CHAPTER XVIII

ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ACARINA—HARVEST-BUGS—PARASITIC MITES—TICKS—SPINNING MITES—STRUCTURE—METAMORPHOSIS—CLASSIFICATION

=Order IX. Acarina (Acari, Acaridea).=

Arachnids with unsegmented, non-pediculated abdomen. Respiration by tracheae, or by the general surface of the body. Month parts suctorial, but frequently capable of biting or piercing. Metamorphosis always observable.

The Acarina or Mites are remarkable not so much for the number of their species, which is very considerable, as for the vast multitude of individuals of the Order, which is far in excess of that of any other Arachnid group. This fact is correlated with their minute size. Very few Mites exceed half an inch in length, while very many are microscopic creatures, often measuring less than the hundredth of an inch. Taken all round, a millimetre may be considered a large size for a Mite.

There is much variety of habit within the Order. All Mites live principally on fluid nutriment, but it may be obtained from living animals or plants or from decaying organic matter. Some are entirely parasitic upon plants or animals; others attach themselves to animals in their larval stage, but are free when adults; while others, again, live an entirely independent and predaceous life.

The greater number of the Mites are too small to strike the eye. Some of them have, however, contrived to attract attention, in no very agreeable manner. Every one knows the Mite popularly called the “Harvest-bug,” but to this day there is some uncertainty as to its identity. It was described as a separate species under the name of Leptus autumnalis, and Mégnin was the first to show that it was the larval form of one of the Trombidiidae (see p. 472). Most authors have considered it the larva of Trombidium holosericeum, but Murray referred it to the genus Tetranychus. The difficulty is that the minute creature cannot be removed from its victim without such injury as to prevent it from being bred out and the mature form determined. Brucker has recently compared a large number of “Harvest-bugs” taken from human beings with the figures and descriptions of the larvae of certain Trombidiidae given by Henking and Berlese, and he determined them as the larvae of T. gynopterorum. Quite possibly, however, more than one genus is concerned in the production of this pest.

That certain skin-diseases are due to Mites (Demodicidae, Sarcoptidae) is a fact which is widely known. The fruit-grower, too, has to take cognisance of the Order, for his trees may suffer from “Red-spider” (Tetranychus telarius), and his black-currant bushes fail under the attack of the “Gall-mite” (Eriophyes or Phytoptus ribis). The curious swellings or galls which disfigure the leaves of many trees are sometimes of insect origin, but they are often due to Mites.

Domestic pets suffer greatly from Acarine parasites. A large number of species confine their attention exclusively to the feathers of birds (Analges, etc., see p. 466). One curious genus, Syringophilus, is parasitic within the feathers, feeding upon the pith of the quill. Heller of Kiel discovered them in 1879, but the researches of Trouessart first showed their frequent presence and very wide distribution. He found that they entered by the superior umbilicus of the feather, and disappeared by the inferior umbilicus when the feathers moulted or the infested bird died.

It is probable that the comparatively large Mites of the group IXODOIDEA (see p. 468), commonly called “Ticks,” are the most widely known of the order. They attack wild and domestic animals and man, and are nearly always acquired from vegetables, such as brush or herbage. It would seem likely that many of these creatures can never have the chance of attaching themselves to animals, and it has been suggested that animal juices are a luxury but not a necessity to them, and that they can live, if need be, on vegetable sap, but further investigations have quite dispelled this view.

The suspected connection between the North American Tick, Boophilus annulatus, and the cattle disease known as Texas fever or “red water,” since clearly proved by the researches of Smith and Kilborne, led to the careful investigation of the life-history of that creature, and this was undertaken by Curtice.

The female Ticks laid eggs a few days after dropping off the cattle, egg-laying lasting a week or more. The eggs took from three to five weeks to hatch, and the larvae attached themselves to cattle, on which they remained a fortnight, becoming mature and fertilised before they again sought the ground. The whole cycle occupied a time varying from six to ten weeks, a period apparently much exceeded by some members of the family.

Lounsbury has recently made out the life-history of the South African “Bont” tick, Amblyomma hebraeum.

