Aphaniptera Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds.

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 hosts' fur or feathers.

Some species can leap 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 hosts' skin.

Genetic evidence indicates that fleas are a specialised lineage of parasitic scorpionflies (Mecoptera) sensu lato, most closely related to the family Nannochoristidae.

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 that causes bubonic plague.

Major outbreaks included the Plague of Justinian, about 540, and the Black Death, about 1350, each of which killed a sizeable fraction of the world's people.

The larvae are small and pale, have bristles covering their worm-like bodies, lack eyes, and have mouth parts adapted to chewing.

[7] Immediately before the jump, muscles contract and deform the resilin pad, slowly storing energy which can then be released extremely rapidly to power leg extension for propulsion.

In most species, neither female nor male fleas are fully mature when they first emerge but must feed on blood before they become capable of reproduction.

[9] Some species breed all year round while others synchronise their activities with their hosts' life cycles or with local environmental factors and climatic conditions.

[12] They are blind and avoid sunlight, keeping to dark, humid places such as sand or soil, cracks and crevices, under carpets and in bedding.

This can take just four days, but may take much longer under adverse conditions, and there follows a variable-length stage during which the pre-emergent adult awaits a suitable opportunity to emerge.

[5] Large numbers of pre-emergent fleas may be present in otherwise flea-free environments, and the introduction of a suitable host may trigger a mass emergence.

Under ideal conditions of temperature, food supply, and humidity, adult fleas can live for up to a year and a half.

As soon as the baby rabbits are born, the fleas make their way down to them and once on board they start feeding, mating, and laying eggs.

One of the seven orders into which he divided them was "Aptera", meaning wingless, a group in which as well as fleas, he included spiders, woodlice and myriapods.

It wasn't until 1810 that the French zoologist Pierre André Latreille reclassified the insects on the basis of their mouthparts as well as their wings, splitting Aptera into Thysanura (silverfish), Anoplura (sucking lice) and Siphonaptera (fleas), at the same time separating off the arachnids and crustaceans into their own subphyla.

Modern fleas probably arose in the southern continental area of Gondwana, and migrated rapidly northwards from there.

Some families are exclusive to a single host group; these include the Malacopsyllidae (armadillos), Ischnopsyllidae (bats) and Chimaeropsyllidae (elephant shrews).

[26] Flea phylogeny was long neglected, the discovery of homologies with the parts of other insects being made difficult by their extreme specialization.

[20] Hectopsyllidae (inc. jigger) Pygiopsyllomorpha Macropsyllidae, Coptopsyllidae Neotyphloceratini, Ctenophthalmini, Doratopsyllinae Stephanocircidae clade inc. Rhopalopsyllidae, Ctenophthalmidae, Hystrichopsyllidae Chimaeropsyllidae Pulicidae (inc. the cat flea, vector of bubonic plague) Ceratophyllomorpha (inc. the Ceratophyllidae, such as the widespread moorhen flea) As of 2023[update], there are 21 recognized families within the order Siphonaptera, 3 of which are extinct.

[28] Fleas feed on a wide variety of warm-blooded vertebrates including dogs, cats, rabbits, squirrels, ferrets, rats, mice, birds, and sometimes humans.

[34]: 126  This can lead to an eczematous itchy skin disease called flea allergy dermatitis, which is common in many host species, including dogs and cats.

[34]: 126 Fleas are vectors for viral, bacterial and rickettsial diseases of humans and other animals, as well as of protozoan and helminth parasites.

[35]: 74  The chigoe flea or jigger (Tunga penetrans) causes the disease tungiasis, a major public health problem around the world.

[39] Fleas have appeared in poetry, literature, music and art; these include Robert Hooke's drawing of a flea under the microscope in his pioneering book Micrographia published in 1665,[40] poems by Donne and Jonathan Swift, works of music by Giorgio Federico Ghedini and Modest Mussorgsky, a play by Georges Feydeau, a film by Charlie Chaplin, and paintings by artists such as Giuseppe Crespi, Giovanni Battista Piazzetta, and Georges de La Tour.

[42] The comic poem Siphonaptera was written in 1915 by the mathematician Augustus De Morgan, It describes an infinite chain of parasitism made of ever larger and ever smaller fleas.

These circuses, extremely popular in Europe from 1830 onwards, featured fleas dressed as humans or towing miniature carts, chariots, rollers or cannon.