These parasites repay by an unkind action the assistance which they have demanded from us.
Besides the gnats, which belong to the family of Culicidæ, there are also the Ceratopogon, and especially the Simulium molestum, known in North America under the name of Black-flies: “the tormenting black-flies of this country,” as the Americans say. Certain Nemocera, known by the name of Rhagio, put to flight both man and animals.
They are very small; they get into the nostrils, and cause animals to become blind by introducing themselves into their eyes. In addition to these hurtful insects, we find others fatal to the life of animals, and which are a real plague in certain countries.
The numerous travellers who have explored the interior of Africa, have almost all spoken to us of a fly which attacks beasts of burden, and kills them in a few hours; this is the Tsetse (Glossina morsitans). More than one expedition has failed on account of this dipterous fly. It was this which obliged Green to abandon his plan of reaching Libebe, by causing him to lose one after another all his beasts of burden and of draught. The horse, the ox, and the dog are more especially attacked by this terrible fly between the 22nd and 28th degree of longitude, and the 18th and 24th of south latitude. Happily it does not produce any effect upon man.
There is another fly in Mexico which is dangerous to man; it is known by the name of Musca hominivora, or more correctly, Lucilia hominivora. Vercammer, a military surgeon of the Belgian army, relates that a soldier in Mexico had his glottis destroyed, and the sides and the roof of his mouth rendered ragged and torn, as if a cutting punch had been driven into those organs. This soldier threw up with his spittle more than two hundred larvæ of this fly. We give below the figure of the larva and of the perfect insect. He had found this man sick in Michoacan, at a height of 1,866 metres, between Mexico and Morelia.
My son-in-law, Dr. Vanlair, informs me that citric acid or the juice of lemons is efficacious in destroying these insects. Injections of this acid are thrown into the nasal fossæ.
At Brazil, in the province of Minas Geraes, they give the name of Berne to a fly which attacks man and cattle from the month of November until February. It deposits its eggs in the loins, the arms, the legs, or even the scrotum, without the victims perceiving it, and their presence is first shown by a redness, then by a sensation of itching, and a swelling with the formation of pus.
Among those insects which suck the blood, is one which is known by every one, the Breeze-fly, Tabanus bovinus. Happily it seldom attacks any animals except oxen and cows. We give a representation of the insect, the parts of the mouth, and one of the antennæ.
In the same order of diptera are found ordinary flies, among which may be easily distinguished the three species which are here represented, and which differ as much by their external characters as by their mode of life.
Another fly also attacks horses and cattle, and occasionally even man, the Asilus crabroniformis, whose wounds sometimes draw blood. Martins, the birds of the twilight, which fly in flocks above the houses, describing circles and uttering shrill cries, are usually infested by many vermin, among which we find a fly of considerable size, which looks much like a spider, the Ornithomya hirundinis. It moves about among the feathers with astonishing facility, and it is not always confined to the same bird; it quits its host to establish itself upon another, and sometimes throws itself upon man to suck his blood.
Some years ago these insects penetrated in the middle of the night through the open windows into one of the apartments of the military hospital at Louvain, and the next morning the skin of many of the patients, and especially the bed-linen, were covered with stains of blood. The physicians sent me some of these insects, not knowing whence they had come, nor whether they had been the cause of this annoyance. During the night, these Ornithomyæ had quitted their hosts to attack the soldiers.
One of these insects, the banded Syrphus (Syrphus balteatus), when in the larva state, seizes the rose aphides, and sucks their blood with great eagerness.
But it is not precisely a case of parasitism, when the wounds of soldiers are covered with larvæ, of which there were many sad instances in the Crimean war. There are flies which deposit their eggs in pus, as in all kinds of animal matter in a state of decomposition. It is even said that these insects, deceived by the smell of the Arum flower, will lay their eggs on the pistil. The name of Myasis has been given to the presence of these larvæ in a wound.
Every one knows that bats are often literally covered with vermin. Among the many parasites which attack these little animals we find, besides the acaridæ, a Pteroptus of great agility, which seems, as it were, to swim among the fur, and looks like a little spider or a microscopic crab. There are but few bats on which we do not find some of these, and we have sometimes seen them in such abundance, that it was impossible to touch a single hair without disturbing them. This species is usually called Pteroptus vespertilionis. It is constantly in motion, and glides among the fur like a mole in a sandy soil.
