch in length. They are prevalent near fast-moving rivers and streams, where their larvae develop. Female black flies cut the skin to feed, leaving a small puncture wound that can result in swelling and itching, sometimes causing a reaction known as “black fly fever” with symptoms like headache and nausea.
Sand Flies (Phlebotominae)
Sand flies (Phlebotominae) are tiny, pale, hairy flies, about 1/8 of an inch long, that hold their wings in a “V” shape when at rest. They are most active during dusk and dawn and deliver painful, itchy bites that often appear as small, red bumps or blisters. Sand flies are found in tropical and subtropical climates, often in moist environments.
Effects and Management of Fly Bites
Fly bites can cause a range of reactions, from minor irritation to more significant discomfort, and require appropriate management. The immediate effects of a fly bite commonly include pain, itching, swelling, and redness at the bite site. The intensity of these symptoms can vary depending on the fly species and an individual’s sensitivity to the fly’s saliva. Scratching the bites excessively can lead to secondary bacterial infections, as bacteria from the skin can enter the open wound.
While most fly bites are generally harmless, some species can transmit pathogens, though this is uncommon in many regions. For instance, deer flies can transmit tularemia, a bacterial disease, and sand flies are known to carry leishmaniasis in certain parts of the world. Symptoms that warrant medical attention include severe allergic reactions like wheezing, difficulty breathing, or widespread swelling, or signs of infection such as increasing pain, pus, fever, or swollen lymph nodes.
Managing fly bites involves both prevention and treatment of symptoms. To prevent bites, wearing protective clothing, especially light-colored long sleeves and pants, can reduce exposed skin. Using insect repellents containing ingredients like DEET or picaridin can also be effective. Avoiding outdoor activities during peak biting times, such as dawn and dusk for sand flies, and staying away from habitats where biting flies are abundant can also minimize exposure.
For treating bites, cleaning the affected area with soap and water is recommended to reduce the risk of infection. Applying a cold compress or ice pack can help reduce swelling and pain. Over-the-counter anti-itch creams, such as hydrocortisone or calamine lotion, can relieve itching, and oral antihistamines may reduce itching and swelling. If symptoms persist or worsen, or if signs of infection or a severe allergic reaction appear, seeking medical advice is advisable.