Hematophagy (sometimes spelled haematophagy or hematophagia) is the habit of certain animals of feeding on blood (from the Greek words, haima "blood" and phagein "to eat"). Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without enormous effort, hematophagy has evolved as a preferred form of feeding in many small animals such as worms and arthropods. Some intestinal nematodes, such as Ancylostomids, feed on blood extracted from the capillaries of the gut and about 75% of all species of leeches (e.g. Hirudo medicinalis), a free-living worm, are hematophagous. Some fish, such as lampreys, and mammals, especially the vampire bats, also practice hematophagy.

Mechanism and evolution of hematophagy

These hematophagous animals have evolved different specialized mouth parts and chemical agents for penetrating vascular structures in the skin of hosts, mostly of mammals, birds and fish. This type of feeding is known as phlebotomy (from the Greek words, phleps "vein" and tomos "cutting").

Once phlebotomy is performed (in most insects by a specialized fine hollow "needle" called proboscis which perforates skin and capillaries; in bats by sharp incisor teeth that act as a razor to cut the skin), blood is acquired either by sucking action directly from the vases, or from a pool of escaped blood, or by lapping (again, in bats). In order to overcome natural hemostasis (blood coagulation), vasoconstriction, inflammation and pain sensation in the host, biochemical solutions in the saliva for instance, for pre-injection, anesthesia and capillary dilation have evolved in different hematophagous species. In fact, new anticoagulant medicines have been developed on the basis of substances found in the saliva of several hematophagous species, such as leeches (hirudin).

Hematophagy can be classified into obligatory and optional practice. Obligatory hematophagous animals do not have any other type of food besides blood; one such species is Rhodnius prolixus (an assassin bug from South America). Contrast that with optional hematophages, like the many mosquitoes species, such as Aedes aegypti, which may also feed on pollen, fruit juice and other biological fluids besides blood. Sometimes, only the female of the species is a hematophage (this is essential for egg production and reproduction). Coyotes, wolves, and other canids may lick blood.

Hematophagy has apparently evolved independently in many disparate arthropod, annelid, nematode and mammalian taxa. For example Diptera (insects with two wings, such as flies) have eleven families with hematophagous habits (more than half of the 19 hematophagous arthropod taxa). About 14,000 species of arthropods are hematophagous, even including some genera that were not previously thought to be, such as moths of the genus Calyptra. Several complementary biological adaptations for locating the hosts (usually in the dark, as most hematophagous species are nocturnal and silent, in order to avoid detection and destruction by the host) have also evolved, such as special physical or chemical detectors (for sweat components, CO₂, heat, light, movement, etc.).