Together with these Pteropti lives a parasite of gigantic size, which insinuates itself among the fur with equal dexterity, and bears the name of Nycteribia. This has long claws like a spider, and plunges deeply into the fur. These Nycteribiæ are found only on bats. They are often associated on these animals with fleas and mites. Mr. Westwood has written a monograph upon them. Mons. Plateau, our colleague, has quite recently described a new species in the “Bulletins de l’Académie de Belgique.”
Among the insects justly dreaded by man, and which follow him everywhere, is found one of the Hemiptera, known by every one under the name of bed-bug (Cimex lectularia). It is said that this insect was unknown in the capital of Great Britain before the fire of London in 1666. According to some entomologists, it was introduced into Europe in some wood that came from America. It is only necessary to make this slight reference to the Cimices; their congeners are, for the most part, parasites of plants, and live on their sap.
To the same order belongs the singular hemipterous insect of our ponds, the boat-fly (Notonecta). It has some feet suited for swimming, and others for running, and it swims on its back with great rapidity. It is a dangerous neighbour for everything that has life. Always greedy of blood, it attacks great as well as little animals, and sucks the blood of its victim to the last drop, so that it must be closely watched when placed in an aquarium.
Lice, concerning which we are about to add a few words, are also free parasites, and belong to a different order of insects. Their mouth is formed of a sucker contained in a sheath, without articulations; it is armed at the point with retractile hooks, within which are four bristles. They have climbing feet, terminated by pincers, with which they seize the hair of the animals on which they live; their eggs are known by the name of nits. We have represented in Figs. 17, 18, and 19, the complete insect, the head, the sucker, and a claw more highly magnified.
Lice are hatched at the end of five or six days, and reproduce at the end of eighteen days. Leeuwenhoek calculated that two females might become the grandmothers of 10,000 lice in eight weeks. They are all parasites of the mammalia, and three species live at the expense of man: the louse of the head, of which Swammerdam gave a detailed description in his work entitled “Biblia Naturæ”; the body-louse, which lives on the bodies of filthy people, forms a distinct species; the third species is the louse which occasions the disease called pedicularis, or Phthiriasis. These insects were formerly much more common than they are at the present day. In 1825 Dr. Sichel published a monograph concerning them; and there appeared in the “Gazette Médicale” of 1871, a long article on the history of Phthiriasis.
It is stated that several great personages have fallen victims to its attack, but these observations date from a period when it was thought that they could be spontaneously originated. It is in fact difficult to believe, as it has seriously been stated, that lice have been seen to issue from the bodies of men like a spring of water from the earth. A physician of the 16th century, named Amatus Lusitanus, speaks of a great Portuguese nobleman who was so covered with lice that two of his servants were constantly occupied in collecting them and carrying them to the sea. Andrew Murray has published a memoir on the lice of the various races of men.
The name of helminthiasis has been proposed for worm disease in general, and either tæniaceous or lumbricoidian helminthiasis, according to the species which made its appearance. These parasites were considered to be formed spontaneously, and their presence constituted a pathological condition, two errors which have now been recognized, and by which the science of medicine has profited.
The Phthirius pubis is another species which has been found only on white races, and attaches itself especially to the hair on the pubis. Mons. Grimm has published in the bulletins of the Academy of St. Petersburg, an interesting memoir on the embryogeny of this insect; and, more recently, Mons. L. Landois, of Griefswald, has completely studied its habits.
We are now about to refer to certain parasitical insects whose name is usually associated with those which have preceded; they are well known by all, and attack both men and the mammalia with no less ferocity; we allude to fleas, which differ from gnats in this respect, that the male is as eager for blood as the female, and that both of them, like leeches, live by sucking it; besides, the larvæ of fleas live only on what the full-grown insects bring them, whereas the larvæ of gnats get their own living; the mother flea sucks for herself first, and then divides the spoil with her larvæ which as yet have no feet. For a long time it was thought that the fleas of different animals belonged only to a single species, and consequently that the flea of man was not different from that of a cat or a dog.