The eggs are deposited in the soil, ten to twenty thousand eggs in all being laid by one female. The larvae climb neighbouring plants and seize passing animals. After the third day of attachment they begin to distend, and they generally fall off, fully distended, on the sixth day, immediately seeking a place of concealment, where they become torpid. Under natural conditions the nymph does not emerge for at least eleven weeks, and then it behaves in the same way as the larva, again attaching itself to an animal for six days. A new time of torpidity and concealment ensues, again of at least eleven weeks’ duration, when the final moult takes place and the mature tick emerges. The males at once attach themselves to animals, but the females hesitate to fix themselves, except close by a male. For four days after fixation the male appears to exercise no attraction for the female, but after that period he shows great excitement at her approach. She, however, does all the courting, the male remaining fixed in the skin of the host. After pairing, the female distends greatly, attaining her maximum size (nearly one inch in length) in about a week, when she lets go and descends to the earth to lay eggs. If unmated, she detaches herself within a week, and seeks another host. Oviposition lasts from three to nine weeks, and the development of the egg from eleven weeks to six months. At least a year is occupied in the whole cycle. These ticks, and many others, communicate disease by inoculation, conveying it from one animal to another.

No poison-glands have been demonstrated in the Acari, the function of the salivary glands of the Ticks being probably to prevent the coagulation of the blood of their victims.

It is an important point in the mode of life of the Ticks that they can live for a long time without food. Mégnin states that he kept an Argas alive for four years, entirely without nutriment.

In the Tetranychinae (see p. 472), glands apparently homologous with the salivary glands of the Ticks have taken on the function of spinning organs. According to Donnadieu, these glands, which resemble bunches of grapes, and are possessed by both sexes, open into the buccal cavity at the base of the chelicerae. The gummy fluid exudes from the mouth, and is combed into threads by the pedipalps. The legs of these mites are furnished terminally with curious hairs ending in a round knob, which are supposed to have some relation to their spinning habits.

The males are the busiest spinners, the time of the females being largely occupied in laying eggs among the excessively fine threads of silk with which the Mites cover the under surface of leaves. In the Eriophyidae (see p. 464) corresponding glands are thought to furnish an irritant fluid which causes abnormal growths or galls upon vegetable tissues.

=External Structure.=—It is often stated, but erroneously, that there is no distinction between cephalothorax and abdomen in the Mites. Certainly no such division can be made out in the Hydrachnidae (see p. 472) or in some other forms, but in the majority of Acari the cephalothorax is clearly marked off by a transverse groove or suture. In some cases the anterior portion of the cephalothorax is movably articulated with the rest, and forms a sort of false head called a “capitulum.” In most Mites the chitinous integument is soft and non-resistant, but it is otherwise with the Oribatidae or “Beetle-mites” (see p. 467), which are nearly all covered by an extremely hard and coriaceous armature.

Eyes are sometimes absent, sometimes present in varying numbers. They seem here to be of remarkably little systematic importance, as otherwise closely allied species may be either eyed or eyeless.

Normally Mites possess the usual Arachnid appendages, chelicerae, pedipalpi, and four pairs of ambulatory legs. The anterior appendages are, however, subject to a very great degree of modification, while in one Family, the Eriophyidae (Phytoptidae), the legs are apparently reduced to two pairs.

The chelicerae are sometimes chelate, in which case they are two-jointed, the distal joint or movable finger being always articulated below the immovable finger. Sometimes they terminate in a single claw or blade, the movable joint being obsolete. In the Ticks they exist as two long styles or piercing weapons, serrate on the outer edge.

The pedipalpi vary very much in structure, according to the habits of the particular form to which they belong. In the Sarcoptidae (see p. 466) they are hardly recognisable owing to the extent to which they have coalesced with the maxillary plate. In many of the free-living forms they are leg-like feeling organs, but in others they are raptorial, being not precisely chelate, but terminating in a “finger-and-thumb” arrangement which is of use in holding prey. The extreme development of the raptorial palp is found in Cheyletus (see p. 473), in which the whole appendage is remarkably thick and strong, and the “finger” is a powerful chitinous claw, while the “thumb” is replaced by movable pectinated spines of chitin. The Water-mites have a palpus adapted for anchoring themselves to water-weeds, the last joint being articulated terminally with the penultimate joint, and bending down upon it. Finally, in the “Snouted mites” (Bdellidae, see p. 471) the palpi are tactile or antenniform, often strongly recalling the antennae of weevils.

The maxillary plates which arise from the basal joints of the pedipalps are always more or less fused, in the Mites, to form a single median transverse plate, constituting the lower lip or “labium” of some authors. In some of the Oribatidae the fusion of the maxillae is only complete at the base, and the free points are still of some use as masticating organs. In those free living Mites which have undergone no great modification of the mouth-parts two other portions can be distinguished, the upper lip or “epipharynx,” and the “lingua,” which forms the floor of the mouth, and is for the most part concealed by the maxillary plate.

