Yes, some flies do bite humans, but the vast majority of the 150,000 species belonging to the order Diptera cannot pierce skin. The common house fly, for instance, is physically incapable of inflicting a true bite. When a fly “bites,” it is actually engaging in hematophagy—piercing the skin and sucking blood—a feeding behavior limited to specific fly families. This distinction between flies that feed on liquids and those that actively seek a blood meal is determined entirely by the structure of their mouthparts. Understanding this difference helps identify which flies pose a physical threat.
How Fly Mouthparts Determine Feeding
The ability of a fly to bite is dictated by one of two primary mouthpart architectures found across the fly order. Many familiar flies, such as the house fly, are equipped with sponging-lapping mouthparts, which are designed to consume liquid or semi-liquid food. This structure consists of a soft, enlarged tip called a labellum, which acts like a sponge to soak up fluids after saliva is regurgitated onto solid food to dissolve it. These flies cannot puncture skin, meaning they are incapable of delivering a bite to humans or animals.
In contrast, flies that bite possess a rigid, specialized structure known as a piercing-sucking proboscis. This proboscis is a hardened, needle-like or bayonet-shaped instrument used to penetrate the host’s skin. The female flies of these species require a blood meal to gain the protein necessary for egg production, a behavior called obligatory hematophagy. When these flies land, they use this sharp apparatus to access blood vessels beneath the skin and draw blood directly into their bodies.
The difference in feeding mechanism explains why some flies are merely annoying while others deliver a painful sting. A fly with sponging mouthparts will attempt to lap up sweat or residue on the skin’s surface, but a fly with a piercing proboscis will aggressively seek a blood source. The presence of a protruding, rigid proboscis is often the most reliable visual identifier for a fly that is capable of biting.
Identifying Common Biting Flies
Among the flies that bite, the Stable Fly (Stomoxys calcitrans) is frequently encountered, often mistaken for the common house fly due to its similar size and dull-gray coloration. Unlike the house fly, the stable fly has a distinct, stiff, bayonet-like proboscis that projects forward from its head. It uses this to deliver a painful bite, typically targeting the ankles or lower legs of humans. Both male and female stable flies feed on blood, usually resting on nearby vertical surfaces before landing on a host for a quick meal.
Larger and more aggressive are the Horse Flies and Deer Flies, which belong to the family Tabanidae and are known for their extremely painful bites. The females of these species do not use a fine needle to probe, but instead use blade-like mouthparts to slash or lacerate the skin, creating an open wound from which they lap up the pooling blood. Horse flies are robust, often measuring up to 30 millimeters. Deer flies are smaller, known for their colorful, striped eyes and their habit of flying around a person’s head.
Another group of biters includes the tiny Sand Flies, sometimes called biting midges, which measure only 1 to 3 millimeters in length. These minute insects are often unobserved during the bite itself, but their presence is confirmed by the intensely itchy, red welts they leave behind. Sand flies are typically found in tropical and subtropical regions, breeding in moist, organic matter and are most active during the hours around dawn and dusk.
Reactions and Health Risks of Fly Bites
The immediate response to a fly bite is often localized pain and a rapid inflammatory reaction caused by the insect’s saliva. When a biting fly feeds, it injects saliva containing anticoagulant and often anesthetic compounds into the wound to facilitate blood flow. This foreign substance triggers the immune system, resulting in characteristic symptoms like swelling, redness, and persistent itching at the site of the bite. These reactions can range from a mild, temporary welt to a more severe allergic response, sometimes involving hives or significant localized swelling.
Disease Transmission
A more concerning consequence of fly bites is the potential for disease transmission, though this risk varies greatly by species and geographic location. Deer flies, for example, are known to mechanically transmit the bacterium responsible for tularemia, commonly known as “rabbit fever,” which can cause fever, skin ulcers, and headache in humans. Sand flies are vectors for the parasite that causes leishmaniasis, a disease that can lead to chronic skin lesions or a more serious systemic illness, particularly in tropical and subtropical climates.
Secondary Infections
In addition to vector-borne diseases, any fly bite creates a break in the skin, which introduces the risk of a secondary bacterial infection. Scratching the intensely itchy bite site can worsen the wound, allowing common bacteria like Staphylococcus or Streptococcus to enter and cause cellulitis. Immediate care for a fly bite involves cleaning the wound with soap and water to minimize infection risk. Applying a cold compress or topical anti-itch cream can help manage swelling and discomfort.