Daniel Scholten, of Amsterdam, in 1815, showed by his microscopical observations, that fleas differ from each other; and in 1832, Dugès of Montpellier, investigated the distinctive marks of the various species. The observations of Scholten may be found in “Les Materiaux pour une faune de la Néerlande,” by R. T. Maitland.
The ordinary flea is called Pulex irritans, and especially attacks man in Europe and in North America; it may be called a fly without wings, and, together with its congeners, it forms a distinct family under the name of Pulicidæ. Van Helmont treated of these insects, and gave directions for making them, just as though he were describing a recipe for pomade. At that time, naturalists supposed that certain fish could be formed spontaneously, and that nothing but fermentation was necessary in order to bring forth a crowd of living creatures from this molecular disaggregation. Fleas may, perhaps, some day find a place in the chemist’s shop as well as leeches. We see no reason why homœopathic bleedings should not be resorted to, as well as homœopathic medicines; we should certainly have more confidence in the effects of the bites of fleas, than in the efficacy of remedies subdivided into the millionth part of a grain.
Fleas differ much in size, according to the places which they inhabit. Dugès, of Montpellier, gives us a curious instance of this. He devoted himself to researches on the zoological characters of this genus, studying the four species which are the best known, the Pulex irritans of man, Pulex canis of the dog, Pulex musculus of the mouse, and Pulex vespertilionis of the bat.
Fleas of a brown colour, almost black, and of enormous size, are commonly met with on the sandy shores of the Mediterranean, at least, in the neighbourhood of Cette and Montpellier; they are more than half as large as a common fly. These are human fleas, and their presence on the sea-shore during the heats of summer is due solely to the great number of bathers of both sexes and of all classes, which lay their clothes down there. If at some future day these insects were to be placed in the rank of surgical species, it would be necessary to resort to those shores in order to procure them; and we might suppose that, by judicious crossing, we might soon produce races that would be of real service; as yet, however, the therapeutic art has had recourse only to leeches. Since we have seen these insects harnessed and performing their exercises in public, we cannot say that the future may not reserve for us a still greater surprise.
None who saw them can have forgotten the exhibition of learned fleas made by a young lady who had sufficient patience to train them. Walckenaer saw them in Paris, and examined them with the eye of an entomologist; he relates that thirty fleas performed their feats at evening exhibitions, for admission to which the sum of sixty centimes was paid; that these fleas stood on their hind legs, armed with a pike, which was a very thin splinter of wood; some dragged a golden chariot, others a cannon with its carriage, and all were attached by a golden chain on the thighs of their hind legs.
It is curious to see how Leeuwenhoek described, two centuries ago, the history of the flea, with all its details, the accuracy of which can scarcely be surpassed. He observed their entire anatomy, as far as was possible with the instruments of his time (1694), and his descriptions are accompanied by excellent plates; he saw them copulate and lay eggs, and followed their whole development.
The finest fleas, both as to their size and form, inhabit the bats. Fleas are often found on horses. A colonel of cavalry, on his return from the frontier in 1871, sent me some of these insects, with the request that I would examine them. He added that the horses of his regiment were literally eaten up by them. It was the Hematopinus tenuirostris. There is a species peculiar to monkeys, which Mons. Paul Gervais has described under the generic name of Pedicinus.
At the commencement of the last century, a certain physician attributed the cause of almost all diseases to microscopical insects, and gave figures of ninety species which were supposed to produce, in some cases smallpox, in others rheumatism and gout, jaundice and whitlows. Almost all these figures represent imaginary creatures. This opinion has reappeared in modern times; how many persons have been seen to smoke camphor in order to preserve themselves from the invasion of animalcules. I do not speak of the apparatus which has been contrived in order to breathe nothing but air which has been filtered and deprived of its living germs.
There are some of the articulata with four pairs of feet, a kind of microscopic spiders which require to be noticed here; these are the numerous Acari which infest many animals. Some of these wander on the surface of the skin, others in galleries under the epidermis, and many pass from one animal to another without changing their form or mode of life. There is a considerable number of them; no class of the animal kingdom is free from them, neither aquatic nor terrestrial animals, neither vertebrates nor invertebrates. These parasites belong for the most part to the same family, and cause by their presence a disease which was for a long time considered to be peculiar to the skin.