The legs are usually six- or seven-jointed, and are subject to great variation, especially as regards the tarsus or terminal joint. This may bear claws (1–3) or sucking disks, or a combination of the two, or may simply take the form of a long bristle or hair.

The Cheese-mite has a claw surrounded by a sucker—like Captain Cuttle’s hook within his sleeve. The claws of those species which are parasitic on the hairs of animals are sometimes most remarkably modified.

FIG. 239.—Diagram of the viscera of an Oribatid Mite, greatly enlarged. C, C, Lateral caeca of stomach; g, cerebral ganglion; od, od, oviducts; oe, oesophagus; pr.g, pro-ventricular gland; ps, pseudo-stigmatic organ; st, stomach; tr, tr, tracheae. (Partly after Michael.) ]

=Internal Structure.=—The minute size of most Mites has rendered research upon their internal structure a matter of great difficulty, and there are still many obscurities to be removed. Those forms which have been subjected to examination present a tolerable uniformity in the structure of the principal organs, but the brief description here given will not, of course, apply to aberrant groups like the Vermiformia. A marked concentration is noticeable throughout the Order, and is best exemplified by the nervous system.

The mouth leads into a sucking pharynx, which narrows to form the oesophagus. This passes through the nerve-mass in the usual Arachnid fashion, and widens to form the ventriculus or stomach. The oesophagus varies considerably in width in the various groups, being very narrow in those Mites which merely suck blood, but wider in vegetable-feeders like the Oribatidae.

The stomach is always provided with caeca, but these are not nearly so numerous as in some other Orders of Arachnida. There are always two large caeca directed backwards, and there may be others. They are most numerous in the Gamasidae (see p. 470), which sometimes possess eight, some being prolonged into the coxae of the legs, as in Spiders. At the sides of the anterior part of the stomach there are usually two glandular bodies, the pro-ventricular glands. In those Mites in which the alimentary canal is most differentiated (e.g. Oribatidae) three parts are distinguishable behind the stomach, a small intestine, a colon, and a rectum, but in most groups the small intestine is practically absent. The Malpighian tubes, very variable in length, enter at the constriction between colon and rectum.

In some of the Trombidiidae there appears to be a doubt as to the existence of a hind-gut at all. A body having the appearance of the hind-gut, and occupying its usual position, is found to contain, not faecal matter, but a white excretory substance, and all efforts to discover any passage into it from the stomach have been unsuccessful. Both Croneberg and Henking came to the conclusion that the stomach ended blindly, and that the apparent hind-gut was an excretory organ. Michael, in his research upon a Water-mite, Thyas petrophilus, met with precisely the same difficulty, and was led to the belief that what was originally hind-gut had become principally or entirely an excretory organ.

The nervous system chiefly consists of a central ganglionic mass, usually transversely oval, and presenting little or no indication of the parts which have coalesced in its formation. Nerves proceed from it in a radiate manner, but no double nerve-cord passes towards the posterior end of the body. As above stated, it is perforated by the oesophagus.

The vascular system is little understood. In 1876 Kramer wrote that he was able to perceive an actively pulsating heart in the posterior third of the abdomen in specimens of Gamasus which had recently moulted, and were therefore moderately transparent. No other investigator has been equally fortunate, though many capable workers have sought diligently for any trace of a dorsal vessel in various Acarine groups.

It would appear that the blood-flow in most Mites is lacunar and indefinite, aided incidentally by the movements of the muscles, and perhaps by a certain rhythmic motion of the alimentary canal, which has been observed to be most marked during the more quiescent stages of the life-history.

The internal reproductive organs have the ringed arrangement generally observed in the Arachnida. The two testes, which are sometimes bi-lobed, are connected by a median structure which may serve as a vesicula seminalis, and there are two vasa deferentia which proceed to the intromittent organ, which is sometimes situated quite in the anterior part of the ventral surface, but at others towards its centre. The male Mite is often provided with a pair of suckers towards the posterior end of the abdomen, and sometimes accessory clasping organs are present.

In some Mites there is no intromittent organ, and Michael has described some remarkable cases in which the chelicerae are used in the fertilisation of the female, a spermatophore, or bag containing spermatozoa, being removed by them from the male opening and deposited in that of the female. The most remarkable instance is that of Gamasus terribilis, the movable joint of whose chelicera is perforated by a foramen through which the spermatophore is, so to speak, blown and carried as a bi-lobed bag, united by the narrow stalk which passes through the foramen, to the female aperture.

