water flea: see crustacean.

flea, common name for any of the small, wingless insects of the order Siphonaptera. The adults of both sexes eat only blood and are all external parasites of mammals and birds. Fleas have hard bodies flattened from side to side and piercing and sucking mouthparts. Their legs are powerful and adapted for fast movement and jumping, enabling them to find new hosts as well as to escape quickly the attempts of the hosts to remove them. The adults can survive away from a host for several weeks without eating. Flea eggs are usually laid in dirt or in the nest of the host; the larvae feed on organic material and the feces of adult fleas. Metamorphosis is complete; the larvae spin silken cocoons when ready to pupate. Many species are not specific to a particular host species, and cat and dog fleas, as well as the human flea of the warmer parts of Europe and Asia, attack humans. Certain rat fleas transmit typhus and bubonic plague to humans, and another species transmits tularemia from rabbits. Fleas also transmit several species of tapeworms that sometimes infest humans. The chigoe is a flea. Water fleas and beach fleas are crustaceans and not closely related to the insects. Fleas are classified in the phylum Arthropoda, class Insecta, order Siphonaptera.

Any of about 450 species (order Anomopoda) of microscopic, mostly freshwater crustaceans distributed worldwide. Species in the genus Daphnia are ubiquitous in Europe and North America. Water fleas have a discrete head that bears antennae. The carapace (shell) encloses all or most of the body, except on the predatory giant Leptodora (up to 0.7 in. [18 mm] long), whose carapace is just a small brood sac. Most species swim by powerful strokes of the antennae, sometimes producing a hopping-and-sinking motion. All but a few predatory species use specialized thoracic limbs to filter organic matter from the water. Seealso copepod.

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or beach flea

Hopping terrestrial crustacean (family Talitridae). The European sand flea (Talitrus saltator) is about 0.6 in. (1.5 cm) long. The long-horned sand flea (T. longicornis), found on the North American Atlantic coast, has antennae the same length as the waxy white body, up to 1 in. (2.5 cm) long. During the day, sand fleas lie buried near the high-tide mark; at night, they forage for organic debris. The common sand flea (Orchestia agilis, or platensis) lives along Atlantic coasts of Europe and the Americas.

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Any member of the beetle subfamily Alticinae (family Chrysomelidae), found worldwide. It is tiny (less than 0.25 in. [6 mm] long) and dark or metallic in colour. The enlarged hind legs are adapted for jumping. Flea beetles are important pests of cultivated plants (e.g., grapes, cucumbers, melons, tobacco, potatoes, and tomatoes). The adults feed on the leaves, the larvae on the roots. Some flea beetles carry plant diseases (e.g., early potato blight).

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Any member of 2,000 species and subspecies of small, wingless, bloodsucking (parasitic) insects (order Siphonaptera), found from the Arctic Circle to the Arabian deserts. Specialized anatomical structures allow the flea to attach itself to the skin of mammals or birds and consume their blood. Though domestic cats and dogs are well-known hosts, rodents are the mammals most commonly afflicted by fleas. The adult flea is 0.04–0.4 in. (1–10 mm) long and lives from a few weeks to more than a year. Powerful leg muscles allow it to jump distances up to 200 times its body length. Flea infestations have had enormous consequences; fleas were the principal transmission agents of the bubonic plague in the medieval epidemics.

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