A horsefly does not sting; it bites. The horsefly belongs to the family Tabanidae, a group of true flies that includes deer flies and clegs. Unlike stinging insects that use a modified ovipositor to inject venom, the horsefly uses specialized, bladelike mouthparts to cut into the skin to obtain a blood meal.
Identifying the Horsefly
Horseflies are robust, large insects, with many species ranging from 8 to 28 millimeters in length. They possess a stout body and prominent compound eyes that are often brightly colored or iridescent. The wings are typically large and fan-shaped, and some species may have patterned or mottled wings. They are most commonly encountered in warm weather near water sources, which is where they lay their eggs.
Only the female horsefly bites humans and other vertebrates, as she requires blood protein to develop her fertilized eggs. The male horsefly has weaker mouthparts and sustains itself by feeding on nectar and plant exudates. Female horseflies are persistent hunters, often attracted to large, dark, moving objects and the carbon dioxide exhaled by potential hosts. Their aggressive pursuit of a blood meal is a hallmark, and they will frequently continue to bite until successful.
The Mechanism of the Bite
The horsefly uses a method of feeding known as telmophagy. Unlike a mosquito, which uses a delicate, needle-like proboscis to pierce a capillary, the female horsefly employs two pairs of razor-sharp cutting blades. These blades are used to slice or tear the skin, creating an open, cross-shaped wound. This crude, surgical-like action immediately causes a sharp, noticeable pain as tissues and small blood vessels are damaged.
Once the skin is lacerated, the blood pools in the resulting wound, which the horsefly then laps up using a sponge-like part of its mouth. To ensure a continuous flow of blood for its meal, the fly injects saliva containing an anticoagulant into the wound. This substance prevents the blood from clotting, allowing the horsefly to consume the blood that wells up from the damaged capillaries. The lack of an anesthetic compound in the saliva, coupled with the slicing mouthparts, is why the bite is often messy and immediately apparent to the host.
Immediate Effects and Treatment
The immediate reaction to a horsefly bite is usually a sudden, intense pain followed by localized bleeding. The bite site quickly develops a raised, red, and swollen welt, which can be sensitive and itchy. In some cases, a small, dark spot may be visible at the center of the wound.
The first step in treating a horsefly bite is to thoroughly clean the area with soap and warm water to prevent a secondary bacterial infection. Applying a cold compress or an ice pack wrapped in a cloth to the site can help reduce both the pain and the swelling. Over-the-counter oral antihistamines can be used to manage itching and reduce inflammation, while pain relievers like acetaminophen may address residual discomfort.
Medical attention should be sought immediately if a person experiences signs of a severe allergic reaction, such as swelling of the throat or face, dizziness, or difficulty breathing. A person should also consult a healthcare provider if the bite site shows increasing redness, warmth, pus, or if the wound does not improve after a few days, as these are indications of a potential infection.