Flea, in zoology, a genus of insects belonging to the order of aptera. It has two eyes, and six feet fitted for leaping; the feelers are like threads; the rostrum is inflected, fetaceous, and armed with a sting; and the belly is compressed.

The generation of this familiar vermin affords something very curious, first discovered by Sig. Diaciento Celfore. Fleas bring forth eggs, or nits, which they deposit on animals that afford them a proper food: these eggs being very round and smooth, usually slip straight down; unless detained by the piles, or other inequalities, of the clothes, hairs, &c. Of these eggs are hatched white worms, of a shining pearl colour, which feed on the scurfy substance of the cuticle, the downy matter gathered in the piles of clothes, or other like substances. In a fortnight they come to a tolerable size, and are very lively and active; and, if at any time disturbed, they suddenly roll themselves into a kind of ball. Soon after this, they come to creep, after the manner of silk-worms, with a very swift motion. When arrived at their size, they hide themselves as much as possible, and spin a silken thread out of their mouth, wherewith they form themselves a small round bag, or case, white within as paper, but without always dirty, and fouled with dust. Here, after a fortnight's rest, the animalcule bursts out, transformed into a perfect flea; leaving its exuviae in the bag. While it remains in the bag, it is milk-white, till the second day before its eruption; when it becomes coloured, grows hard, and gets strength; so that upon its first delivery it springs nimbly away.

The flea, when examined by the microscope, affords a very pleasing object. It is covered all over with black, hard, and flaky scales or plates, which are curiously jointed, and folded over one another in such a manner as to comply with all the nimble motions of the creature. These scales are all curiously polished, and are befit about the edges with short spikes in a very beautiful and regular order. Its neck is finely arched, and much resembles the tail of a lobster: the head also is very extraordinary; for from the snout-part of it there proceed the two fore-legs, and between these is placed the piercer or sucker with which it penetrates the skin to get its food. Its eyes are very large and beautiful, and it has two short horns or feelers. It has four other legs joined all at the breast. These, when it leaps, fold short one within another; and then, exerting their spring all at the same instant, they carry the creature to a surprising distance. The legs have several joints, and are very hairy, and terminate in two long and hooked sharp claws. The piercer or sucker of the flea is lodged between its fore-legs, and includes a couple of darts or lancets; which, after the piercer has made an entrance, are thrust farther into the flesh, to make the blood flow from the adjacent parts, and occasion that round red spot, with a hole in the centre of it, vulgarly called a flea-bite. This piercer, its sheath opening side-wise and the two lancets within it, are very difficult to be seen; unless the two fore-legs, between which they are hid, be cut off close to the head: for the flea rarely puts out its piercer, except at the time of feeding, but keeps it folded inwards; and the best way of seeing it is by cutting off first the head, and then the fore-legs, and then it is usually seen thrust out in convulsions.

By keeping fleas in a glass tube corked up at both ends; but so as to admit fresh air, their several actions may be observed, and particularly their way of coupling, which is performed tail to tail; the female, which is much the larger, standing on the male. They may also be thus seen to lay their eggs, not all at once, but ten or twelve in a day, for several days successively; which eggs will be afterwards found to hatch successively in the same order. The flea may easily be dissected in a drop of water; and by this means the stomach and bowels, with their peristaltic motion, may be discovered very plainly, as also their testes and penis, with the veins and arteries, though minute beyond all conception. Mr Lieuwenhoek affirms also, that he has seen innumerable animalcules, shaped like serpents, in the femen maculium of a flea. This blood-thirsty insect, which fattens at the expense of the human species, prefers the more delicate skin of women; but preys neither upon epileptic persons, nor upon the dead or dying. It loves to nestle in the fur of dogs, cats, and rats. The nests of river-swallows are sometimes plentifully stored with them.

