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usually on a moveable footstalk; their palpigerous mandibles; and their four antennæ, at least in the great majority. But the principal difference consists in the internal organisation and the fountains of vitality; for the crustacea have a double circulation, the fountain of which is a heart in the middle of their thorax. They have too a kind of gizzard and liver, at least the decapods, and their respiration is by gills. Genuine insects terminate their existence after they have laid their eggs; but the crustacea live longer and lay more than once.' See our articles CANCER, MONOCULUS, ONISCUS, &c.

Although the adjective apterous is frequently applied more generally, no insect, it should be observed, can be included in the aptera of the Linnæan system, unless both sexes, when arrived at their last state of being, are destitute of wings. No insect of this order, except the common flea, is supposed to undergo those successive changes which are usual in most other insects. They are almost entirely produced from the egg in their perfect form, and cannot therefore be said to have any larva. In the flea, however, the small worm which corresponds with the larva in other insects is clearly ascertained; and if we may place sufficient reliance on the evidence of certain naturalists, one or two more of the apterous genera exist for a short period in a state somewhat analogous either to the larva or pupa. Leuwenhoek observed that the mite was oviparous, laying very small oval whitish eggs, from which proceeded the young animals, resembling their parents in all respects, except in the number of their legs, these being only six in number, instead of eight; and after they cast their skin, another pair of legs appear, a fact that evidently implies two periods of transformation, the first of which may be compared with that of larva. An advancement, or progressive increase in the number of feet, is also well known to take place in species both of the julus and scolopendra genera, after they are produced from the egg state. The julus fabulosus, when first excluded, is furnished with three pairs of legs, which are situated at each side near the head; some days after about fourteen become visible, and the remainder, to the amount of 120 on each side, are gradually acquired afterwards. The same remark will apply to certain species, if not the whole of the scolopendra genus. Scorpions are produced by the female parent in a living state; they are at first very small and white, and become of a darker hue in a few days; these, like the spiders cast their skin as they increase in size.

Our countryman, Dr. Leach, has contended that the first five of the Linnæan genera of this order, are the only insects, properly speaking, to be found in it, i. e. the lepisma, podura, pediculus, pulex, and termes. The genera, acarus, phalangiuni, aranea, and scorpio, he refers to his class arachnoida. Genera cancer, monoculus, and oniscus, to the crustacea; scolopendra and julus, to myriapoda. We adopt the names of these classes in our subdivision of this order, but make no further alteration in the system of Linné.

Class I.-INSECTA.

six legs; a head distinct from the body, and fur nished with two antennæ: they are all produced from eggs. Some undergo no metamorphosis, others but a partial change, whilst the remainder pass through three stages of existence, after having been hatched from the egg.

Genus 1. Lepisma.-Gmelin, to whom the admirers of Linné are indebted for one of the best editions of the Systema Naturæ, defines this genus as having four palpi, two of which are setaceous, and two capitated; the lip membranaceous, roundish, and.emarginated; antenna setaceous; body imbricated with scales; tail ending in setaceous bristles; legs six, and formed for running.

This character combines the more essential parts of that of Fabricius. Lamarck proposes further alteration; and Latreille, in whose arrangement they form the first family (lepismena) of the order thysanoura, divides them into two distinct genera: lepisma and machilis. The true lepisma, according to the new definition, has the antennæ inserted between the eyes, the body flat, and the tail ending in three equal bristles.

This genus walks and runs. In the genus machilis (which moves by skipping) the antennæ are seated under the eyes, the body convex, and the middle bristle of the tail larger than those at the sides. All the species, except the L. saccharinum, are natives of Europe. In their various states of growth they prey on decayed wood, and moist or rotten substances; and are most commonly found in damp cellars, neglected water-courses, and similar situations.

L. saccharinum is scaly, silvery; tail triple, and originally a native of America, whence it has been introduced and naturalised in Europe. The antennæ are as long as the body, which is oblong and tapering; tail terminating in three bristles, and two pairs of smaller ones beneath. It secretes itself among books, old furniture, &c., and runs, when disturbed, with great agility: it is often found in sugar, and is thus supposed to have found its way to Europe. In some places of this country it is common under stones; its eight pairs of strings (one on each ventral segment of the abdomen) give it, says Kirby, a wonderful power of leaping. The L. polypoda also, inhabiting among stones and rubbish on sandy sea-shores, possesses the faculty of leaping in the same way to a prodigious height. Linné only described another species, viz. L. terrestris: naked, tail triple.

Gmelin notices the close affinity this species bears to the podura tribe. It is entirely white and cylindrical, with obtuse antennæ half the length of the antennæ. Fabricius adds L. villosa, brown, with a triple, villous tail, a native of China; L. collaris, black, with a snowy band on the neck and end of the abdomen: tail triple and villous, an inhabitant of the South American Islands: and L. lineata, tail triple, body brown, with two white fillets. Natives of Helvetia.

