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 mm or .12 in 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. Fleas’ larvae are worm-like with no limbs; they have chewing mouthparts and feed on organic debris left on their host’s skin.
The Siphonaptera are most closely related to the snow scorpionflies, or snow fleas in the UK, formally the Boreidae, placing them within the Endopterygote insect order Mecoptera. Fleas arose in the early Cretaceous, most likely as ectoparasites of mammals, before moving on to other groups including 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.