There are more than 2200 species and subspecies of fleas in about 220 genera, but only relatively few are important pests of humans. About 94% of species bite mammals while the remainder are parasitic on birds. Fleas occur almost worldwide, but many have a more restricted distribution; for example the genus Xenopsylla, which contains important plague vectors, is confined to the tropics and warmer parts of some temperate countries.

Medically the most important fleas are Xenopsylla species, such as X. cheopis, which is a vector of plague (Yersinia pestis) and flea-borne murine typhus (Rickettsia typhi). Fleas in the genus Ctenocephalides may be intermediate hosts of cestodes (Dipylidium caninum, Hymenolepis diminuta). Fleas may also be vectors of tularaemia (Francisella tularensis), and the chigoe or jigger flea (Tunga penetrans) ‘burrows’ into people's feet.

External morphology

Adult fleas are relatively small (1–6 mm), more or less oval insects, compressed laterally and varying from light to dark brown (Plate 16). Wings are absent, but there are three pairs of powerful legs, the hind pair of which are specialized for jumping. The legs, and much of the body, are covered with bristles and small spines.

The head is approximately triangular, bears a pair of conspicuous eyes (a few species are eyeless), and short three-segmented more or less club-shaped antennae which lie in depressions behind the eyes. The mouthparts point downwards.