While most fly species are harmless, certain types bite humans, often to obtain a blood meal. These interactions can range from minor irritations to more significant health concerns. Understanding which flies bite and why can help manage these encounters.
Identifying Biting Flies
Several types of flies bite humans, each with distinct characteristics. Horse flies are large, often measuring up to 1.25 inches, and can have green heads or be entirely black. They are strong, fast fliers often found near water. Deer flies are smaller than horse flies, about the size of a housefly, and often have dark bands on their wings, appearing yellow-black or brown. Both horse and deer flies inflict painful bites with scissor-like mouthparts that cut the skin.
Stable flies, sometimes called biting house flies, resemble common house flies but possess a bayonet-like proboscis that protrudes forward from their head. They are about 0.25 inches long, gray, and have a checkerboard pattern on their abdomen. Black flies are small, stout-bodied insects, less than 0.125 inches long, with a distinctive humpbacked appearance. They are commonly found near flowing water sources like rivers and streams. Biting midges, also known as “no-see-ums” or “punkies,” are tiny, less than 0.0625 inches, making them difficult to spot.
Why Flies Bite
Female flies bite humans and other animals for a blood meal, which provides necessary protein and nutrients for egg development. Male flies feed on nectar and do not bite.
Biting flies have specialized mouthparts to pierce skin and access blood vessels. Deer flies, for instance, use blade-like mouthparts to cut into the skin, causing blood to flow, which they then lap up. When a biting fly feeds, it often injects saliva containing anticoagulants to prevent blood clotting, ensuring a continuous meal. This explains why bites can be painful and may continue to bleed slightly after the fly departs.
Effects of Fly Bites
Fly bites can cause immediate and lasting effects. The most common symptoms include pain, swelling, redness, and itching at the bite site. Horse fly bites, for example, often result in sharp pain and noticeable welts, which can be accompanied by a large, red, raised rash. Stable fly bites may feel like a sharp prick and appear on the feet, ankles, or legs, causing red rashes and small bumps. Black fly bites can leave a small puncture wound, leading to swelling that can vary from slight irritation to a golf-ball-sized bump, sometimes accompanied by headache, nausea, fever, and swollen lymph nodes, a condition known as “black fly fever.”
Beyond localized irritation, some flies can transmit pathogens. Deer flies have been linked to the transmission of tularemia, a bacterial disease, particularly in the western United States. Horse flies can also transmit tularemia and, in rare instances, anthrax. Black flies are known vectors for onchocerciasis, or “river blindness,” caused by a parasitic worm, primarily in parts of Africa, South America, and the Arabian Peninsula.
Stable flies can mechanically transmit animal diseases like anthrax and equine infectious anemia. Individuals may experience allergic reactions to fly saliva, manifesting as hives, dizziness, weakness, or wheezing, requiring immediate medical attention.
Protecting Yourself
To minimize fly bites, several protective measures can be taken. Wearing long sleeves and pants in light colors can help deter flies. Flies are often attracted to darker colors and movement, so lighter attire makes you less noticeable. Insect repellents containing DEET, picaridin, or oil of lemon eucalyptus provide a barrier against biting flies. Applying these to exposed skin and clothing significantly reduces bites.
Avoiding areas where biting flies are prevalent, such as marshy or wooded environments, especially during peak activity times like dawn and dusk, can also be beneficial. Maintaining a clean environment around homes and outdoor spaces by removing decaying organic matter and standing water reduces breeding sites for many fly species.