Why do flies bite

Overview

Flies biting humans is a behavior exhibited by specific dipteran species within the order Diptera, which contains over 160,000 described species worldwide. The evolutionary adaptation of blood-feeding (hematophagy) developed independently in multiple fly lineages, with fossil evidence suggesting biting flies existed as early as the Jurassic period approximately 150 million years ago. Among the approximately 120 families of true flies, only a few contain species that bite humans, including Tabanidae (horse flies), Simuliidae (black flies), Ceratopogonidae (biting midges), and Muscidae (stable flies). Female mosquitoes (family Culicidae), technically a type of fly, are the most medically significant biters, with Anopheles species alone responsible for malaria transmission throughout human history. The historical impact of biting flies is substantial, with records from ancient civilizations describing fly-borne illnesses, and military campaigns throughout history being affected by fly-borne diseases.

How It Works

Biting flies employ specialized mouthparts adapted for piercing skin and accessing blood vessels. Female flies require blood meals primarily for reproduction, as the protein in blood is essential for egg development. Different species have evolved distinct biting mechanisms: horse flies (Tabanidae) use scissor-like mandibles to cut the skin, then lap up blood with a sponge-like labellum; stable flies (Stomoxys calcitrans) have rigid proboscises that function like hypodermic needles to penetrate skin; black flies (Simuliidae) have cutting mouthparts that create small wounds from which they feed; and mosquitoes have stylets that include six separate parts for piercing, injecting saliva, and sucking blood. The biting process typically involves locating hosts through chemical cues like carbon dioxide, body heat, and skin odors, followed by injection of saliva containing anticoagulants and anesthetic compounds to facilitate feeding. After obtaining a blood meal, which can range from 0.001 to 0.01 milliliters depending on species, females utilize the nutrients for oogenesis (egg production).

Why It Matters

Biting flies have significant impacts on human health, agriculture, and economies worldwide. Medically, they transmit numerous pathogens including malaria parasites (Plasmodium species via Anopheles mosquitoes), dengue virus (Aedes mosquitoes), and filarial worms (black flies transmitting river blindness). The World Health Organization estimates vector-borne diseases account for more than 17% of all infectious diseases globally. Economically, biting flies affect livestock production through reduced weight gain and milk production in animals, with estimated annual losses exceeding $2 billion in the United States alone. Environmentally, biting flies serve as food sources for numerous species and contribute to ecosystem balance, though their population control remains challenging due to insecticide resistance documented in over 60 countries. Understanding fly biting behavior informs public health strategies, including the development of repellents, traps, and integrated pest management approaches.