Superkingdom Eukaryota
Kingdom Animalia
Phylum Arthropoda
Superclass Hexapoda
Class Insecta
Subclass Pterygota
Series Neoptera
Order Siphonaptera

Common name(s): flea

Description: Small, highly modified, laterally compressed ectoparasites. Mouthparts are modified for piercing and sucking, and have no mandibles; a stylet is derived from the epipharynx and two elongate serrate lacinial blades within a sheath formed from the labial palps. The adult flea feeds only on the blood of its host. Antennae lie in grooves. The body has many backwards-directed setae and spines, some modified as combs to maintain position. Legs are strong, and end in strong claws for grasping the hairs of the host. Compound eyes are absent and ocelli range from absent to well-developed. Immature stages (larvae) are terrestrial, are legless (apodous) and have a distinctive head capsule.

Fun facts: The word Siphonaptera is derived from the Greek words for 'sucking' and 'wingless'. Fleas are secondarily wingless, which means that their evolutionary ancestors had wings, but they evolved to lose their wings as part of their ectoparasitic lifestyle.

Siphonaptera is an order of approximately 2400 species, all of which are highly modified, apterous and laterally compressed ectoparasites. Development is holometabolous.

As with most ectoparasites, fleas are very small (although they are, on average, larger than lice). One of the largest flea species is the mole flea, which measures almost 6 mm in length.

The flea's gut has a salivary pump to inject saliva into the wound, and pumps to suck up blood. Many adults can survive for weeks, even months, without access to a host. Large eggs are laid into the host's nest, where the free-living worm-like larvae develop on shed skin debris from the host. High temperatures and humidity are required for development by many flea larvae, including those of the domestic cat, dog and human. Both sexes of the adult take blood from a host. Some species are monoxenous (restricted to one host species), but many others are polyxenous (having more than one host species). The plague flea Xenopsylla chiopis is polyxenous, facilitating the transfer of plague from rat to human hosts. Fleas do transmit other diseases of minor significance to humans, such as murine typhus and tularaermia. There have been some recorded cases of several species of tapeworm that can infect humans after using the flea as an intermediary host. Apart from plague, the most common human health threat is from allergic reactions to bites from the fleas of domestic pets (cats and dogs).

Fleas predominantly use mammal hosts, with very few birds having fleas, these being derived from many lineages of mammal flea. Some hosts have been reported to harbour more than 20 different species of flea, and conversely, some fleas have over 30 recorded hosts, so host specificity is clearly less than for lice.

Siphonaptera are traditionally thought of as the sister group to Diptera, but recent evidence from molecular research and musculature suggests an intriguing relationship to the family Boreidae (winter scorpion-flies) within the order Mecoptera, making the latter order paraphyletic.

sourced, in part, by The Insects: An outline of entomology, second ed. Gullan, P.J. and P.S. Cranston. Blackwell Science, Great Britain, 2000.
Parasitic Insects. Askew, R. R. Heinmann Educational Books, London, 1971.

Thanks be to Gritchka for pointing out to me that Siphonaptera is derived from the Greek, not the Latin.

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