the Louse, in zoology, a genus of insects belonging to the order of aptera. It has six feet, two eyes, and a sort of sting in the mouth; the feelers are as long as the thorax; and the belly is depressed and sublobated.

Many animals both of the quadruped and flying kinds are subject to lice; but these are of peculiar species on each animal, and are not at all like those which infest the human body. Nay, even insects are infested with vermin which feed on and torment them. Several kinds of beetles are subject to lice; but particularly that kind called thence the lousy beetle. The lice on this are very numerous, and will not be shook off. The earwig is often infested with lice, just at the setting on of its head; these are white, and shining like mites, but they are much smaller; they are round-backed, flat belled, and have long legs, particularly the foremost pair. Snails of all kinds, but especially the large naked sorts, are very subject to lice; which are continually seen running about them, and devouring them. Numbers of little red lice, with a very small head, and in shape resembling a tortoise, are often seen about the legs of spiders, and they never leave the animal while he lives; but if he is killed, they almost instantly forsake him. A sort of whitish lice is found on humble-bees: they are also found upon ants; and fishes are not let subject to them than other animals.

Kircher tells us, that he found lice also on flies, and M. de la Hire has given a curious account of the creature which he found on the common fly. Having occasion to view a living fly with the microscope, he observed on its head, back, and shoulders, a great number of small animals crawling very nimbly about, and often climbing up the hairs which grow at the origin of the fly's legs. He with a fine needle took up one of these, and placed it before the microscope used to view the animalcules in fluids. It had eight legs; four on each side. These were not placed very distant from each other; but the four towards the head were separated by a small space from the four towards the tail. The feet were of a particular structure, being composed of several fingers, as it were, and fitted for taking fast hold of any thing; but the two nearest the head were also more remarkable in this particular than those near the tail; the extremities of the legs for a little way above the feet were dry and void of flesh like the legs of birds, but above this part they appeared plump and fleshy. It had two small houses upon its head, formed of several hairs arranged closely together; and there were some other clutters of hairs by the side of these horns, but they had not the same figure; and towards the origin of the hinder legs there were two other such clutters of hairs which took their origin at the middle of the back. The whole creature was of a bright yellowish red; the legs, and all the body, except a large spot in the centre, were perfectly transparent. In size, he computed it to be about \(\frac{1}{400}\)th part of the head of the fly; and he observes, that such kind of vermin are rarely found on flies.

The louse which infests the human body makes a Pediculus very curious appearance through a microscope. It has such a transparent shell or skin, that we are able to discover more of what passes within its body than in most other living creatures. It has naturally three divisions, the head, the breast, and the tail part. In the head appear two fine black eyes, with a horn that has five joints, and is surrounded with hairs standing before each eye; and from the end of the nose or snout there is a pointed projecting part, which serves as a sheath or case to a piercer or sucker, which the creature thrusts into the skin to draw out the blood and humours which are its destined food; for it has no mouth that opens in the common way. This piercer or sucker is judged to be 700 times smaller than a hair, and is contained in another case within the first, and can be drawn in or thrust out at pleasure. The breast is very beautifully marked in the middle; the skin is transparent, and full of little pits; and from the under part of it proceed six legs, each having five joints, and their skin all the way resembling shagreen, except at the ends where it is smoother. Each leg is terminated by two claws, which are hooked, and are of an unequal length and size. These it uses as we would a thumb and middle finger; and there are hairs between these claws as well as all over the legs. On the back part of the tail there may be discovered some ring-like divisions, and a sort of marks which look like the strokes of a rod on the human skin; the belly looks like shagreen, and towards the lower end it is very clear, and full of pits; at the extremity of the tail there are two semicircular parts all covered over with hairs, which serve to conceal the anus. When the louse moves its legs, the motion of the muscles, which all unite in an oblong dark spot in the middle of the breast, may be distinguished perfectly, and so may the motion of the muscles of the head when it moves its horns. We may likewise see the various ramifications of the veins and arteries, which are white, with the pulse regularly beating in the arteries. But the most surprising of all the sights is the peristaltic motion of the guts, which is continued all the way from the stomach down to the anus.