The ovaries are fused in the middle line, and connected by oviducts with the tube (vagina or uterus) which passes to the exterior. There is often an ovipositor.

Professor Gené of Turin described, in 1844, some remarkable phenomena in connection with the reproduction of Ticks. The male Ixodes introduced his rostrum into the female aperture, two small white fusiform bodies emerging right and left from the labium at the moment of introduction. On retraction they had disappeared. When the female laid eggs, a bi-lobed vesicle was protruded from beneath the anterior border of the scutum and grasped the egg delivered to it by the ovipositor, appearing to manipulate it for some minutes. Then the vesicle was withdrawn, and the egg was left on the rostrum, and deposited by it in front of the animal. When the vesicle was punctured, and so rendered useless, the unmanipulated eggs quickly shrivelled and dried up.

Lounsbury has recently confirmed Professor Gené’s observation as to oviposition in the case of a South African Tick, Amblyomma hebraeum.

The respiratory organs, if present, are always in the form of tracheae. These are usually long and convoluted, but not branching. The spiral structure is difficult to make out in these animals, and in the Oribatidae at least, instead of the external sheath being fortified with a spiral filament of chitin, there is a very delicate enveloping membrane with an apparently unbroken chitinous lining, which can, however, by suitable treatment, be resolved into a ribbon-like spiral band. The position of the stigmata is very variable, and is utilised to indicate the main groups into which the Mites have been divided.

The Oribatidae possess two curious cephalothoracic organs which were for a long time considered respiratory. These are in the form of two bodies, like modified hairs, which protrude from sockets on the dorsal surface of the cephalothoracic shield. Michael has shown that these have no connection with the tracheae, and he regards them as sensory organs— possibly olfactory. They are generally referred to as the “pseudo-stigmatic” organs.

In the Oribatidae, at all events, well-developed coxal glands are present. In many Mites, especially the Ixodoidea or Ticks, the salivary glands are large and conspicuous.

=Metamorphosis.=—All Mites undergo a metamorphosis, varying in completeness in the different groups. Altogether six stages can be recognised, though they are seldom or never all exhibited in the development of a single species. These are ovum, deutovum, larva, nymph, hypopial stage, and imago.

THE OVUM.—All Mites lay eggs. It is frequently stated that the Oribatidae are viviparous exceptions, but though some of them are perhaps ovoviviparous, most deposit eggs like the rest of the Order. A phenomenon which has probably helped to foster this erroneous view is the occasional emergence from the dead body of the mother of fully-formed larvae. Towards winter it is not unusual for the mother to die at a time when her abdomen contains a few ripe eggs, and these are able to complete their development internally.

THE DEUTOVUM.—In a few cases (Atax, Damaeus) a stage has been observed in which the outer envelope of the egg becomes brown and hard, and splits longitudinally, so as to allow the thin inner membrane to become visible through the fissure. More room is thus obtained for the developing larva, which is, moreover, protected, over most of its surface, by a hard shell. The deutovum stage may occur either within the body of the mother, or after the egg has been laid.

THE LARVA.—Omitting, for the moment, the very aberrant Vermiformia (see p. 464), it is the almost universal rule for the egg to hatch out as a hexapod larva. The larvae of the genus Pteroptus are said to be eight-legged. Winkler has stated that the early embryo of Gamasus possesses eight legs, of which the last pair subsequently atrophy, but this observation requires confirmation.

THE NYMPH.—The nymph-stage commences on the acquisition of eight legs, and lasts until the final ecdysis which produces the imago. This is the most important period of Acarine life, and is divided into a prolonged active period, during which the animal feeds and grows, and an inert period, sometimes prolonged, but at others very short, and differing little from the quiescence observable at an ordinary moult, during which the imago is elaborated. In many species the nymph is strikingly different from the imago; in others there is a close resemblance between them. It would appear, from the cases which have been most thoroughly investigated, that the imago is not developed, part for part, from the nymph, but that there is an “histolysis” and “histogenesis” similar to that which occurs among certain insects (see vol. v. p. 165). There may be more than one nymphal stage.

THE HYPOPIAL STAGE occurs in the Tyroglyphinae, the “Cheese-mite” sub-family. Here some of the young nymphs assume an entirely different form, so different that it was for a long time considered to constitute a separate genus, and was named Hypopus. The animal acquires a hard dorsal covering. The mouth-parts are in the form of a flat blade with two terminal bristles, but with no discernible orifice. The legs are single-clawed, and all more or less directed forward, and they are articulated near the middle line of the ventral surface. Suckers are always present under the hind part of the abdomen.

