Flies interact with humans primarily to secure a blood meal for reproduction or to gather surface nutrients and moisture. The perception of a fly “attack” depends entirely on the species involved and its specific biological needs. Only a minority of fly species possess the specialized mouthparts required to pierce human skin. Their motivation is strictly to obtain protein for egg development, not aggression. Many common nuisance flies are instead attracted to the chemical signals, warmth, and moisture that humans emit.
The Flies That Seek a Blood Meal
A select group of flies, known as hematophagous species, are equipped to bite humans and other mammals for a blood meal. This behavior is almost exclusively limited to female flies, who require the protein and iron in blood to mature their eggs. This reproductive necessity is the sole driver of the biting behavior.
Families like Tabanidae, which include horse flies and deer flies, are known for their painful bites. These flies have powerful, bladelike mouthparts that slice into the skin to create a wound. They are “pool feeders,” meaning they lap up the blood that pools at the surface of the laceration, often leaving a noticeable wound.
Other biting flies, such as the stable fly (Stomoxys calcitrans), which resembles a house fly, use a different mechanism. Both male and female stable flies are blood feeders, possessing a rigid, piercing proboscis that they insert directly into the host’s tissue to draw blood. Black flies (Simuliidae) also bite to access blood, using complex mouthparts that cut the skin. They inject anticoagulant saliva to ensure blood flow. This saliva can also trigger an allergic reaction in the host.
Flies That Land for Moisture and Nutrients
The majority of flies that interact with humans do not bite but are drawn to the human body for surface feeding. Common house flies (Musca domestica) and face flies lack piercing mouthparts. They instead have sponging mouthparts adapted for liquefying and soaking up liquids. Their attraction is based on the rich mix of compounds available on the skin’s surface.
Flies are highly sensitive to the chemical components in human sweat, which provides them with salt, proteins, and carbohydrates. Tears and mucus are also attractive sources of moisture and nutrients, explaining why flies often hover around a person’s eyes and mouth. The warmth and carbon dioxide that humans exhale signal our presence, guiding these insects to a readily available food source.
To consume solid material on the skin, a house fly will regurgitate digestive enzymes onto the surface to dissolve it, then sponge up the resulting liquid. This behavior, while non-aggressive, is a mechanism for the transfer of pathogens. Their landing is an indiscriminate search for a quick meal, viewing the human body as a mobile source of moisture and organic residue.
Potential Health Risks from Fly Interaction
Both biting and non-biting fly interactions pose distinct risks to human health, ranging from mild irritation to serious infection. The primary concern with non-biting flies, such as the house fly, is their role in mechanical transmission of disease. These flies frequently contact unsanitary matter like feces and decaying material, picking up pathogens on their hairy legs and bodies.
When a fly lands on human food or skin, it can deposit millions of bacteria, including organisms responsible for typhoid, dysentery, and E. coli infections. This mechanical transfer happens through physical contact and the fly’s feeding habit of regurgitating fluids. Biting flies can also act as biological vectors, where a disease-causing organism multiplies within the fly before being injected into a host during a blood meal.
Beyond disease transmission, a risk associated with certain fly species is myiasis: the parasitic infestation of living human tissue by fly larvae, or maggots. Flies like the human botfly (Dermatobia hominis) lay their eggs on a host. The resulting larvae burrow under the skin to develop, causing painful lesions. Allergic reactions to the saliva of biting flies can also occur, leading to localized swelling, itching, and discomfort.