Blood feeding evolved again and again, from finches to moths
Blood is rich in protein and fat and easy to take, so evolution keeps inventing ways to drink it. Leeches, lampreys, vampire bats and roughly 14,000 kinds of arthropods do it. So do a handful of birds, including the vampire finch, and even a moth genus nobody once suspected.
Hematophagy, from Greek for blood and eating, has arisen independently in arthropods, segmented worms, nematodes and mammals. Flies alone account for eleven blood-feeding families. About three quarters of leech species feed this way. One jumping spider, Evarcha culicivora, gets vertebrate blood secondhand by preferring mosquitoes that have just fed.
Some species cannot live on anything else. The South American assassin bug Rhodnius prolixus and the human bed bug are obligate blood feeders. Mosquitoes such as Aedes aegypti are facultative: both sexes sip pollen and fruit juice, but females need a blood meal to make eggs. Most blood feeders are nocturnal and silent, locating hosts in the dark with detectors tuned to sweat chemicals, carbon dioxide, heat and movement.
Getting the meal takes kit. Insects pierce skin and capillaries with a fine hollow proboscis, while vampire bats slice with razor-sharp incisors and lap up the pooling blood. Saliva injected beforehand counters clotting, constriction of the vessels, inflammation and pain, and some species add anaesthetic. There is subtler traffic too: a study found that a human microRNA swallowed by Aedes aegypti travels into the mosquito's fat body and helps regulate a yolk protein gene used for eggs. The same open wound lets pathogens pass the other way, which is why blood feeders spread malaria, dengue, Lyme disease, plague and sleeping sickness.
Their chemistry has also proved useful. Hirudin, an anticoagulant from leech saliva, inspired medicines, and a drug engineered from the saliva of the common vampire bat, desmoteplase, improved recovery in a study of stroke patients. Researchers are also eyeing a weakness shared by mosquitoes, ticks and bed bugs: they rely on an enzyme to break down excess tyrosine after feeding, and blocking it with nitisinone kills them. Humans have their own traditions, from the Maasai staple of cow blood mixed with milk to blood sausage.
Source: Hematophagy
