Siphonaptera ordo_authority = Latreille (Pierre Andr� Latreille), 1825 subdivision_ranks = Infraorders subdivision = Ceratophyllomorpha Hystrichopsyllomorpha Pulicomorpha Pygiopsyllomorpha synonyms = Aphaniptera

Flea is the common name for insects of the order (order (biology)) Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood. Fleas are external parasites (parasitism), living by hematophagy off the blood of mammals (including humans) and birds.

In the past, it was most commonly supposed that fleas had evolved from the flies (fly) (Diptera (fly)), based on similarities of the larvae. (Some authorities use the name Aphaniptera because it is older, but names above family rank (circumscriptional name) need not follow the ICZN (International Code of Zoological Nomenclature) rules of priority, so most taxonomists use the more familiar name). Genetic (Genetics) and morphological evidence indicates that they are descendants of the Scorpionfly (Mecoptera) family Boreidae (snow scorpionfly), which are also flightless; accordingly it is possible that they will eventually be reclassified as a suborder within the Mecoptera. 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 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)