Flea is the common name for insects of the order Siphonaptera which are wingless insects whose mouthparts are adapted for piercing skin and sucking blood. (some authorities use the name Aphaniptera because it is older, but names above family rank need not follow the ICZN rules of priority, so most taxonomists use the more familiar name). Fleas are external parasites, living by hematophagy off the blood of mammals and birds. Genetic and morphological evidence indicates that they are descendants of the Scorpionfly family Boreidae, which are also flightless; accordingly it is possible that they will eventually be reclassified as a suborder within the Mecoptera. In the past, however, it was most commonly supposed that fleas had evolved from the flies (Diptera), based on similarities of the larvae. In any case, all these groups seem to represent a clade of closely related insect lineages, for which the names Mecopteroidea and Antliophora have been proposed.

Some well known flea species include:

• Cat flea (Ctenocephalides felis),
• Dog flea (Ctenocephalides canis),
• Human flea (Pulex irritans),
• Northern rat flea (Nosopsyllus fasciatus),
• Oriental rat flea (Xenopsylla cheopis).

Morphology and behavior

left Fleas are small (1/16 to 1/8-inch (1.5 to 3.3 mm) long), agile, usually dark colored (for example, the reddish-brown of the cat flea), wingless insects with tube-like mouth-parts adapted to feeding on the blood of their hosts. Their bodies are laterally compressed (human anatomical terms), permitting easy movement through the hairs or feathers on the host's body (or in the case of humans, under clothes). Their legs are long, the hind pair well adapted for jumping (vertically up to seven inches (18 cm); horizontally thirteen inches (33 cm) Fleas, HYG-2081-97 William F. Lyon, Ohio State University entomology page. Accessed 28 December 2006) - around 200 times their own body length, making the flea one of the best jumpers of all known animals (in comparison to body size), second only to the froghopper. The flea body is hard, polished, and covered with many hairs and short spines directed backward,Fleas - P.G. Koehler and F. M. Oi. Printed July 1993, revised February 2003. Provided by the University of Florida which also assists its movements on the host. Its tough body is able to withstand great pressure, likely an adaptation to survive attempts to eliminate them such as scratching. Even hard squeezing between the fingers is normally insufficient to kill the flea; it may be necessary to capture them with adhesive tape, crush them between the fingernails, roll them between the fingers, or put them in a fire-safe area and burn them with match or lighter. They can also be drowned.