Popular Science Monthly/Volume 14/February 1879/Mites, Ticks, and Other Acari
THE acari constitute a large order of minute animals, including mites, ticks, itch insects, etc, with which, in some of their forms, every one is more or less familiar, though, owing to their small size and obscure ways of living, but little is known of their structure and habits. They are often spoken of as insects, but are by scientific classification separated from the true insects, the most marked distinction being the possession of eight legs instead of six. Usually they are furnished with a suctorial apparatus. They are parasitic on both the animal and vegetable kingdoms. No class of animals is free from them. They prey upon each other; insects are infested with and support them at the cost of their lives; they attach themselves to fish and cold-blooded reptiles, and colonize on the whole range of birds and mammals. Some live under the skin, burrowing in the muscles; others dig corridors beneath the epidermis; while others, again, wander only on the surface. They are found in the lungs, air-passages, and intestinal canals of vertebrates. They are very prolific, and, though small, awkward, and slow-moving, they are transferred in a surprising way, often suddenly appearing about houses in enormous numbers, infesting plants and domesticated animals, or swarming on articles of food. Sometimes, especially in the tropics, they inflict considerable suffering; but it is probable that, on the whole, they are beneficial to man. A few directly injure him, and none directly benefit him, but indirectly they do him service by preying, as will be seen, on insect-pests and by acting as scavengers. The number of species in this family is so great as to forbid any detailed description, or even the merest mention of all of them. For the most part they are seldom seen except by those who make them a study. But a brief account of what is known about those of them that come into relations with man closely enough to affect his comfort and welfare, may be of interest and value.
The species thus referred to are found in the following classes: 1. Spinning and harvest mites, red spiders, etc. (Trombidiinæ); 2. Insect mite-parasites, sometimes called ticks (Gamasidæ); 3. True ticks (Ixodidæ); 4. Cheese and sugar mites, itch mites, etc. (Acaridæ)—the portion of the family commonly included under the name acari.
The spinning mites, like their relatives the spiders, can spin webs. They are of small size and semi-transparent. The most common is the red mite (Fig. 1), Tetranychus telarius, so well known to all who have the care of house plants. It is one twentieth of an inch long, yellowish, with two red spots on the sides, though its color varies with the plant on which it is found, and in different individuals on the same plant; a rusty color shows maturity. This mite, like most of the genus, spins a web on the back of the leaves so delicate that a single strand can not be seen even with the aid of a common magnifier. When the web-work is done, however, it is plainly visible,
|Fig. 2.—Trombidium parasiticum.|
|Fig. 1.—Tetranychus telarius—perfect insect, male.||Fig. 3.—Trombidium sericeum.|
and under it the mites congregate in numbers, and feed and multiply very fast. The leaves soon look sickly, are marbled with gray and yellow above, the underside being white and shiny, with recurved edges. A microscope will show hundreds of mites of all ages, together with unhatched eggs pasted to the web; they are draining the leaf-vessels of their sap, and choking the breathing-pores with excrement. The remedies used to get rid of them are various, but sulphur seems the most potent of them all. It is recommended to lay flowers of sulphur on the heating-pipes in the hot-house, or it may be mixed with soapsuds, and applied to the leaves. Plain soap and water is effectual if made to reach the insect, but a bent syringe or some other means must be employed to reach the underside of the leaves, otherwise the mites will remain in comfort and safety through a hard drenching. Luckily for the florist, they are preyed upon by other mites and insects—the grub or larvæ devouring whole colonies of them very quickly. The cucumber, cacti, vine, etc., as well as the shrubs and herbage of wood and field, support various species of this genus; they are all of similar habits while confined to vegetation, and to be got rid of by similar methods. But some of them transfer themselves to the skin with seeming ease, and when thus lodged cause great discomfort, producing itching and redness, and sometimes eruptions similar to those of the itch. Two species known in the Mississippi Valley as jiggers are very annoying to those who have to expose themselves where they abound. Prof. Riley names and describes them as follows: T. Americanus, a mite barely visible to the naked eye, moves readily, and is found more frequently upon children than upon adults. It lives mostly upon the scalp and. under the armpits, but is frequently found on other parts of the body. It does not bury itself in the flesh, but works its head under the skin, causing irritation, followed by a red pimple. T. irritans, the best known and most troublesome of the two, causes intense irritation and swelling on all parts of the body, but more especially upon the legs and around the ankles. By the aid of its strong jaws, the lower pair of which are elbowed, it buries itself completely in the flesh, resulting in a red swelling, with a pustule containing watery matter. If the mite be not removed the irritation lasts several days, the pustule refilling as often as it is broken. It is seldom noticed, being so small, and the uninitiated are often alarmed at the symptoms. Saleratus water or salt water will allay the irritation, but sulphur ointment effects a speedy cure.
