Hematophagy is the biological practice where certain animals feed on blood, which is a rich fluid tissue. This specialized feeding behavior is particularly prevalent among arthropods. For many species, especially female insects, a blood meal provides the concentrated protein necessary to produce a batch of eggs, a process known as vitellogenesis. This requirement has led to the evolution of sophisticated mechanisms for locating hosts and successfully consuming blood.
Identifying the Major Blood Feeders
The most commonly encountered blood-feeding arthropods belong to two major classes: insects and arachnids. True insects include notorious species like mosquitoes, which are slender flies with piercing-sucking mouthparts. Only female mosquitoes require blood for egg development, while males feed on plant nectar. Fleas are small, laterally compressed insects that use powerful legs to jump and feed through specialized mouthparts.
Lice and bed bugs are also true insects that rely exclusively on blood. Bed bugs are small, oval-shaped parasites that must consume blood to survive. Lice are adapted to specific host environments, with head lice gripping hair shafts and body lice living in clothing seams, both feeding with piercing mouthparts.
Ticks and mites, however, are not insects but arachnids. Ticks are of particular concern because they attach to a host for an extended period, often hours or days, unlike insects that feed quickly and depart.
The Science of Blood Feeding
Successful blood feeding requires a series of biological adaptations, beginning with the ability to locate a host, often in the dark. Blood feeders use specialized sensory organs to detect the chemical signatures of a host, including carbon dioxide exhaled in breath and various compounds in sweat. They also sense body heat, which helps them pinpoint a location for feeding.
Once a host is located, the arthropod uses specialized mouthparts to access the blood supply, categorized into two main strategies. Vessel feeders (solenophages), such as mosquitoes, use a fine, hollow proboscis to directly probe and feed from a capillary or small vessel. Pool feeders (telmophages), like ticks and some flies, use cutting mouthparts to lacerate the skin and surrounding capillaries, creating a small pool of blood from which they drink. Ticks also use a harpoon-like structure to anchor themselves securely into the host’s skin during feeding.
The injection of saliva is crucial because the host’s body naturally defends against blood loss with hemostasis. The saliva contains pharmacologically active molecules that counteract these defenses. Anti-coagulants, such as the protein Aegyptin found in mosquitoes, prevent the blood from clotting by interfering with platelet aggregation. The saliva also contains vasodilators, which widen blood vessels to increase localized blood flow and shorten the feeding time. Some species also inject anesthetic agents to numb the bite site, preventing the host from noticing the feeder.
Vectors of Disease
The most significant public health consequence of hematophagy is the role of these arthropods as disease vectors. A vector is an organism that transmits a pathogen from an infected host to a non-infected host. Transmission can occur in two primary ways: mechanical or biological.
Mechanical transmission happens when the pathogen is simply carried externally on the vector’s body, like a housefly transporting bacteria to food. Biological transmission is more complex, as the pathogen must enter the vector, replicate or undergo part of its life cycle inside the arthropod, and then be transmitted to a new host, usually through a bite.
Mosquitoes are the most well-known biological vectors, transmitting diseases like Malaria, Dengue fever, and West Nile Virus. Ticks are responsible for spreading bacterial infections such as Lyme Disease.