An English naturalist, Mr. George Johnson, carefully studied the parasitical and free acaridæ of Berwickshire. Mons. Ehlers has written a very interesting work, with fine illustrations, on the acaridæ of birds, published in the “Archives of Troschel.” There is more than one species which lives at the expense of man, and one of them produces a disease known in every country and at all times under the name of the itch; until 1830 its true nature was still unknown. It is not an affection of the skin, as was thought, but merely the result of the presence of these animalcules. The director of the special Hospital for Skin Diseases at Paris was so fully convinced that the acaridæ are not the cause of the itch, that he offered a prize to any one who could render these insects visible. A student of medicine, a Corsican by birth, had happened to see these itch-insects sought for in his own country, and was the first to prove, in 1834, the real cause of the disease. A resident student had given, in a thesis which he sustained at Paris before the faculty of medicine, a drawing of a cheese-mite instead of the itch-insect, and this error had caused it to be supposed that the species peculiar to this disease did not exist. We give in Figures 21, 22, 23, representations of the male and female insect, greatly magnified.[2] Of course, all the treatment necessary for the cure consists in getting rid of the animalcules and their eggs, and in cleansing the skin and the clothes of the patient. Petroleum oil has been judiciously prescribed in order to destroy the mite, but the remedy which seems the most efficacious is Balsam of Peru.
Fig. 21.—Sarcoptes scabiei, or male acarus of the itch; the lower surface.
Fig. 23.—Sarcoptes scabiei, male; the dorsal surface.
Most mammals have their peculiar species of acari, and the horse has two which give rise to different skin affections. Since the presence of these animals constitutes the disorder, it may be easily caught; man may communicate it to domestic animals, and they may give it to him. The itch-insect of man bears the name of Sarcoptes scabiei, and no other species than those of Sarcoptes can be transferred from animals to man. These animalcules have at different times been diligently studied by many naturalists, and Dr. Füestenberg has lately published a folio volume, under the title of “Die Krätzmilben der Menschen und Thiere,” with large lithographic plates, and illustrations in the text. It is possible that the pustular disease which prevails at Sierra Leone is originated by some peculiar acarus. Another acarus parasitical on man, the Persian Argas, is fortunately unknown in Europe. It is said to be common at Miona, and prefers to attack strangers. Its stings produce acute pain, and travellers assure us that they may be the cause of death. This acarus remains but a short time on the person, and generally makes its appearance during the night. It is called also the Miona bug. Fischer of Waldheim has published a very interesting memoir on this parasite. Justin Goudot has also observed another Argas (A. Chinche) which torments man in the temperate regions of Columbia.
These Arachnida, for they are articulata with four pairs of legs, often make their appearance where we should not expect to find a living organism, and naturalists, under these circumstances, have, with the best faith possible, supposed that they had seen these mites produced spontaneously without parents. We have seen a remarkable instance of this in the Acarus marginatus of Hermann. On the 18th Thermidor, an 2, they were making a post mortem examination at Strasburg of a man who had died of fracture of the skull, and when opening the dura mater, they saw on the corpus callosum, a mite running about which became the type of the species. The appearance of this acarus under such conditions made, as may be supposed, much noise at the time, but we should not be surprised if it had been introduced during the operation by a fly seeking to lay its eggs.
In this group is found another interesting acarus, which is developed in man in the sebaceous crypts of the nostrils. The name of Simonea has been given to it, from Dr. Simon of Berlin, who made it his especial study. This genus leads us by its form to the Linguatulæ, the structure of which has been so long doubtful. The Simonea folliculorum belongs to the family of the Demodicidæ.
The dog harbours a demodex (D. Caninus) which causes it to lose its hair. Some years ago, the sheep in Belgium were attacked by one of the acaridæ, the Ixodes reduvius, which had been introduced from a neighbouring country, and had multiplied with frightful rapidity. Packard has given an account of an Ixodes bovis on the Erethizon epixanthus, and on the Lepus Bairdii, and an Argas Americana on cattle coming from Texas; this was published in the sixth report of the United States’ Geological survey (1873).