Fleas are apterous; walk but little, but leap to a height equal to 200 times that of their own body. This amazing motion is performed by means of the elasticity of their feet, the articulations of which are so many springs. Thus it eludes, with surprising agility, the pursuit of the person on whom it riots.—Among the memorabilia of fleas, one, they say, has been seen to draw a small silver piece of ordnance to which it was fastened, the firing of the gun nowile daunting its intrepidity. The owner carried it about in a little box lined with velvet, every now and then placing it on her arm to let it feed; but winter put an end to the being of this martial flea. Another flea that became slave to an Englishman, had, for its daily and easy task, to drag its golden chain and padlock, of the weight of one grain. A third flea served as a thrall-horse to an English artist, who had made an ivory coach and fix, that carried a coachman with his dog between his legs, a postilion, two footmen, and four inside riders. At Surat fleas, bugs, and other voracious vermin, are in so great veneration, that they have an hospital endowed, where every night a poor fellow, for hire, suffers himself to be preyed upon. He is fastened naked on a bed, when the feast begins at his expense. In Turkey there is a similar foundation for decayed dogs; an institution less ridiculous than the other. Mercureal ointment, brimstone, a fumigation with the leaves of penny-royal, or fresh-gathered leaves of that plant sewed up in a bag, and laid in the bed, are remedies pointed out as destructive of fleas.

Pulex-Arbores, in natural history, the name given by Mr Reaumur to a very large genus of small animals. They are a kind of half-winged creatures: they have granulated antennae; and some of them, in their most perfect state, have complete wings. These are distinguished from the others by the name of musca-pulex or the winged-pulex.

The several species of these creatures are of different colours: some are brown, others yellow; but the most frequent are green. They all feed upon the leaves of trees, which become withered and curled up on their eroding them; and they are so common, that wherever a leaf of a tree is found curled up, or of a different form from the others, it is highly probable these animals animals are on it, or that it is their work. Among trees the willow and the rose are the most infected by them; and among plants, the bean and the poppy. They live a social life, multitudes of males and females being found together. The females are easily distinguished from the males, by their being thicker in the body, and having larger bellies.

It is very wonderful, that of all the known animals of the winged kind, there are the only ones which are viviparous. This is easily seen beyond a possibility of doubt; for, on examining a cluster of them together, it is a common thing to see, by the help of a small magnifier, a female in the act of parturition; and the author of this account frequently saw the young pullex protruded out, from a pallage near the anus of the female, perfectly formed. He had suspected this from the total want of eggs among so numerous a tribe of animals, and from their remarkably speedy propagation, and was thus convinced of it by ocular demonstration.

They are armed with a tender and flexible proboscis; with which they seize hold of the young shoots of the tree they live upon, twisting the proboscis round it. These creatures are always seen naked and exposed, hanging on the outside of the stalks and leaves, and sucking in their juices for nourishment with their proboscis. But there is another species of them, which are alike viviparous, and agree with them in all respects except in their manner of living. These get into the inner substance of the leaves, like the worms called acariides; and feed on the parenchyma, being defended from all injuries by living between the integuments. In this case, the leaves they bury themselves in become februous and deformed, and produce a sort of galls: so that Malpighi erred in supposing all the galls of trees to be produced by the animals hatched of the eggs of ichneumon flies; since these animals, which are viviparous, and are of a very different kind from the worms of the ichneumon flies, equally produce them. A female of the species here treated of has been seen to bring forth seven young ones in a day; and thus from residing alone in the tubercles which she had formed on a leaf, she in a little time becomes the mother of a numerous family; each of which raises its own tumour or gall on the leaf, which at first are small and round, and of a beautiful red like kermes.

Such of these as are of the male species have a certain time of rest, in which they lie buried in a silky matter, and afterwards become winged, flying nimbly about; whereas the females never are able to fly, but remain always half-winged. It is to be observed, however, that there is a different species of winged insects frequently found flying about the female pulices, as well as their own males; so that all the small-winged insects about them are not to be thought of their own species. These do not greatly differ in figure; but the one are harmless, and the others have stings, and hurt any part of the body on which they fix.

