Hematophagy

Mechanism and evolution

Hematophagous animals have 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", the 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 veins or capillaries, from a pool of escaped blood, or by lapping (again, in bats). To overcome natural hemostasis (blood coagulation), vasoconstriction, inflammation, and pain sensation in the host, hematophagous animals have evolved chemical solutions, in their saliva for instance, that they pre-inject—and anesthesia and capillary dilation have evolved in some hematophagous species. Scientists have developed anticoagulant medicines from studying substances in the saliva of several hematophagous species, such as leeches (hirudin).

Hematophagy is classified as either obligatory or facultative. Obligatory hematophagous animals cannot survive on any other food. Examples include Rhodnius prolixus, a South American assassin bug, and Cimex lectularius, the human bed bug. Facultative hematophages, meanwhile, acquire at least some portion of their nutrition from non-blood sources in at least one of the sexually mature forms. Examples of this include many mosquito species, such as Aedes aegypti, whose both males and females feed on pollen and fruit juice for survival, but the females require a blood meal to produce their eggs. Fly species such as Leptoconops torrens can also be facultative hematophages. In anautogenous species, the female can survive without blood but must consume blood in order to produce eggs (obligatory hematophages are by definition also anautogenous).

As a feeding practice, hematophagy has evolved independently in a number of 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. Hematophagy in insects, including mosquitoes, is thought to have arisen from phytophagous or entomophagous origins. Several complementary biological adaptations for locating the hosts (usually in the dark, as most hematophagous species are nocturnal and silent to avoid detection) have also evolved, such as special physical or chemical detectors for sweat components, CO₂, heat, light, movement, etc.

In addition to these biological adaptations that have evolved to help blood-feeding arthropods locate hosts, there is evidence that RNA from host species may also be taken up and have regulatory consequences in blood feeding insects. A study on the yellow fever mosquito Aedes aegypti has shown that human blood microRNA has-miR-21 are taken up during blood feeding and transported into the fat body tissues. Once in the fat body they target and regulate mosquito genes such as vitellogenin, which is a yolk protein used for egg production.