Another red mite, belonging to a different genus, makes itself useful by preying on insects. One species, Trombidium parasiticum (Fig. 2), is found on the house-fly, which in some seasons is so infested that hardly one can be caught that is not loaded with mites clinging to the base of its wings. This or closely related species are found on spiders and mosquitoes, which they help to kill off. One species Dr. Packard found doing good work in eating the plant-lice off his rose-bushes; another attack the Rocky Mountain locust, the grasshopper scourge of the West; they attach themselves to the body under the wings, and suck it to a dry shell; while still another species, T. sericeum (Fig. 3), creep in great numbers into the holes where the locusts' eggs are and eat them.
The Gamasidæ are for the most part parasitic on insects, as the beetle, humblebee, wasp, etc., though the kinds here noticed are found on warm-blooded animals. They are sometimes called ticks, and are closely allied to that family. The mite shown in Fig. 4, Dermanyssus avium, infests domestic poultry, canaries, and other cage-birds, living in henneries and aviaries, and ready, like many of its congeners, to migrate to persons the first opportunity. They are small, white, and quite agile. A case is related in which these mites settled upon the skin of a woman otherwise healthy. She was constantly infested with little, louse-like animals, which were supposed to be bred on her body, as the greatest cleanliness failed to extirpate them. It was at last found that she went several times a day into a cellar over which the hen-roost was located. As often as she passed the fowls flew up to their roosts, and by this means sprinkled the woman with mites. The removal of the hen-roosts cured her. Fowls, when allowed to roost in stables, sometimes convey these mites to horses, torturing the poor brutes nearly to death, the owner being unable to decide what malady afflicts them. The mite is easiest served in the cages of singing birds. That they feed upon the bird is shown by their digestive organs being full of blood. They harbor in the recesses of the cage and the perches, particularly those made of hollow cane, and sally out at night to settle on the sleeping bird.
|Fig. 4.—Dermanyssus avium.||Fig. 5.—Tyroglyphus siro.|
Scrupulous cleanliness should be exercised to prevent and remove them. Several members of this family are parasitic on cats. One species is found on dead bodies. A case has been cited where one was observed on the brain of a dead soldier, which had just been opened. Although the observer thought that the mite had been domiciled there, and upon the strength of this case the statement is often made that they are sometimes found in the brain, it is probable that the mite was in some way conveyed to the brain after it had been opened.
