any of a group of bloodsucking insects that are important carriers of disease and can be serious pests. Fleas are parasites (parasitism) that live on the exterior of the host (i.e., are ectoparasitic). As the chief agent transmitting the Black Death (bubonic plague) in the Middle Ages, they were an essential link in the chain of events that resulted in the death of a quarter of the population of Europe.

Fleas are small, wingless insects with a tough cuticle bearing many bristles and frequently combs (ctenidia) of broad, flattened spines. The adult flea varies from about 0.1 to 1 cm (0.039 to 0.39 inch) in length and feeds exclusively on the blood of mammals (including man) and birds. With about 2,000 species and subspecies known, the order is still a small one compared with many other groups of insects. However, it is widely distributed with some—such as the rat flea and the mouse flea—having been carried all over the world by man. Native species of fleas are found in polar, temperate, and tropical regions.

Details of the life cycle are available for only a few species. The four life stages are the egg, larva, pupa, and adult. Pearly white, oval eggs are deposited on the body of, or in the nest or habitat of, the host animal. The larva is small and legless and feeds on organic debris, such as dried excrement, dried bits of skin, dead mites, or dried blood found in the host's nest. The adult fleas pass freshly imbibed blood rapidly through their gut to produce fecal matter for the nourishment of their offspring, which is essential to the successful metamorphosis of certain species of flea larvae. After three (exceptionally, two) molts, the larva spins a silk cocoon that includes debris from the nest and enters the pupal stage. The pupa emerges as an adult some days or months later. Some species can enter an arrested state of development at the end of the pupal stage and will not emerge as an adult until a host is present. Depending upon the species or environmental conditions, the time required for a complete life cycle varies from two weeks to several months. The life-span of the adult flea varies from a few weeks (e.g., Echidnophaga gallinacea) to a year or more (Pulex irritans). The life cycle of the European rabbit flea (Spilopsyllus cuniculi) and its host are perfectly synchronized. The sexual development of male and female fleas is under the direct control of the rabbit's sex hormones. Thus, the eggs of the female flea mature successfully only if she is feeding on a pregnant doe rabbit. When the young rabbits are born, both flea sexes mature and leave the parturient doe for the nestlings and the nest, where they copulate and lay their eggs, thus assuring the flea larvae a suitable habitat for development. If the sex hormones of the female rabbit are controlled artificially by the administration of synthetic progestins (contraceptives), the sexual development of the female flea is also arrested. Although no similar case is yet known among other species of fleas, it has been recorded that rat fleas are less fertile if fed on baby mice than on their parents and that the mouse flea (Leptopsylla segnis) is more prolific if reared on family units rather than on individual adult mice. It seems likely, therefore, that the influence of the hosts' hormones is more widespread than had been suspected.

Anatomically, adult fleas are a rather uniform but distinctive group, with many interesting modifications and few obvious links with other orders. The compressed body enables them to move swiftly through hairs or feathers of the host, while the backwardly projecting spines or combs serve to anchor them within fur, hair, or feathers. The mouthparts are modified for sucking blood and include barbed stylets that aid both in penetration of the flea into the host skin and in attachment of species that spend long periods fixed to the host (e.g., the sticktight fleas). Generally fleas that live on diurnal hosts have well-developed eyes, whereas species that parasitize subterranean hosts (e.g., moles) or nocturnal animals (e.g., bats) have poorly developed eyes or none at all. The most impressive adaptations are highly developed jumping legs and the powerful muscles that control them. During their evolution, fleas, like the majority of parasitic insects, lost their wings. However, certain parts of the flight mechanism have been retained and incorporated in the jumping mechanism. Because fleas are able to leap horizontal or vertical distances 200 times their body length and to develop an acceleration of 200 gravities, they have been described as insects that fly with their legs. Certain species that live in nests high above the ground or in other unusual habitats crawl rather than jump. An incidental use of the unusual strength of fleas is in “flea circuses” in which they pull miniature carts and perform other feats.

Small, wingless, laterally compressed insects; adults ectoparasitic on warm-blooded vertebrates; eyes present or lacking; antennae short and stout, reposing in grooves; mouthparts adapted to piercing and sucking, maxillary and labial palps present; thoracic segments free; legs with large coxae, tarsi five-segmented; larvae elongated, legless, caterpillar-like; pupae with appendages free, enclosed in cocoons.

G.P. Holland, “The Siphonaptera of Canada,” Tech. Bull. Dep. Agric. Can. 70 (1949), the best work on identification, affinities and host relations of relevant North American fauna; “Evolution, Classification and Host-Relations of Siphonaptera,” A. Rev. Ent., 9:123–146 (1964), a critical review of the literature; G.H.E. Hopkins and M. Rothschild, An Illustrated Catalogue of the Rothschild Collection of Fleas (Siphonaptera) in the British Museum (Natural History), 5 vol. (1953–71), indispensable for global identification and general morphology of families treated to date; W.L. Jellison, “Fleas and Disease,” A. Rev. Ent., 4:389–414 (1959), a summary of the medical importance of fleas; P.T. Johnson, “A Classification of the Siphonaptera of South America with Descriptions of New Species,” Mem. Ent. Soc. Wash., 5:1–299 (1957), sole opus on South American fleas, well prepared and well illustrated; M. Rothschild, “Fleas,” Sci. Am. 213:44–53 (Dec. 1965), an account of interesting aspects of the biology and behaviour of fleas; “The Rabbit Flea and Hormones,” in The Biology of Sex, ed. by A. Allison, pp. 189–199 (1967), a review of findings on hormonal interrelationships of the rabbit flea and its host, and with T. Clay, Fleas, Flukes and Cuckoos, 3rd ed. (1957), information on bionomics and habits of bird fleas and other bird parasites; R. Traub, Siphonaptera from Central America and Mexico (1950), includes generic revisions and the first study on comparative anatomy of the aedeagus of fleas; United States Department of Agriculture, “Fleas—How to Control Them,” Leaflet No. 392 (n.d.), one of a series of pamphlets on methods of control; Allan H. Benton, An Atlas of the Fleas of the Eastern United States (1980).The Hon. Miriam Louisa Rothschild Robert Traub