Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that survive as external parasites of mammals and birds. Fleas live by consuming blood, or hematophagy, from their hosts. Adult fleas grow to about 3 millimetres (1⁄8 inch) long, are usually brown, and have bodies that are "flattened" sideways or narrow, enabling them to move through their host's fur or feathers. They lack wings, but have strong claws preventing them from being dislodged, mouthparts adapted for piercing skin and sucking blood, and hind legs extremely well adapted for jumping. They are able to leap a distance of some 50 times their body length, a feat second only to jumps made by another group of insects, the superfamily of froghoppers. Flea larvae are worm-like with no limbs; they have chewing mouthparts and feed on organic debris left on their host's skin.
Genetic evidence indicates that fleas are a specialised lineage of parasitic scorpionflies sensu lato, most closely related to Nannochoristidae. The earliest known fleas are known from the Middle Jurassic, though modern looking forms do not appear until the Cenozoic. Fleas likely originated on mammals before later parasitising birds. Each species of flea is more or less a specialist with respect to its host animal species: many species never breed on any other host, though some are less selective. Some families of fleas are exclusive to a single host group; for example, the Malacopsyllidae are found only on armadillos, the Ischnopsyllidae only on bats, and the Chimaeropsyllidae only on elephant shrews.
The oriental rat flea, Xenopsylla cheopis, is a vector of Yersinia pestis, the bacterium which causes bubonic plague. The disease was spread to humans by rodents such as the black rat, which were bitten by infected fleas. Major outbreaks included the Plague of Justinian, c. 540 and the Black Death, c. 1350, both of which killed a sizeable fraction of the world's population.
Fleas appear in human culture in such diverse forms as flea circuses, poems like John Donne's erotic "The Flea", works of music such as by Modest Mussorgsky, and a film by Charlie Chaplin.
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