The Ixodes or true ticks, are known by the tough, leathery skin of their abdomen and legs, their fastening on warm-blooded animals, and sucking their blood, for which they have a special mouth-piece. The body of the female is capable of great distention as she gorges. Their habit is herbivorous at first, for it is from the herbage that they find their way to the creatures on which they fix. They mount to the summit of blades of grass, or tips of leaves, and, holding on by their forelegs only, stretch the other three pairs out so as to fasten on any animal that comes in their way. Once settled, they plunge their proboscides into the skin, and suck the blood until their flat bodies become of a globular form, varying from the size of a number-eight shot to that of a bean. The process of filling is slow, sometimes taking days. Little irritation is felt, but the proboscis is liable to be broken off and remain in the wound, when serious sores are formed. Care should be used in taking them off; a little tobacco-juice will generally make them let go. Dogs after running in the woods often bring home quantities of them, and they may come to be permanently located in the dog-kennels. The common cattle-tick of the West is very annoying to horned cattle. Various troublesome species abound in the tropics, and these exotic tick-mites are not infrequently imported on plants, and in the moss and sphagnum which horticulturists use in packing their wares. The habits of this group are somewhat anomalous; at first they are unquestionably herbivorous, but they greedily avail themselves of every opportunity of sucking blood instead of sap. No special adaptation is needed for this, as the sucking-apparatus is suited to both the liquid blood and the juice of plants, and is similar to that of all suctorial insects, which some use on plants, some on animals, and some on both. Of the immense swarms of ticks, gnats, mosquitoes, bed-bugs, etc., the majority probably never taste animal food at all, although so greedy for it when it comes in their way.
The sub-family, Acaridæ, is divided into two sections: the cheese-mites and their allies, and the itch-mites with their relatives. The first section produces a number of vegetable-feeders; several species are found on the scales of some species of Liliaceæ, principally hyacinths, and on dahlia-roots, cyclamen, potatoes, mushrooms, etc.; these occasionally cause itching and irritation to those handling them. The smallest species of this group, Tyroglyphus entomophagus, is a pest too well known to entomologists, upon whose collections it preys. It lives chiefly in the inside of the insects which it attacks, gnawing the soft parts of their bodies, and destroying the ligaments which hold the articulations together, allowing them to fall apart.
Edible mushrooms, especially the cultivated sorts, are often attacked by a moist black rot, which, until lately, has been regarded as spontaneous. But it has been shown that it is caused, or at least aided, by a species of mite remarkable for fecundity. The activity of their agency in this decomposition is shown by the fact that mushrooms on which they have been placed become, in forty-eight hours, a black, putrescent mass, on which myriads of these creatures swarm; while other mushrooms subjected to the same conditions, except the inoculation, dry up, and take from eight days to a fortnight to decompose. The presence of mites in great numbers on some of the common articles of food is well known. These are chiefly the cheese-mites, which are characterized by a soft, smooth, fleshy, whitish body, with generally a single claw, surrounded by a vesicle or fine sucker, like a sieve. T. siro (Fig. 5) is the principal mite which commits ravages upon cheese, living upon all kinds when a little decayed, and especially the harder parts. They hibernate during winter, crowded in heaps in chinks and hollows in the cheese; but with warm weather their activity begins, and they gnaw away, reducing the cheese to powder. This powder is composed of excrement in little grayish balls, eggs, egg-shells, larvae, cast skins, perfect mites, fragments of cheese, and numerous spores of microscopic fungi. A mite-tainted cheese is not objected to by epicures, and the creatures are sometimes introduced to give a premature ripeness; but, if left to work unchecked, they will spoil a cheese very soon.
Linnæus described a mite under the name Acarus dysenteriæ, to the presence of which was ascribed a persistent dysentery that attacked one of his pupils, and some later authors have thought it may have been the cheese-mite, or some of its allies; but this is improbable. Cheese-mites are constantly eaten in great quantities, and, so far as can be judged, with perfect impunity; if they did cause mischief, numerous cases would have been noted and reported, whereas there are none recorded. A mite is also found in flour, sometimes in abundance; Hassell, however, says it is never present unless the flour is damaged. Assuming it to be a separate and distinctive species, it was described by Linnæus and named A. farinæ, and has been so figured and spoken of by naturalists down to the present time. But Mr. Andrew Murray, whose excellent hand-book on "Economic Entomology" is freely used in this article, asserts that the flour-mite and the milk-mite are not distinct species, but are identical with the cheese-mite. Not, he explains, that every species found on cheese or flour is this species—for both are doubtless infested with others—but that the old authors have made two or three species out of one. The cheese-mite has been met with in very old linseed-meal, and has been found on wounds that had been dressed with poultices made of linseed-meal. Of course, its presence under such circumstances must be mischievous. Another species which lives on cheese, T. longior (Fig. 6), is distinguished from the above by its more rapid movements, larger size, and longer and rounder body. Its habits and diet are much the same, though they are not found, together.