Genus 2. Podura.-Antennæ filiform; palpi, four, subclavate; lip bifid; two eyes, composed of eight facets; the body scaly; tail forked, bent under the body and acting as a spring; from which the genus has its name, podura or spring

Dr. Leach defines this class as having uniform' tailed: it has six legs, formed for running.

There are thirty species enumerated by Gmelin, all comparatively small insects, found in general in damp places, under stones, on the bark of trees, &c. When disturbed they suddenly spring to a small distance by the help of their spring which is ordinarily doubled under the abdomen, and thrown out during the act of leaping. They feed on the leaves of various plants. Their spring will even enable these animals to leap on water. A minute species P. aquatica is sometimes found in boundless numbers on the water of hollows and ruts. When disturbed they resemble in their leaping up the appearance of steel-filings or gunpowder suddenly thrown into motion. One species, sminthurus fuscus, Lat. besides the caudal fork has another singular organ which seems designed to prevent its falling off from a perpendicular surface. Between the ends of the fork is an elevated cylinder or tube from which this little creature can protrude at pleasure two long filiform transparent threads, covered with a slimy secretion.

Genus 3. Pediculus (the louse).-Antenna, length of the thorax; mouth with a retractile recurved sucker without proboscis; no palpi; two eyes; the abdomen depressed; six legs formed for running.

The insects of this genus live by extracting animal juices. Some infest the bodies of quadrupeds, others of birds, and some even of insects themselves. It must, however, be observed, that many insects, infesting other animals, have sometimes been referred to the genus pediculus, which, in reality, belong to those of acarus, monoculus, &c. Gmelin enumerates sixty-six species. Leuwenhoeck has proved that the nits or eggs of ordinary lice are not hatched until the eighth day after they are laid; and that they never commence laying them until a month old: he also ascertained that a single louse may in eight weeks witness the birth of 5000 descendants. A pedicular disease (phthiriasis), is said to have carried off Antiochus Epiphanes, Sylla, two of the Herods, and the emperor Maximian. Mr. Kirby, however, disputes this, because, as he contends, this genus is never subcutaneous, for which he quotes the authority of Mead and Willan If these observations be allowed their due weight, he continues, it will follow, that a disease produced by animals residing under the cuticle cannot be a true phthiriasis, and therefore the death of the poet Alcman, and of Pherecydes Syrius the philosopher, mentioned by Aristotle, must have been occasioned by some other kind of insect.' For speaking of the lice to which he attributes these catastrophes, he says that they are produced in the flesh in small pustule-like tumors, which have no pus and from which when punctured they issue. For the same reason, the disorder which Dr. Heberden has described in his Commentaries, from the communications of Sir E. Wilmot, under the name of morbus pedicularis, must also be a different disease, since, with Aristotle, he likewise represents the insects as inhabiting tumors, from which they may be extracted when opened by a needle. He says, indeed, that in every respect they resemble the common lice, except in being whiter; but medical men, who were not at the

same time entomologists, might easily mistake an acarus for a pediculus. Dr. Willan, in one case of prurigo senilis, observed a small number of small insects on the patient's skin and linen. They were quick in their motions, and so minute that it required some attention to discover them. He took them at first for small pediculi; but under a lens they appeared to him rather to be a nondescript species of pulex; yet the figure he gives has not the slightest likeness to the latter genus, while it bears a striking resemblance to the former. It is not clear whether his draughtsman meant to represent the insect with six or eight legs: if it had only six, it was probably a pediculus; but if it had eight, it would form a new genus between the acaride and the hexapod aptera. Dr. Bateman in reply to some queries put to him at my request, by our common and lamented friend Dr. Reeve, relates that he understood from Dr. Willan, in conversation, that the insect in question jumped in its motion. This circumstance he regards as conclusive against its being a pediculus; but such a consequence does not necessarily follow, since it not seldom happens that insects of the same genus either have or have not this faculty; for instance, cyphon hemisphæricus, acarus scabiei, &c. Dr. Willan has quoted with approbation two cases from Amatus Lusitanus, which he seems to think correctly described as phthiriasis. In one of them, however, which terminated fatally, the circumstances seem rather hyperbolically statedI mean, where it is said that two black servants had no other employment than carrying baskets full of these insects to the sea!!'

Genus 4. Puler (the flea).-Antennæ projecting, and moniliform; no palpi or maxillæ ; a long, inflected proboscis, covered at the base with two ovate laminæ; the sheath two-valved, five-jointed, and containing one bristle; lip rounded, and fringed with reflected prickles; eyes two; abdomen compressed; legs six, formed for leaps.

