FLEA, an Anglo-Saxon name, probably derived from " fieogan," to fly, typically applied to Pulex irritans, a blood-sucking insect-parasite of man and other mammals, remarkable for its powers of leaping, and unfortunately too well known, even in our own temperate climate. Its position in classification has long been somewhat undecided; a separate order was erected for it and its allies under the various names of Suctoria, Siphonaptera, JRophoieira, and Aphaniptera, by De Geer, Latreille, Clairville, and Kirby respectively ; and it is included in the Áptera of Lamarck, MacLeay, and others, and in the Rhyngota of Fabricius. Various affinities have also been attributed to the group, which has been supposed to have relations with the Hymenoptera, Hemiptera, and Anoplura, and even with the Coleóptera (beetles). As regards the latter order, it is noteworthy that a recently discovered allied group, Platy-psyllidce, founded upon a parasite on the beaver (and considered as a separate order by Westwood, under the name Achreioptera), has been with considerable show of reason referred to the beetles by so sound an entomologist as Dr Leconte. As regards the flea, however, it is now generally accepted that its true affinities are with the Díptera or two-winged flies, as suggested by Lamarck and Strauss Durckheim. In that order, it is by some held to occupy, as a family Pidicidce (with the Platypsyllidw), the position of a sub-order, at the end, under the name Aphaniptera (see DÍPTERA, vol. vii. p. 256); but by others it is placed before the Mycetophilidce, at the head of the tribe Eucephala of the Nematocerous section of the great division Orthorhapha. The community of parasitic habits with the Hippoboscidm (forest-flies and sheep-ticks) is to a certain extent the reason for the former position; but the earlier transformations seem to indicate a stronger relationship with the Eucephala. The general characters have been given under DIPTERA ; but the structure of the mouth of the perfect insect may be specified. The labrum is obsolete; the mandibles are represented by two flat and long processes, strengthened by a mid rib, and having very finely toothed edges, and uniting with the slender central lingua to form a puncturing lancet. When not in use, this is protected by the labial palpi, which form a sort of tube. The maxillae are small leathery plates, and their palpi, which are four-jointed and large, have been mistaken for antennae. The power of leaping, as well known, is very great; but there is no apparent development of the hind femora to account for it (as in many jumping beetles), although the posterior legs are saltatorial. The great muscular power of fleas has been long turned to account by public exhibitors in all countries, who have, under the pretence of taming or educating these minute creatures, made use of various contrivances to render the natural efforts of the insect to escape assume the appearance of trained action. An account of the methods employed will be found in the American Naturalist, vol. xi. p. 7, from the pen of Mr W. H. Dall. In some cases the steady carriage of the flea is to be traced to fracture of its jumping legs. The female flea lays a few oblong white eggs, in dirty places on floors frequented by domestic animals. The larvae, before hatching, have a frontal point, used in breaking the shell of the egg. They are long and worm-like, without feet, but with two small hooks at the tail, and short antennae and mouth organs at the head. They are very active, and apparently feed upon animal substances, forming, when full grown, a silky cocoon.
Many species are known, parasitic upon various animals and birds. They have been recorded as infesting the inside of rabbits' ears, and the neck of a fowl, from hedgehog, mar-mot, cat, dog, bat, squirrel, dormouse, ferret, weasel, hare, rabbit, rat, mouse, field-mouse, shrew, moor-hen, jackdaw, thrush, missel-thrush, blackbird, jay, bullfinch, chaffinch, yellow-hammer, pipit, blackcap, whitethroat, skylark, willow-wren, long-tailed titmouse, siskin, stock-dove, wood-pigeon, common pigeon, starling, swallow, ifec. (though it is by no means certain that these are all necessarily of distinct specific value); and a species has been described from the common fowl in Ceylon. A large species is often found in sandy pits, near the openings of the nests of sand-martins, and a very large one sometimes occurs in wet and marshy places, probably living upon the mole. For the latter parasite, and others in which the antennae exhibit certain supposed peculiarities, a separate genus, Oeratopsyllus, has been proposed. In another flea, also found on the mole, no trace of eyes could be found, even under a high power. A very large species has been found on the Australian porcupine in Tasmania; and Kirby, in the Fauna Boreali-Americana, described as Pulex gigas a flea two lines long, which he believed to be the largest known. This was taken in 65° N. lat. Westwood has recorded 17 British species; and oddly enough the same number are noted from the Netherlands by Bitsema. Any notice of these parasites would be incomplete without a reference to the "jigger," "chigoe," " bicho de p6," "nigua," or " earth -flea,"Dermatophilus, Sarcopsylhcs, or Rhynchoprion penetrans, so well known as a burrower into the naked feet of men, in sandy localities in the West Indies and South America. So great is this pest, that serious trouble has been occasioned by it even to military expeditions in South America ; and the French army in Mexico was much troubled by it. The entry is effected usually under the nail, by the impregnated female, which thereupon becomes enormously distended with an immense number of eggs. Inflammation and ulceration follow this attack, and unless great care is taken in extracting the insect, serious illness and even death result. A good plate of the metamorphoses of this species is given in the volume of the American Naturalist above quoted, p. 754. For an account of the medical aspects, see Dr Laboulbène's article on the " Chique," in the Dictionnaire Encyclopédique des Sciences Médicales (Paris, 1875), p. 239 ; and for an exhaustive history, Guyon's memoir in the Revue et Magasin de Zoologie for 1868 and 1869. Much attention does not appear as yet to have been paid to the Pulicidce by naturalists, except as regards the anatomy of the common speci es. Dugès's ' ' Recherches sur les Caractères Zoologiques du genre Pulex," in the Annales des Sciences Naturelles, vol. xxvii. (1832) p. 145, and J. Kiinckel's observations in the Annales de la Société Entomologique de France, 5e sér. iii. p. 129, and, as regards P. irritons, W. H. Furlonge's descriptions in the journal of the Quekett Microscopical Club, 1871, p. 189, and 1873, p. 12, and C. Bitsema's in the Album der Natuur, xi. (1872), p. 65, may be specially noticed. The name "flea" is frequently erroneously applied to many jumping or lively insects not allied to the Ptdi-cidce ; for instance, the " turnip-flea " is a small beetle, Phyllotreta undulata, one of the Halticid. (E. C. R.)