If one of these creatures, when hungry, be placed on the back of the hand, it will thrust its sucker into the skin, and the blood which it sucks may be seen passing in a fine stream to the fore-part of the head; where, falling into a roundish cavity, it passes again in a fine stream to another circular receptacle in the middle of the head; from thence it runs thro' a small vessel to the breast, and then to a gut which reaches to the hinder part of the body, where in a curve it turns again a little upward; in the breast and gut the blood is moved without intermission, with a great force; especially in the gut, where it occasions such a contraction of the gut as is very surprising. In the upper part of the crooked ascending gut above mentioned, the propelled blood stands still, and seems to undergo a separation, some of it becoming clear and watery, while other black particles are pushed forward to the anus. If a louse is placed on its back, two bloody darkish spots appear; the larger in the middle of the body, the lesser towards the tail; the motions of which are followed by the pulsation of the dark bloody spot, in or over which the white bladder Pediculus, seems to lie. This motion of the systole and diastole is best seen when the creature begins to grow weak; and on pricking the white bladder, which seems to be the heart, the creature instantly dies. The lower dark spot is supposed to be the excrement in the gut.

Lice have been supposed to be hermaphrodites; but this is erroneous; for Mr Lieuwenhoek observed, that the males have flings in their tails, which the females have not. And he supposes the smarting pain, which those creatures sometimes give, to be owing to their flinging with these flings when made uneasy by pressure or otherwise. He says, that he felt little or no pain from their suckers, though six of them were feeding on his hand at once.

In order to know the true history and manner of breeding of these creatures, Mr Lieuwenhoek put two female lice into a black stocking, which he wore night and day. He found on examination that, in six days one of them had laid above 50 eggs; and, upon dissecting it, he found as many yet remaining in the ovary: whence he concludes, that in 12 days it would have laid 100 eggs. These eggs naturally hatch in six days, and would then probably have produced 50 males, and as many females; and these females coming to their full growth in 18 days, might each of them be supposed after 12 days more to lay 100 eggs; which eggs, in six days more, might produce a young brood of 5000; so that in eight weeks, one louse may see 5000 of its own descendents.

Signior Rhedi, who has more attentively observed these animals than any other author, has given several engravings of the different species of lice found on different animals. Men, he observes, are subject to two kinds; the common louse, and the crab-louse. He observes also, that the size of the lice is not at all proportioned to that of the animal which they infest; since the starling has them as large as the swan.

Some kinds of constitutions are more apt to breed lice than others; and in some places of different degrees of heat, they are certain to be destroyed upon people who in other climates are overrun with them. It is an observation of Oviedo, that the Spanish sailors, who are generally much afflicted with lice, always lose them in a certain degree of latitude in their voyage to the East Indies, and have them again on their returning to the same degree. This is not only true of the Spaniards, but of all other people who make the same voyage; for though they set out ever so lousy, they have not one of those creatures by the time they come to the tropic. And in the Indies there is no such thing as a louse about the body, though the people be ever so nasty. The sailors continue free from these creatures till their return; but in going back, they usually begin to be lousy after they arrive at the latitude of the Madeira islands. The extreme sweats, which the working people naturally fall into, between the latitude of Madeira and the Indies, drown and destroy the lice; and have the same effect as the rubbing over the lousy heads of children with butter and oil. The sweat, in these hot climates, is not rank as in Europe, and therefore it is not apt to breed lice; but where people return into latitudes where they sweat rank again, their nastiness subjects them to the same visitations of these vermin as before. The people in general in the Indies are very subject to lice in their heads, tho' free from them on their bodies. The reason Pediluvium of this is, that their heads sweat less than their bodies, and they take no care to comb and clean them. The Spanish negroes wash their heads thoroughly once every week with soap, to prevent their being lousy. This makes them escape much better than the other negroes who are slaves there; for the lice grow so numerous in their heads, that they often eat large holes in them.