According to the observations of Mons. Megnin, the Tyroglyphi, the Hypopi, the Homopi, and the Trichodactyli, are transitory forms which ought not to be preserved as generic divisions among the acaridæ. We have found on the small bat (Pipistrella) an acaride (Caris elliptica) and a new Ixodes which we have described in a special memoir on the parasites of the Cheiroptera. Mr. Lucas caught an ixodes on a dog, and kept it alive long enough distinctly to see it lay eggs which proceeded from an oviduct. These eggs formed masses attached to the abdomen of the mother.
An acarus (Dermanyssus avium) is found on birds, and multiplies with such rapidity that it completely exhausts those on which it has established itself. It has been seen accidentally on man. An instance is recorded of a woman who could not get rid of these parasites, because she passed every day through her henhouse in order to get to her cellar, and the frightened fowls threw down upon her a perfect shower of acaridæ. Not long ago mention was made at the Academy of Medicine at Paris, of a sarcoptes (S. mutans), which produces a disease among fowls, especially on the cock and hen, and which passes from these to the horse and other domestic animals. This sarcoptes prefers to live under the epidermis of the feet. Reptiles are not free from its attacks, for it is often seen on lizards and serpents. We have found a very curious one on the skin of a gecko from the south of France.
Many insects are always covered with certain species of acaridæ. Every entomologist knows that the body of the “watchman” beetle always has some of these, like little living pearls, which wander especially on the under side of the abdomen. It is the same with a small coleopterous insect that is found abundantly wherever there is any decomposing matter. Léon Dufour gave himself up to the study of some of the parasites of insects, and mentions, among others, a species belonging to the muscidæ, the Limosina lugubris, which does not measure a line in length, and which harbours as many as fifteen pteropti under its abdomen.
Bees, which give us their wax and their honey in exchange for the shelter which we afford them, have a mortal enemy, an acarus, which attaches itself to them, not in order to gain any advantage from them, but to cause their death. It is not so much a parasite as an assassin, and we may be excused from describing it. We have found acaridæ on certain polyps, the Campanulariæ and Sertulariæ of our coasts, and some years ago we described one which is very curious, and inhabits the southern whale, in the midst of its Cyami and Tubicinellæ. The anodonts of our ponds, as well as the Uniones usually have the skin of their feet and that of their mantle encrusted with acari of every age, to which the name of Atax ypsilophora has been given. The species which live on the anodonts are not the same as those which inhabit the Uniones; and Mons. E. Bessels, who has so fortunately returned from his voyage to the North Pole, on board the Polaris, has seen the species of the anodonts crossed with those of the Uniones.
There are also Arachnida which are parasitical only while young, as the Trombidions and certain Hydrachnæ (Fig. 24) which frequent aquatic animals. The Leptus autumnalis, known in France, at least in some localities, by the name of Rouget, is an acarian which throws itself upon man, and especially attaches itself to the roots of the hair: fortunately, it is only found in the country districts. The Acarus (Cheyletus) eruditus (Fig. 25) lives in books and collections, as well as on fruits and all kinds of bodies more or less damp, left in dark places; it has been studied by Van Der Hoeven. Mons. Leroy de Méricourt found in pus, which was running from the ear of a sailor, acaridæ which Mons. Robin refers to the genus Cheyletus, rather than to that of the Acaropses.
[2] Hardy, in his Leçons sur les maladies de la peau (Paris 1863), devotes a special chapter to parasitical diseases, and gives the complete history of the itch-mite.
We have brought together in the former chapter the animals which live at the expense of their neighbours, without seeking for anything except shelter. They seize their prey as they pass, are nourished by the blood of their neighbours, but never think of establishing themselves in their organs during any period of their life. They are almost as much carnivora as parasites, and only differ from the former class because they spare the life of their victims. They are unlike ordinary parasites, since they are contented with their food alone; and their appearance from the period of their entrance into the world is that of free animals. Those whose history we are now about to sketch, live in freedom like the preceding during all the time that they are young; like them, they are completely independent during the first period of their life; but when they have arrived at mature age, when the endless cares entailed by their young ones come upon them, they change their costume and accommodate themselves as well as they can to the new lodging which they have chosen. There is often not the least resemblance between these creatures in their youth and their adult state. All these parasites have lived a joyous life before choosing the host which is to serve them as a cell; but though in many species we see both sexes shut themselves up as in a cloister, some species are to be found in which the female alone seeks for extraneous aid; which is not surprising, since she alone undertakes all the charge of the family, and this would be beyond her strength, and would endanger the life of her offspring, if she did not receive help and protection.