Pulex Aquaticus aurorum (monoculus pulex of Linnaeus), in entomology, is a species of the genus Monoculus, which see. It is a most curious insect of the size of a flea, and has been noticed by many writers who have examined its parts with accuracy, and is that which, uniting together in vast numbers, occasions the beautiful red patches which may be observed in a dry summer season on stagnant waters, giving rise to reports of water being turned to blood, and in the minds of the less informed thought to portend dire events. The other species of the same genus collect on waters in a similar way, and occasion a similar appearance, as has been mentioned under the generic name, to which we refer our readers. See also Swammerdam's Book of Nature, p. 39.; Baker's Employment for the Microscope, p. 32.; Schöffer's Icon. Inf.; Sulz. Inf., p. 32.; De Seer's Inf., vol. 7., &c. where there are also excellent figures of it. We have given a figure of it magnified and drawn from life: 'The outward form of the body, Swammerdam says, is a kind of square; under the eye there is a sharp beak; on the breast are a kind of arms divided into branches like the boughs of trees, and in the abdomen there is a transparent substance with the legs and tail, and in the hinder part of the body, its legs appear placed as it were on the middle of the back: The eyes are almost close together, and are reticulated; the beak is transparent.

It appears that insects of this tribe are enabled to bear the extremes of heat and cold: for Ray, in his Historia Insectorum, p. 41., observes, that the pulex flavinotus was met with by Mr Willoughby in a hot bath near Vicenza in Italy, the temperature of which was such as to prevent any other living therein; and, on the contrary, O. Fabricius, in his Fauna Greenlandiae, p. 264., mentions the circumstance of the monoculus pulex being frequently found under the ice in the stagnant waters of Greenland.

The chergo, or pulex minimus, extem penetrans, Americanus of Cateby, is a very small animal found in warm climates. It is a very troublesome insect, especially to negroes and such as are lovey or go barefooted. They penetrate the skin, under which they lay a bunch of eggs, which swell to the bigness of a small pea or tare. They are exceedingly painful; and unless great care is used in taking them out, they are dangerous. It is about one-fourth the size of a common flea; the figure is considerably magnified. From the ccccxxvi. mouth issues a hollow tube like that of a common flea, between a pair of antennae. It has six jointed legs, and something like a tail. Under it is one of its eggs, which is scarcely visible to the naked eye. These animals are a great nuisance to most parts of America between the tropics. See Sir Hans Sloane's History of Jamaica, Introd. p. cxiv. and vol. ii. p. 191, 192.

Pulex-Eaters, a name given by naturalists to a sort of worms frequently found on the leaves of trees, where they devour the animals called pulices arborei.

Of these there are several species, which owe their origin to the eggs of different creatures; for there are none of them in their ultimate state in this their time of feeding. According to the different animals whose eggs they are hatched from, these are of different form and structure. Some are hexapods, or ended with six feet; these belong to the beetle-tribe, and finally change into beetles like the parent animal from whose eggs they sprung. Others have no legs, and are produced from the eggs of flies of various kinds. And, finally, others are genuine caterpillars, though small; but there are the most rare of all.

The two general kinds are the hexapods, or beetle-worms; worms; and the apodes, or fly-worms. The fly which gives origin to the last of these is a four-winged one; and takes care always to deposit her eggs in a place where there are plenty of the pulices, usually on the stalk or young branches of a tree in the midst of large families of them. The worm, as soon as hatched, finds itself in the midst of abundance of food, preying at pleasure on these animals, which are wholly defenceless. The stalks of the elder and woodbine are frequently found covered over with these pulices; and among them there may usually be found one or more of these destroyers feeding at will, sucking in the juices from their bodies, and then throwing away the dry skins. Besides the worms of this four-winged fly, there is one of a two-winged wasp fly, very destructive of these animals.