It appears that these remarkably modified nymphs are entrusted with the wider distribution of the species, and that they are analogous to the winged individuals which occur in the parthenogenetic generations of the Aphidae. The ordinary Tyroglyphus is soft-bodied, and requires a moist environment, and exposure to the sun or prolonged passage through the air would be fatal to it. The hypopial form is much more independent of external conditions, and its habit is to attach itself by its suckers to various insects, and by this means to seek a new locality, when it resumes the ordinary nymph-form and proceeds with its development.

=Classification.=—There is no generally accepted classification of the Acarina, though several eminent Arachnologists have attempted of late years to reduce the group to order. Widely different views are held concerning the affinities of certain groups, and there is no agreement as to the value to be accorded to the characters which all recognise. Thus Canestrini allows thirty-four families, while according to Trouessart there are only ten.

Trouessart’s scheme of classification is in the main followed in the present chapter.

=Sub-Order 1. Vermiformia.=

This Sub-order includes the lowest and most aberrant forms of the Mites. They are entirely parasitic, and of very small size. The abdomen is much elongated, and is transversely striated. There are two families, Eriophyidae (Phytoptidae) and Demodicidae.

=Fam. 1. Eriophyidae (Phytoptidae).=—These are the so-called Gall-mites. The curious excrescences and abnormal growths which occur on the leaves and buds of plants are familiar to every one. Various creatures are responsible for these deformities, many being the work of insects, especially the Cynipidae among the Hymenoptera, and the Cecidomyiidae among the Diptera. Others, again, are due to Eriophyid Mites.

Though the galls originated by Mites are often outwardly extremely similar to those of insect origin, they can be at once distinguished on close examination. Mite-galls contain a single chamber, communicating with the exterior by a pore, usually guarded with hairs, and the Mites live gregariously within it, apparently feeding upon the hairs which grow abundantly on its inner surface. In Insect-galls each insect larva lives in a separate closed chamber.

FIG. 240.—Vermiform Mites, highly magnified. =A=, Demodex folliculorum; =B=, Eriophyes (Phyptoptus) ribis. ]

The Eriophyidae are unique among Mites in possessing only two pairs of legs, situated quite at the anterior part of the body. The mouth-parts are very simple.

There are three genera, Eriophyes (Phyptoptus) with about one hundred and fifty known species, Monochetus with a single species, and Phyllocoptes with about fifty species.

Among the best known examples are Eriophyes tiliae, which produces the “nail-galls” on lime-leaves, and E. ribis, the “black-currant Gall-mite,” which feeds between the folded leaves of the leaf-buds, and gives rise to swelling and distortion.

=Fam. 2. Demodicidae.=—The single genus Demodex which constitutes this family consists of a few species of microscopic Mites which inhabit the hair-follicles of mammals, and are the cause of what is known as “follicular mange,” some other forms of mange being due to members of the succeeding family. Demodex possesses eight short, three-jointed legs, each terminated by two claws. The abdomen is much produced, and is transversely striated. About ten species have been described, but of these five are probably varieties of D. folliculorum (Fig. 240, A), which infests Man.

=Sub-Order 2. Astigmata.=

The Astigmata are Mites of more or less globular form, with chelate chelicerae and five-jointed legs. All members of the group are eyeless. Their habits are very various, some feeding on vegetable matter and others on carrion, while a large number are parasitic on animals. Tracheae are absent. There is only one family.

FIG. 241.—=A=, Leg of a fowl infested with “leg-scab”; =B=, female of Sarcoptes mutans, greatly magnified. (After Neumann.) ]

=Fam. 1. Sarcoptidae.=—No tracheae or stigmata. Apical rostrum. Oviparous or ovoviviparous. The seventy genera and 530 odd species of this family are divided into a number of sub-families, of which the principal are the Sarcoptinae, the Analgesinae, and the Tyroglyphinae.

(i.) The Sarcoptinae are the so-called “Itch-mites.” They are minute animals, with bodies transversely wrinkled and legs terminating in suckers or bristles. The genus Sarcoptes, which includes about fifteen species, lives in tunnels which it burrows in the skin of mammals.

(ii.) The Analgesinae are the “Birds’-feather Mites.” The principal genera are Pterolichus (120 species), Pteronyssus (33 species), Analges (23 species), Megninia (42 species), and Alloptes (33 species).

