Certain species of flies do bite humans, though the term “bite” refers to a specific biological action. These insects are obligate blood feeders, meaning they must pierce the host’s skin to extract a blood meal, a process known as hematophagy. The bite is not a defensive action but a necessary step for the female fly to acquire the protein and nutrients required to develop her eggs. This ability is determined by specialized, sharp mouthparts that are very different from those of the common house fly.
Identifying the Biting Culprits
The flies capable of piercing human skin belong to several distinct families, each with specific behaviors and habitats.
Horse Flies and Deer Flies
The Tabanidae family includes the large Horse Flies and the smaller Deer Flies (Chrysops genus). Both use scissor-like mouthparts to slash the skin, causing a painful wound. Deer flies are often encountered near wooded areas, streams, and lakes.
Stable Flies and Black Flies
The Stable Fly (Stomoxys calcitrans) closely resembles the non-biting house fly but possesses a stiff, bayonet-like proboscis for piercing. These flies are often found around livestock and decaying organic matter, inflicting sharp, stabbing pain, frequently targeting the ankles and lower legs. Black Flies (Simuliidae family) are small but inflict painful bites. They often swarm near running water sources like creeks and rivers where their larvae develop.
Biting Midges and Sand Flies
Even smaller are the Biting Midges (Ceratopogonidae), commonly known as “no-see-ums” or “punkies.” Their minute size allows them to penetrate standard window screens, and their bites can be irritating. Sand Flies (Psychodidae family) are small and hairy, and can transmit serious diseases in tropical and subtropical regions.
How Biting Flies Are Different
The fundamental difference between biting and non-biting flies lies in the morphology of their mouthparts, which dictates their feeding strategy. Non-biting species, such as the common House Fly (Musca domestica), possess a sponging proboscis. This mouthpart ends in fleshy, sponge-like lobes called labella, which are covered in tiny channels.
Non-biting flies cannot pierce the skin; instead, they liquify solid food by regurgitating saliva and then use the labella to soak up the resulting liquid. In contrast, a biting fly has a complex, hardened structure designed to penetrate the host’s dermal layer. For example, the female Horse Fly uses sharp, blade-like mandibles and maxillae that cut the tissue, causing a pool of blood to form.
Other biting flies, like the Stable Fly, have a piercing proboscis that acts like a hypodermic needle to puncture the skin and directly access a capillary. In both cases, the fly injects a salivary secretion into the wound that contains anticoagulants. This prevents the blood from clotting, ensuring a successful meal, a mechanism entirely absent in non-biting flies.
Health Risks Associated with Fly Bites
The immediate consequence of a fly bite is localized pain, often described as a sharp sting or slice, followed by a reaction to the fly’s saliva. This injected saliva causes the body to release histamine, leading to localized swelling, redness, and the formation of an itchy welt. In some individuals, the reaction can be severe, involving extensive swelling or a systemic allergic response.
Disease Transmission
Certain biting flies are medically significant because they can act as vectors for pathogens. This transmission occurs when the fly feeds on an infected animal and then carries the pathogen on its mouthparts to a human host. Deer Flies, for instance, are known to transmit the bacterium Francisella tularensis, which causes tularemia.
In tropical regions, Sand Flies are vectors for leishmaniasis, a parasitic disease that can lead to severe skin sores or affect internal organs. Any break in the skin from a bite also introduces a risk of secondary bacterial infection if the site is scratched or poorly cleaned. Scratching allows common bacteria from the skin or environment to enter, potentially leading to cellulitis.