FLEA (O. Eng. fléah, or fléa, cognate with flee, to run away from, to take flight), a name typically applied to Pulex irritans, a well-known blood-sucking insect-parasite of man and other mammals, remarkable for its powers of leaping, and nearly cosmopolitan. In ordinary language the name is used for any species of Siphonaptera (otherwise known as Aphaniptera), which, though formerly regarded as a suborder of Diptera (q.v.), are now considered to be a separate order of insects. All Siphonaptera, of which more than 100 species are known, are parasitic on mammals or birds. The majority of the species belong to the family Pulicidae, of which P. irritans may be taken as the type; but the order also includes the Sarcopsyllidae, the females of which fix themselves firmly to their host, and the Ceratopsyllidae, or bat-fleas.
Fleas are wingless insects, with a laterally compressed body, small and indistinctly separated head, and short thick antennae situated in cavities somewhat behind and above the simple eyes, which are always minute and sometimes absent. The structure of the mouth-parts is different from that seen in any other insects. The actual piercing organs are the mandibles, while the upper lip or labrum forms a sucking tube. The maxillae are not piercing organs, and their function is to protect the mandibles and labrum and separate the hairs or feathers of the host. Maxillary and labial palpi are also present, and the latter, together with the labrum or lower lip, form the rostrum.
Fleas are oviparous, and undergo a very complete metamorphosis. The footless larvae are elongate, worm-like and very active; they feed upon almost any kind of waste animal matter, and when full-grown form a silken cocoon. The human flea is considerably exceeded in size by certain other species found upon much smaller hosts; thus the European Hystrichopsylla talpae, a parasite of the mole, shrew and other small mammals, attains a length of 51 millimetres; another large species infests the Indian porcupine. Of the Sarcopsyllidae the best known species is the “jigger” or “chigoe” (Dermatophilus penetrans), indigenous in tropical South America and introduced into West Africa during the second half of last century. Since then this pest has spread across the African continent and even reached Madagascar. The impregnated female jigger burrows into the feet of men and dogs, and becomes distended with eggs until its abdomen attains the size and appearance of a small pea. If in extracting the insect the abdomen be ruptured, serious trouble may ensue from the resulting inflammation. At least four species of fleas (including Pulex irritans) which infest the common rat are known to bite man, and are believed to be the active agents in the transmission of plague from rats to human beings. (E. E. A.)