The host resembles in some respects a lying-in hospital, especially when the female alone seeks for herself a resting-place and her food, which is not always the case. We find, in fact, in a considerable number of Lernæans, that the microscopic male passes unperceived upon his female, and when he renounces his bachelor life, she feeds him with her own blood. There cannot be a more faithful husband, since he only plays the part of a spermatophore. We find a still more curious example in this respect, and in which the dignity of the male is not less compromised; we refer to the Bonelliæ which live freely in the sand, and whose males establish themselves parasitically on the sexual organs of the female. She herself lives by her own industry, nourishes her husband, and alone provides for all the requirements of maternity.
In a later part of this work, we shall mention worms which live in freedom in damp earth, and whose direct progeny, entirely composed of females and hermaphrodites, can only exist as parasites. These worms do not resemble their mother but their grandmother, and if their descent had not been traced, they would doubtless have been taken for species entirely distinct from each other. Thus it is not always the whole family which is modified; the male often preserves all the attributes of his sex and of his youth, while the female changes entirely her appearance and her mode of motion, especially at the approach of the period when the interest of the species prevails over that of the individual.
We can nowhere find more graceful and regular forms during the whole of their early youth than those of many of these parasites; we can never see more ungraceful, we might almost say more comical, attitudes than those of the greater part of these creatures when full grown. One might take them for some misshapen excrescence, or some scrap of wasted flesh on the body of their host. A certain number of insects are found which lead this singular kind of life, but this is more especially the case among the crustaceans, particularly the copepod crustaceans. Among all these we find the most absurd recurrent forms; in fact these animals instead of carrying on their evolution, like the caterpillar which becomes a butterfly, retrograde rather than advance, and acquire an appearance and character which prevent us from recognizing their origin. Many of these are at present known, whose graceful form is so completely changed, that without referring to the study of their embryo state, one could not tell to what class they belong. Nothing remains of their organs except the sexual apparatus and a shapeless skin. These curious parasites live also on the surface of bodies, and sometimes in the cavity of the mouth; but in fishes they are most frequently found in the branchial membranes. They look like natural setons, and it is not impossible that they sometimes fulfil the same functions.
We will first examine some insects, then certain isopode crustaceans, an order to which the Cloportidæ (wood-lice) belong, many of which require uninterrupted assistance; then we will turn to the Lernæans, which surpass all the rest in their many and bizarre transformations.
We have first to speak of the Chigoe, an insect, the female of which alone demands lodging and provisions, the male being contented, like those of the preceding chapter, with pillaging his victim as he passes by. This parasite of man inhabits South America, and has received the name of Pulex penetrans, or, according to the latest nomenclature, of Rhyncoprion penetrans. It is a very small species, which pierces the shoes and the clothes with its pointed beak (Fig. 27), and penetrates into the substance of the skin; the male (Fig. 26) is contented with sucking the blood, and then resumes its wanderings, like the parasites of which we have spoken in the preceding chapter; while the female finds for herself a hiding-place, and becomes of such a monstrous size that the entire insect is nothing more than an appendage of the abdomen, as may be seen in the annexed figure. This insect is well known, since it attacks man, and usually establishes itself on his toes, but it occasionally fixes itself in the same manner on the dog, the cat, the pig, the horse, and the goat. It has also been seen upon the mule. Mons. Guyon has paid much attention to it, but we owe the last observations to Mons. Bonnet, a French navy surgeon, who passed three years in Guiana, and has ascertained that the chigoe fortunately does not extend beyond the 29th degree of south latitude. Another parasite, well known by sportsmen, is the tick. It is not an insect like the flea, but an arachnid, a kind of acarus, which passes through its last stages of development under the skin of a mammal. It is called Ixodes ricinus, and Professor Pachenstecher has carefully studied its organization. The ticks especially attack dogs, but are also found on the roebuck, the sheep, the hedgehog, and even on bats.