It is the species most commonly met with in stores of cantharides, which are very subject to attacks of mites. This species, Mr. Murray says, gave rise about 1837 to a good deal of talk among scientific people, as having been supposed to be produced by electricity. There was at that time in the semi-scientific world a vague idea in favor of electricity being the source of many of the phenomena of life. The limits and extent of its power were, of course, even less known than at present, and all sorts of wild experiments were tried. One gentleman set up lines of electrical wires over portions of his estate, with a view of ascertaining whether the plants would not thrive better under what he supposed would be an increased flow of electricity. Others made similar experiments in different directions. One gentleman, Mr. Cross, tried to produce organic beings by the aid of electrical apparatus. His process was to operate on volcanic stone kept moist by a solution of silicate of potash and muriatic acid, constantly subjected to electricity. After carrying this on for some time, he was rewarded by finding some of these mites wandering about his apparatus, and arrived at the conclusion that they had been produced by his electrical batteries. The species, therefore, enjoyed an ephemeral fame as a human creation. It was sent to M. Turpin, in Paris; having but a single dead specimen to work from, he believed it to be a new species, but by no means endorsed the idea that it was created by Mr. Cross or his electrical apparatus. It was not only a highly organized animal, and nearly allied to a well-known species, but it proved to be a female, containing eggs, which, as he dryly remarked, seemed an unnecessary complication in a new creation.
|Fig. 6.—Tyroglyphus longior.||Fig. 7.—Sugar-Mite (Tyroglyphus sacchari).|
The sugar-mite, T. sacchari (Fig. 7), is most commonly found in brown sugars. It is large enough to be seen with the naked eye, and sometimes appears as white specks in the sugar. It may be detected by dissolving two or three spoonfuls of sugar in warm water, and allowing the solution to stand for an hour or so; at the end of the time the acari will be found floating on the surface, adhering to the sides of the glass, and lying mixed with the grit and. dirt that always accumulate at the bottom. In ten grains of sugar as many as 500 mites have been found, which is at the rate of 350,000 to the pound. Those who are engaged in handling raw sugars are subject to an eruption known as "grocers' itch," which is doubtless to be traced to the presence of these mites. They are almost invariably present in unrefined sugars, and. may be seen in all stages of growth and in every condition, alive and dead, entire or broken in fragments. Refined sugars are free from them; this is in part due, perhaps, to the crystals being so hard as to resist their jaws, but principally to the absence of albumen, for without nitrogenous matter they can not live. The sugar-mite is also found on the surface of jellies and preserves that have begun to dry, and on the sugar of dried fruit, such as figs, prunes, cherries, etc. They sometimes accumulate in the mouth of wooden taps used for wine and beer. In dismissing this part of the subject, which refers to mites infesting food, it seems proper to say that the common assertion that living forms are a matter of course in everything that we eat and drink is without warrant. While it is true that water and one or two other articles usually contain microscopic organisms, the idea commonly conveyed by such statements is erroneous. These degraded and disgusting forms are not proper food-stuff; nor is their consumption unavoidable. Pure articles, in an undamaged condition, do not contain them; and their presence in numbers in any article of food is proof that it is unfit for human use, and should be rejected.