(iii.) The Tyroglyphinae have received the popular name of “Cheese-mites,” from the best known example of the group. They are smooth-bodied, soft-skinned white Mites, with legs usually terminating in a single claw, sometimes accompanied by a sucker. They are for the most part carrion-feeders, living upon decaying animal or vegetable matter, but a few are parasitic on mammals, insects, and worms.

There are sixteen genera, including about fifty species. Tyroglyphus siro and T. longior are common Cheese-mites. Other species live in decaying vegetables and food-stuffs. Some of the genus Glycyphagus (G. palmifer, G. plumiger) are very remarkable for the palmate or plumose hairs which decorate their bodies. The remarkable hypopial stage in the development of Tyroglyphus has been mentioned on p. 463. The Tyroglyphinae are the lowest of the free-living Acarine forms.

=Sub-Order 3. Metastigmata.=

The four families which constitute this sub-order comprise a large number of Mites in which the tracheae open near the articulation of the legs, and consequently in a somewhat posterior situation. The families are Oribatidae, Argasidae, Ixodidae, and Gamasidae.

=Fam. 1. Oribatidae.=—The Oribatidae or “Beetle-mites” are free-living Acari, with tracheae of which the stigmata are concealed by the articulation of the legs. The cephalothorax is distinctly marked off from the abdomen, and bears dorsally two “pseudo-stigmatic” organs. The rostrum is inserted below the cephalothorax. These Mites gain their popular name from the beetle-like hardness of their integuments. They are oviparous or ovoviviparous. Eyes are always absent.

FIG. 242.—Oribatid Mites. =A=, Cepheus ocellatus, × 24; =B=, ventral view of Hoploderma magnum, closed, × 20. (After Michael.) ]

These are small creatures, seldom attaining the twentieth of an inch in length. They are vegetable-feeders (except, perhaps, Pelops), and are to be found in dead wood or vegetable débris, under bark, or among moss and lichen. In winter they often take refuge under stones. It is impossible at present to estimate the number of existing species, for only a few localities have been systematically worked for them, and their small size has prevented their inclusion, in any numbers, in the collections of scientific expeditions. Our knowledge of the group is likely, however, to be largely extended, for it has been found that they reach England alive and in good condition from the most remote regions if moss or other material in which they live is collected when not too dry, and hermetically sealed up in tin cases.

About twenty genera and more than 220 species are at present known. Pelops has much elongated chelicerae, with very small chelae at the end. There are ten species, found in moss and on bushes. Oribata numbers about fifty species, found in moss and on trees. Notaspis, in which the last three legs are inserted at the margin of the body, has about thirty species, found among moss and dead leaves. Nothrus is a short-legged genus with flat or concave dorsal plate, often produced into very remarkable spiny processes. There are twenty-two species found under bark and among moss and lichen. Hoploderma (Hoplophora) is remarkable for its power of shutting down its rostrum and withdrawing its legs in a manner which leaves it as unassailable as a tortoise or an armadillo.

Though the Oribatidae are all eyeless, they are distinctly sensitive to light, not wandering aimlessly till they reach a shadow, but apparently making straight for a dark spot when subjected to strong illumination. Some species have a curious habit of collecting dirt and débris on their backs, so as entirely to obscure the often very remarkable disposition of the spines and processes with which they are furnished.

FIG. 243.—Capitulum of Boophilus australis; ventral view. p^1, p^2, p^3, p^4, The four articles of the palp; m, the mandible or chelicera; d, its digit; n, the hypostome. ]

The next two families include the animals commonly known as Ticks, the largest and most familiar of the Mite tribe. Of recent years they have attracted much attention as the conveyers, to man and domestic animals, of certain diseases due to blood-parasites (see p. 457, n.), and our knowledge of their structure and habits has greatly increased in consequence. Hitherto they have generally been considered to constitute a single family, the Ixodidae, but a section of them so differ from the rest as to require their removal to another family, the Argasidae, so that it is necessary to employ a super-family name—IXODOIDEA—to embrace the whole group.

Ticks are parasitic on mammals, birds, and reptiles, some shewing a marked partiality for a particular host, others being much more catholic in their tastes. Both sexes in the Argasidae, but the females only of the Ixodidae, are capable of great distension, but when unfed they are all somewhat flat animals with laterally extended legs and rather crab-like movement.

