Not all flies bite; only certain species possess specialized mouthparts and a biological drive to feed on blood. This feeding habit is primarily linked to their reproductive cycle, as blood provides essential nutrients for their offspring. Understanding which flies bite and the biological reasons behind their actions can clarify this common outdoor experience.
Identifying Biting Flies
Several fly species are known for their biting behavior, differing in size, appearance, and how they obtain a blood meal. Mosquitoes, belonging to the family Culicidae, are perhaps the most recognized, using a slender proboscis to pierce the skin. Stable flies (Stomoxys calcitrans) resemble common house flies but possess a rigid, bayonet-like proboscis for piercing, and both males and females of this species bite.
Deer flies (Chrysops) and horse flies (Tabanus) are larger, often recognized by their robust bodies. These flies use scissor-like mouthparts to cut into the flesh, causing blood to pool, which they then lap up. Black flies (Simuliidae) are small, humpbacked insects, with females also having scissor-like mouthparts that cut the skin to obtain blood. Biting midges (Ceratopogonidae), often called “no-see-ums” due to their minute size, also have tiny cutting blades to break the skin.
The Biological Need for Blood
For most biting fly species, the act of biting is intrinsically linked to the female’s reproductive cycle. Female flies require a blood meal for the development and maturation of their eggs. This blood provides a rich source of proteins, particularly for the production of vitellin, a yolk precursor protein essential for nourishing developing embryos.
Blood also supplies iron and other micronutrients unavailable from their typical diets of nectar or plant fluids. While both male and female flies may feed on sugary substances like nectar for energy, only the females of most species require blood for reproduction. Without this protein-rich meal, many female flies are unable to produce viable eggs, a condition known as anautogeny.
A sufficient blood meal triggers hormonal responses within the female fly. These cascades direct nutrients toward the ovaries, facilitating egg formation. This biological imperative underscores why these flies seek out hosts and engage in biting behavior, directly supporting the continuation of their species.
Targeting and Biting Mechanisms
Biting flies employ a range of sensory cues to locate their hosts. They are attracted to carbon dioxide exhaled by animals, as well as body heat, certain body odors, and visual cues such as movement or dark colors. Once a host is identified, the flies utilize specialized mouthparts designed for blood feeding. The specific structure of these mouthparts varies among different fly groups.
Mosquitoes, for instance, possess a complex proboscis containing multiple slender stylets that pierce the skin and locate a blood vessel. Horse flies and deer flies have robust, scissor-like mouthparts that cut through skin tissues, creating a wound from which they then sponge up the flowing blood. Black flies also use scissor-like mouthparts with serrated edges to incise the skin. Stable flies have a rigid, piercing proboscis that projects forward, allowing them to stab into the host.
During the biting process, most biting flies inject saliva into the host. This saliva contains substances that prevent blood from clotting, ensuring a continuous flow of blood for the fly to consume. Some species’ saliva may also contain compounds that act as anesthetics, minimizing the host’s immediate sensation of the bite, allowing the fly to feed undetected for a longer duration.
What Happens After a Bite
After a biting fly feeds, the host typically experiences a localized reaction at the bite site. Common symptoms include pain, itching, swelling, and redness. These reactions are primarily a result of the host’s immune system responding to the foreign proteins and other compounds present in the fly’s saliva. The body releases histamine, a chemical that increases blood flow and causes inflammation, leading to the characteristic itch and swelling.
The severity of the reaction can vary among individuals, depending on their immune sensitivity to the specific fly’s saliva. While often a nuisance, fly bites can also carry a potential for disease transmission. Some species can inadvertently transfer pathogens from an infected host to a new one during feeding.