There are several species, placed by later authors in the same genus with the sugar-mites, whose normal habitat seems to be on food, insect collections, and in the dust and mold of cellars and damp places, but which, when transferred to animal bodies, become parasitic, causing curious and painful disorders. Signor Moriggia figures a singular horny excrescence which grew from the hand of a lady. It was nearly eight inches long, tapering upward from a wide base, and curved toward the wrist. Its cavities were swarming with a species of acari.
|Fig. 8.—Cheyletus eruditus.|
Another species was found by Hering in the hind feet of a horse, that, although young and in other respects sound, had to be killed. The hoofs were quite disorganized, the frog and sole consisting of a soft, fibrous mass, secreting an offensive liquid; in the end, the sore spread to the flexors and muscles of the fetlock. A negro inmate of the Seaman's Hospital, London, suffered from large and peculiar sores on the soles of the feet. Examination by Prof. Busk revealed the presence of a mite, which was doubtless the cause of the trouble. The negro attributed the disease to wearing a pair of shoes which were lent for a few days to another negro who was similarly afflicted. The latter came from Sierra Leone, and inquiry proved the existence of a pustular disease, native to that place, called craw-craw, a species of itch very troublesome to cure.
The genus Cheyletus is a very remarkable type. It is unquestionably carnivorous, its palpi being adapted for holding its prey. They are not gregarious in their habits; but, like all animals that live by rapine, are solitary. When placed in company with the cheese-mites, they seize them between their palpi, and, plunging their beaks into the body, suck up the juices. The C. eruditus (Fig. 8) appears also to have poisoning powers similar to those of the spider. They seize their victim by the leg or any other part, who, after the lapse of fifteen or twenty seconds, becomes paralyzed, makes no resistance to the devourer, and remains passive till nothing is left but an empty and shrunken shell. The poison has no effect on its own species: they frequently feed on each other; but in that case the prey struggles so long as any fluid seems to be left in the body. It is found in the dust of hay, old grain, meal, and flour, on collections, and also on books, from which latter it derives its name; but, in the opinion of Mr. Murray, it is so found because other gregarious acari congregate there, and "we should no more think that it was there for the purpose of feeding on those stuff's in which it is found than we should admit that a cat eats hay because it is found in a rat-infested hay-stack."
The occurrence of the phenomena of parthenogenesis among acarids has been shown by this species. Mr. Beck succeeded in rearing three successive generations from a female without any intervention of the male. The female is very careful of her eggs: she lays them in a heap, and rests brooding over them, guarding them from attack.
The itch-mites, Sarcoptes, infest the larger mammals. The commonest species, S. scabiei, which preys on man, when seen with the naked eye, looks like a. white, shining globe, or a little bladder of
|Sarcoptes scabiei— Male.||Sarcoptes scabiei— Female.|
water, 75 of an inch long and 100 of an inch wide. The microscope brings to view (Fig. 9) a round, flat body, with a head not unlike that of a tortoise, provided with powerful jaws and nippers, the structure of which is well adapted to mining the skin. The body is set with strong, short legs, terminating in long spines (Figs. 10 and 11). The feet and nippers are provided with suckers. They have no eyes; but, with their habits of life, the absence of these organs causes them no inconvenience. The impregnated female digs a long burrow in the skin, depositing her eggs as she moves. If one of the early vesicles of the itch is examined, a small spot may be seen upon some point of the surface; this is the opening made by the mite to begin its tunnel; leading from this, a white, fluted line may be traced, which is the cuniculus or burrow of the acarus, and the fluted or dotted appearance is due to the eggs, the white dots showing where they lie (Fig. 12).