Some years ago it was propagated in an extraordinary manner on roebucks in the woods of the Duke of Arenburg, in the environs of Louvain. They are sometimes found also on man. We know of two instances: the first is that of a lady at Antwerp, who had a small tumour on her shoulder, which was removed, and enclosed a living tick. Leeuwenhoek gives an instance of a woman of the lower classes who had a tick in the middle of her stomach. Moquin-Tandon relates that Raspail found some on the head of a little girl four or five years old. He also gives an instance of a young man who, returning from hunting, found a tick under his arm; and while on the site of a sheep market, a servant found one morning three attached to the skin of his breast. Delegorgue speaks of some very small reddish ticks in Africa, which cover the clothes by thousands, and produce distressing itching. Others are found in different parts of the globe, and twenty-four species have been described. Several new American Ixodes have been noticed lately by Mr. Packard on the stag, the monax marmot, the Lepus palustris, &c. These arachnida live at first in freedom in the bushes, but after fecundation the female attacks the first mammal which she finds in her way, and establishes herself upon it; dogs become infested with it by running in and out among the brushwood.
The Argas reflexus lives on pigeons, and is allied to the Ixodes. R. Buchholz has lately studied many new acaridæ found on different birds.
If the forms are not so varied among the isopods as elsewhere, many among them present nevertheless the most extraordinary appearance, the most unexpected contour. Most of the parasitic isopods instal themselves in the thoracic cavity under the carapace of a neighbour, and make themselves contented in the small space which remains to them. After having disposed of their luggage, they arrange themselves scrupulously according to the extent of the lodging which they occupy, and, rather than interfere with the branchiæ, they raise up the walls of the cephalothorax, thus forming a sort of tumour which betrays the presence of the intruder. Others are found which are not contented with a natural cavity; they raise the scale of the skin of a fish, perforate or hollow out the true skin, or even pierce through the walls of the abdomen, in order to establish themselves in the intestines, still keeping up a communication with the exterior. A very common species of this class is called Bopyrus. We often see beautiful prawns, which are usually remarkable for their fine rose colour, exposed for sale in shop windows. If we examine them at certain seasons, especially in France, we perceive that the carapace at the side is raised; and if we take it off with some precaution, we discover underneath an irregular flattened body, which fishermen take for a young sole on account of its shape. This is the female bopyrus. The many appendages of the thorax, the division into rings, the symmetry of the body, all have disappeared, and the claws, the traces of which are scarcely seen, are no longer similar on the right and left sides. The male remains small and independent, and preserves the livery of the order to which he belongs. On the coast of Labrador, a bopyrus behaves in the same manner towards a Mysis. We have found under the carapace of a pagurus a female bopyrus full of eggs, so much flattened that it might have been taken for a leaf accidentally introduced into this cavity.
Fritz Müller has divided the Bopyridæ in the following manner:—
1. Those which fix themselves on the appendages or in the branchial cavity of decapods; these are the Bopyri, Iones, Phryxi, Gyges, Athelgi, &c.
2. Those which live in the thoracic cavity of some Brachyuri, as the Entoniscus.
3. Those which live in the cirrhipeds, like the Cryptoniscus, as well as the Liriopes.
4. Those which live on copepods as true parasites, as the Microniscus (M. Fuscus).
The Iones thoracicus, the Cepes distortus, the Gyges branchialis, and so many others live, like the Bopyri, in the thoracic cavity of different decapod crustaceans, and the females throw off at the same time their organs of sense and all their fishing and travelling apparatus.
Rathke, a learned professor of Königsberg, was the first to notice an isopod, known under the name of Phryxus paguri, which lives on the stomach of a pagurus, attached to it by its back, so that the stomach of the parasite is turned, like that of the pagurus, towards the partitions of the shell. The tail with the branchial appendages is always directed towards the orifice of the shell. The male is very small and never leaves the female. The Athelca cladophora is another bopyrian living on the abdominal region of a pagurus, which always chooses shells infested by Alcyonia. Another bopyrian, the Prosthetes cannelatus, lives on the abdomen of an ordinary pagurus.