All Ticks possess a small, movable “false head” or capitulum bearing mouth-parts which are exceedingly characteristic of the group. The chelicerae are cutting instruments with their distal ends serrated outwardly, and there is always present a hypostome beset with recurved teeth which serve to maintain a firm hold on the tissues into which it is thrust. On either side of the chelicerae are the four-jointed palps, leg-like in the Argasidae, but more rigid and rod-like in the Ixodidae, where their inner margin is often hollowed so as to enclose the chelicerae and hypostome when the palps are apposed. There is a conspicuous pair of spiracles near the coxae of the fourth pair of legs.

FIG. 244.—Ornithodoros talaje, under surface, × 5. (After Canestrini.) ]

=Fam. 2. Argasidae.=—The Argasidae are leathery Ticks without a shield or scutum, and with free, leg-like palps. The capitulum is never more than partially visible when the adult animal is viewed dorsally. Their hosts are always warm-blooded animals. Two genera are usually recognised, Argas and Ornithodoros, though recent discoveries of new forms have tended towards their fusion. Argas reflexus and A. persicus have been proved to convey a Spirochaete disease to fowls, and the latter, under the name of the “Mianeh Bug” has long possessed an evil reputation for the “poisonous” effect of its bite on human beings. In Mexico the “Turicata” (Ornithodoros turicata) and the “Garapata” (O. megnini) are greatly dreaded, while human “tick fever” on the Congo has been traced to the instrumentality of O. moubata.

FIG. 245.—Female Sheep-tick, Ixodes ricinus. ]

=Fam. 3. Ixodidae.=—These are the more familiar Ticks, possessing a scutum or shield, which covers the whole back of the male, which is capable, therefore, of little distension, whereas it forms only a small patch on the front part of the body of the distended female. There are ten genera, Ixodes, Haemaphysalis, Dermacentor, Rhipicentor, Rhipicephalus, Boophilus, Margaropus, Hyalomma, Amblyomma, and Aponomma.

Ixodes ricinus is the common English sheep-tick. Species of Boophilus are parasitic on cattle the world over, and B. annulatus is the transmitter of Texas fever. Rhipicephalus and Amblyomma are large genera which include several species of economic importance. For example, R. sanguineus conveys canine piroplasmosis, and A. hebraeum causes “heart-water” in South African cattle. The genus Aponomma confines its attention to reptiles, and some of its species are exceedingly ornate.

Neglecting Margaropus and Rhipicentor, which include only a very few aberrant forms, the following entirely artificial key will serve to differentiate the genera of the Ixodidae:—

1. A pair of eyes on the lateral borders of the scutum 2

No eyes 6

2. Capitulum long, much longer than broad 3

Capitulum short 4

3. Unicolorous, ♂ with chitinous plates near anus Hyalomma

Generally ornate, ♂ without anal plates Amblyomma

4. Generally ornate, ♂ without anal plates, but with enlarged 4th coxae Dermacentor

Unicolorous, ♂ with anal plates and normal coxae 5

5. Palpi very short, spiracle circular Boophilus

Palpi medium, spiracle comma-shaped Rhipicephalus

6. Capitulum short; 2nd article of palp projecting laterally Haemaphysalis

Capitulum long 7

7. Unicolorous, elongate, on birds or mammals Ixodes

Generally ornate, broad-oval, on reptiles Aponomma

Neumann has recently revised the Ixodoidea in a series of papers published in the Mémoires de la Société zoologique de France, but the work is not obtainable as a whole. A monograph, by Nuttall, Warburton, Cooper, and Robinson, is now in course of publication at the Cambridge University Press.

=Fam. 4. Gamasidae.=—The Gamasidae are carnivorous Mites, either free-living or parasitic on animals. The chelicerae are chelate, and the palps are free. The tarsi have two claws, accompanied by a “caruncle” or sucking disc. They are mostly pale-coloured Mites, with a smooth, more or less scutate covering. The three principal sub-families are Gamasinae, Uropodinae, and Dermanyssinae.

Of the GAMASINAE, Gamasus coleoptratorum is the well-known Beetle-parasite so frequently seen on Geotrupes. It is often confounded with another species of similar habits, G. crassipes.

The curious Beetle-parasites attached to their victim by a thread belong to the genus Uropoda of the UROPODINAE. The connecting filament, which the Mite can sever at will, for a long time puzzled observers. It was variously construed as a silken cord of attachment, and as a sort of umbilical cord, through which the Mite drew nourishment from the Beetle. On more careful investigation it proved to be connected with the anus of the Mite, and to consist of its consolidated excrement.

The DERMANYSSINAE are all parasitic on warm-blooded animals, principally birds and bats. Dermanyssus avium is the common parasite infesting fowls and cage-birds.