|Fig. 10.—Two of the Posterior Legs of the Male Sarcoptes scabiei.||Fig. 11.—Two of the Posterior Legs of the Female Sarcoptes scabiei.|
The burrow varies in length, sometimes reaching half an inch, and at its end, under a slight elevation, lies the mite. The cut shows a burrow with a number of empty shells scattered along the line, with one unhatched egg close to the mite. Close inspection will reveal a small, dark point at the end of the burrow; and, if the skin be raised there with the point of a needle, the creature can be brought to view and easily extracted. The itch-mite is never found in a vesicle or pustule; indeed, there is no connection between the later vesicles and the burrow; they are evidently caused by the proximity of the mite. The larvae and young females hide themselves in broken surfaces, or burrow a short distance into the skin; the male retreats under any protecting edge. The advance of civilization, with its increased use of soap and water, has done much toward exterminating this pest; the "Jackson itch" and "seven-year itch" are much less heard of than formerly; but wherever, as in army life, men are crowded together, and personal cleanliness is neglected, it reappears in a most flourishing condition. The disease must, of course, be conveyed by transference of the mite from those who are infected to those who are sound; but the mode of transmission has been a puzzle, it being observed that among doctors and hospital attendants infection was comparatively rare. This is explained by the nocturnal habits of the mite—or, if not truly nocturnal, its activity is promoted by a certain degree
of warmth, so that it lies dormant during the day, when the body is cool, to sally forth in search of new fields when stimulated by the genial warmth of the bed. This explains also why the itching is most violent at night. The mode of getting rid of the parasite is obvious; we have only to kill it; and, fortunately, the means is cheap, easily applied, and perfectly effectual: sulphur, which seems to be a deadly bane to all insects, in whatever shape it can be given. It is only necessary to expose the insect to its influence, and it surely dies.
One curious point regarding these mites is, how they produce the physiological effects that characterize their attacks. Their incessant gnawings and nibblings are undoubtedly sufficient to cause great irritation; but there are the different symptoms induced by different species—scurfy, inflamed surfaces, coarse, leprous crusts, deep ulcers, etc.; can these be accounted for by degrees of mechanical irritation, or are they possibly the result of some special poisonous virus? The itch-mite is found on the lion, dog, sheep, ox, horse, pig, and other animals, as well as on man. There are several other species, as the S. scabei-crustosœ, or Norwegian itch-mite, most common in Norway, but not wholly confined to that country. The mite is much like the common species, but smaller and darker. The malady is similar to that of the common itch, though all the symptoms are intensified, the tubercles and crusts growing enormously. It yields, however, to cleanliness and the sulphur treatment. S. cati infests the domestic cat, the points of attack being the base of the nose, the lips, the ears, and the eyes, but spreading in all directions; the animal is reduced to a pitiable condition, being literally devoured alive by the parasites, if not destroyed before this extreme stage is reached. This complaint is chiefly seen in great towns. S. mutans (Fig. 13) is parasitic on birds and
|Fig. 13.—Sarcoptes mutans—Male, underside.|
domestic fowls, appearing on the feet, on the comb, and about the beak, gradually spreading until the whole body is involved. Other species have been found on other domesticated and wild animals, while a large genus—the louse-mites, Myobia—infest the smaller mammals, as rats, mice, cats, etc.
The Demodex folliculorum is a minute animal, 100 to 50 of an inch in length, that lives in sebaceous sacs and hair follicles of the human skin, particularly about the nose (Fig. 14). Its habits are in some respects similar to those of the itch-mites. It is not a normal inhabitant of the follicles or glands, but appears to enter them from without. If pressure be made upon any one of the sebaceous follicles that are enlarged and whitish, with a terminal black spot, matter will be forced out, consisting mostly of accumulated
sebaceous secretions, in which the parasite, if present, will be imbedded together with its eggs and young. The secretion may be softened with oil, so that these may be separated and the animal removed with a pointed brush. They do not seem to be present in all persons, occurring in two or three cases out of ten, and seeming to prefer thick, greasy skins. They are entirely harmless, and their presence is no indication of disease. The family roll of the acari closes with the gall-mites, Phytoptidæ, a curious and little-known group that causes the abnormal growths known as galls on the leaves and other parts of various plants. Any extended reference to them here would be out of place. The whole of the order Acarina afford interesting objects of investigation, being in close relations to man, and yet very imperfectly known. Some of their forms are always attainable, and an examination of them under the lower powers of the microscope is one of the most amusing objects of study that can be presented to the young.