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.

Siphonaptera An order of secondarily wingless insects comprising the fleas. The body of a flea is laterally compressed and bears numerous backward-directed spines. Fleas live as blood-sucking ectoparasites of mammals and birds, having mouthparts adapted to piercing their host, injecting saliva to prevent clotting, and sucking up the blood. The long bristly legs can transmit energy stored in the elastic body wall to leap relatively long distances (over 300 mm horizontally). Apart from causing irritation, fleas can transmit disease organisms, most notably bubonic plague bacteria, which can be carried from rats to humans by the rat flea (Xenopsylla cheopsis). The whitish wormlike legless larvae feed on organic matter. After two moults the larva spins a cocoon and undergoes metamorphosis into the adult.

Siphonaptera (fleas; class Insecta, subclass Pterygota) An order of wingless, parasitic insects in which the adult body is flattened laterally. The larvae (maggots) feed on organic debris, undergo complete metamorphosis, and hatch from the pupa when this is disturbed by a possible host. Most adults are host-specific, although some have a wide range of hosts and others will feed on any host when very hungry. There are 1400–1500 species, of world-wide distribution, grouped in two suborders: Pulicida and Apulicida. The Pulicida includes the families Pulicidae (most flea parasites of mammals, some of which are vectors for serious diseases, e.g. pneumonic plague) and Tungidae (the jigger or sand flea).

flea the flea is often taken as the type of something small and contemptible, or as a sign of dirt and degradation.
as fit as a flea in very good health. Recorded from the late 19th century, an example of the emphasis given by alliteration (compare as fit as a fiddle).
flea bite an insignificant inconvenience or cost (literally, a small red mark caused by the bite of a flea).
a flea in one's ear a sharp reproof administered to someone.

see also big fleas have little fleas, lie down with dogs, you will get up with fleas, nothing should be done in haste but gripping a flea.

flea (flee) n. a small wingless bloodsucking insect with a laterally compressed body and long legs adapted for jumping. Adult fleas are temporary parasites on birds and mammals and those species that attack humans (Pulex, Xenopsylla, and Nosopsyllus) may be important in the transmission of various diseases. Their bites may become a focus of infection.

flea / flē/ • n. a small wingless jumping insect (order Siphonaptera) that feeds on the blood of mammals and birds, including the human flea (Pulex irritans) and the cat flea (Ctenocephalides felis). ∎  a water flea (see daphnia).

flea Any of 1000 species of wingless, leaping insects found worldwide. They are external parasites on warm-blooded animals. In moving from one host to another, they can carry disease. Length: to 1cm (0.4in). Order Siphonaptera.

flea OE. flēa(h), corr. to MLG., MDu. vlō (Du. vlo), OHG. flōh (G. floh), ON. fló; perh. rel. to FLEE.

fleas See Siphonaptera.

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