This genus is oviparous, and the common species, P. irritans, deposits its nits on animals that afford them a proper food; these, being round and smooth, usually slip straight down, unless detained by folds, or other inequalities of clothes, hairs, &c. They 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. These eggs first produce a white worm, of a shining pearl color, which feeds on the scurfy substance of the cuticle, the downy matter gathered in the piles of clothes, or like substances. In a fortnight they come to a tolerable size, and are very lively and active: if at any time disturbed, they roll themselves into a kind of ball. Soon after they creep, after the manner of silkworms, with a very swift motion. When arrived at full size, they hide themselves as much as possible, and spin a silken thread out of their mouth, wherewith they form a small round bag, or case, white within as paper, but without always dirty. Here, after a fortnight's rest, the animalcule bursts out, transformed into a perfect flea; leaving its exuviæ in the bag. While it remains in the bag it is milk-white, till the second day before its eruption; when it becomes colored, grows

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hard, and gets strength; so that, upon its first delivery, it springs nimbly away.—Phil. Trans.

No. 249.

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. Leuwenhoeck affirms also, that he has seen innumerable animalcules, shaped like serpents, in the semen masculinum of a flea. Baker's Micros. p. 191, and 194. On dissection, this animal is far from being a disagreeable object. It is covered with curious black shelly scales or plates, admirably jointed, and folded one over another in such a manner as to comply with all the nimble motions of the creature. They are all also curiously polished, and set round the edges with short spikes in a very regular manner. Its neck is finely arched; 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 obtain its nutriment. Its eyes are large and handsome, 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 at the same instant, they carry the creature to a surprising distance. Aristophanes ridicules Socrates for having measured the leap of a flea. (Nubes, act i. sc. 2). Scientific men have however persevered in ascertaining the wonderful power and strength of this little creature, which will leap two hundred times the length of its own body, unaided, as we see, by wings. Mr. Kirby (vol. iv. p. 188) furnishes us with the following anecdotes of the strength of this species.-Mouffet relates that an ingenious English mechanic, named Mark, made a golden chain of the length of a finger, with a lock and key, which was dragged by a flea ;-he had heard of another that was harnessed to a golden chariot, which it drew with the greatest ease. Another English workman made an ivory coach with six horses, a coachman on the seat with a dog between his legs, a postilion, four persons in the coach, and four lacqueys behind-which also was dragged by a single flea. At such a spectacle one would hardly know which most to admire, the strength and agility of the insect, or the patience of the workman. Latreille mentions a flea, of a moderate size, dragging a silver cannon on wheels, that was twenty-four times its own weight, which, being charged with powder, was fired without the flea appearing alarmed.

The legs have several joints, are very hairy, and terminate in two long and hooked sharp claws. The flea lodges its piercer or sucker 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 red round spot, with a hole in the centre of it, vulgarly called a flea-bite This piercer, its sheath opening sideways, 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, when it is usually thrust out in convulsions.

'Dear Miss,' said a lively old lady to a friend of mine (says Mr. Kirby), who had the misfortune to be confined to her bed by a broken limb, and was complaining that the fleas tormented her, dont you like fleas? Well, I think they are the prettiest little merry things in the world. I never saw a dull flea in all my life.' The celebrated Willoughby kept a favorite flea; which used at stated times to be admitted to suck the palm of his hand; and enjoyed this privilege for three months, when the cold killed it. And Dr. Townson, from the encomium which he bestows upon these vigilant little vaulters, as supplying the place of an alarum and driving us from the bed of sloth, would seem to have regarded them with feelings much more complacent than those of Dr. Clarke and his friends, when their hopes of passing one night free from the attacks of vermin,' were changed into despair by the information of the laughing sheik, that the king of the fleas held his court at Tiberias:' or than those of M. M. Lewis and Clarke, who found them more tormenting than all the other plagues of the Missouri country, where they sometimes compel even the natives to shift their quarters.

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P. penetrans, the chigoe of South America, has a proboscis as long as the body. The body is of a reddish-brown color; abdomen of the female, when gravid, orbicular, and swollen to nearly a hundred times its usual size. It is very troublesome in the sugar colonies, penetrating into the skin of the inhabitants, particularly of the negroes, where it lodges its eggs, and causes malignant, and often fatal, ulcers.

'All disputes,' observe Messrs. Kirby and Spence, concerning the genus of this insect, would have been settled long before Swartz's time (who first gave a satisfactory description and figure of it, proving it to be a puler, as has been observed above), had success attended the patriotic attempt of the Capuchin friar, recorded by Walton in his History of St. Domingo, who brought away with him from that island a colony of these animals, which he permitted to establish themselves in one of his feet; but unfortunately for himself, and for science, the foot entrusted with the precious deposit mortified, was obliged to be amputated, and with all its inhabitants committed to the waves.' According to Ulloa, and his opinion is confirmed by Jussieu, there are two South American species of this mischievous insect. It is described as generally attacking the feet and legs, getting without being felt, between the skin and the flesh, usually under the nails of the toes, where it nidificates and lays its eggs; and, if timely attention be not paid to it, which, as it occasions no other uneasiness than itching, (the sensation at first I am assured is rather pleasing than otherwise), is sometimes neglected, it multiplies to such a degree, as to be attended by the most fatal consequences, often, as in the above instance, rendering amputation necessary, and sometimes causing death. The female slaves in the West Indies are frequently

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