=Sub-Order 4. Heterostigmata.=

=Fam. Tarsonemidae.=—This is the sole family of the sub-order. It comprises a number of minute vegetable-feeding Mites which have been little studied, though they are probably the cause of considerable injury to the leaves and buds of plants.

=Sub-Order 5. Prostigmata.=

In these Mites the stigmata are situated anteriorly, in the rostrum or the thorax. In the Water-mites the tracheae have atrophied, but these creatures are clearly Trombidiidae which have taken to an aquatic life.

FIG. 246.—Bdella lignicola, x about 50. (After Canestrini.) ]

=Fam. 1. Bdellidae.=—The Bdellidae are sometimes known as the “Snouted Mites” on account of the very prominent forwardly-directed “capitulum” or false head. They have chelate chelicerae and tactile palps, which are often “elbowed,” like the antennae of weevils. Eyes may be present or absent. They are usually of a bright red colour, and are free-living and predaceous, though in their larval stages they may often be found attached to the limbs of insects and spiders.

The minute active scarlet Mites of the genus Eupodes and its allies perhaps come within this family. Their legs are six-jointed.

The remaining families of the Prostigmata (Halacaridae, Hydrachnidae, and Trombidiidae) all have raptorial palps, and clawed or piercing chelicerae.

=Fam. 2. Halacaridae.=—This is a small group of marine Mites. In their usually prominent capitulum they resemble the Bdellidae. In some respects they recall the Oribatidae, having hard integuments, and their legs being articulated near the margin of the body. They do not swim, but crawl upon weeds and zoophytes, or burrow in the mud.

FIG. 247.—Atax alticola, x 16. (After Canestrini.) ]

=Fam. 3. Hydrachnidae.=—The Hydrachnidae are the Fresh-water Mites. Their legs are provided with long close-set hairs, and thus adapted for swimming. They are predaceous, and in their young stages are often parasitic on water insects. A familiar example is Atax bonzi, which lives within the shell of the fresh-water mussel.

=Fam. 4. Trombidiidae.=—The predaceous palps of the Trombidiidae are generally of the “finger-and-thumb” type. The tarsi are two-clawed, without caruncle. This group may be divided into six sub-families.

FIG. 248.—Tetranychus gibbosus, x 50. (After Canestrini.) ]

(i.) The LIMNOCHARINAE or “Mud-mites” connect the Hydrachnidae with the typical Trombidiidae. They are usually velvety and of a red colour. They do not swim, but creep. The larva of Limnocharis aquaticus is parasitic on Gerris lacustris.

(ii.) The CAECULINAE bear a strong general resemblance to the Harvestmen or Phalangidae. Caeculus is so similar to the Phalangid genus Trogulus that it was considered by Dufour to belong to the same order.

(iii.) The TETRANYCHINAE or “Spinning mites” are phytophagous, and do much injury to plants, sucking the sap from the leaves and giving them a blistered appearance. Tetranychus telarius is the “Red-spider” of popular nomenclature.

(iv.) The CHEYLETINAE are remarkable Mites with fleshy, semi-transparent body, and enormously developed raptorial pedipalpi, which are extremely formidable weapons of attack. They do not creep or run like most Mites but proceed by a series of short leaps. Cheyletus is the principal genus.

The curious genus Syringophilus, which is parasitic in the interior of birds’ feathers, appears to be a degenerate Cheyletine.

(v.) The ERYTHRAEINAE are minute, active Mites, usually red in colour, free-living and predaceous.

(vi.) The TROMBIDIINAE include most of the moderate-sized, velvety red Mites which are commonly known as “Harvest-mites,” and their larvae, the so-called Harvest-bugs, frequently attack man. Trombidium holosericeum is a well-known example.

=Sub-Order 6. Notostigmata.=

This sub-order has been established for the reception of the curious genus Opilioacarus.

=Fam. Opilioacaridae.=—Mites with segmented abdomen, leg-like palps, chelate chelicerae, and two pairs of eyes. There are four dorsal abdominal stigmata. Four species of the sole genus Opilioacarus have been recorded, O. segmentatus from Algeria, O. italicus from Italy, O. arabicus from Arabia, and O. platensis from South America.

APPENDICES TO ARACHNIDA I. AND II.

TARDIGRADA AND PENTASTOMIDA

ARTHUR E. SHIPLEY, M.A., F.R.S.

Fellow and Tutor of Christ’s College, Cambridge, and Reader in Zoology in the University