Arachnida Embolobranchiata (Scorpions, Spiders, Mites, Etc.) (part 2)

Sub-Fam. 1. Cheliferinae.—These Chernetidea have the cephalothorax slightly narrowed in front, and generally marked dorsally with two transverse striae, while the abdominal plates are bisected by a dorsal longitudinal line. With the exception of Chelifer cancroides, which is always found in houses, all the species are to be sought under bark, though occasionally they are discovered under stones.

The two genera of this sub-family are Chelifer and Chernes, the species of Chelifer being two-eyed, and those of Chernes blind.

As already stated, Simon does not consider the possession of the two—often very feebly developed—eyes of generic importance, and admits only the genus Chelifer.

Five species of Chelifer (including Ch. cancroides) and five of Chernes have been recorded in England.

Sub-Fam. 2. Garypinae.—The Garypinae have the cephalothorax greatly contracted in front and often projecting considerably.

There are three genera, Chiridium, Olpium, and Garypus. Chiridium is eyeless, and appears to have only ten segments in the abdomen, the tergal plates of which are bisected. C. museorum is found in England, and is the only Chernetid except Chelifer cancroides which habitually lives in houses. C. ferum is found under bark in the south of France.

Neither Olpium nor Garypus, which both possess four eyes and eleven abdominal segments, have as yet been found in this country. Garypus, like Chiridium, has the dorsal abdominal plates bisected. There is a transverse stria on the cephalothorax, and the eyes are far from the anterior border. In Olpium the dorsal plates are undivided and the eyes more anterior.

Sub-Fam. 3. Obisiinae.—The cephalothorax of the Obisiinae does not narrow—and is, indeed, sometimes broadest—anteriorly. The chelicerae are notably large, and the dorsal abdominal plates undivided. They are the most active of the Chernetidea, ordinarily running backwards or sideways with their pedipalpi closely folded up against the body. Four genera usually admitted fall within this group:—Obisium, Roncus, Blothrus, and Chthonius.

Obisium has four eyes, parallel-sided cephalothorax, and curved chelae on the palps. Roncus is like Obisium except in having only two eyes, and is therefore disallowed by Simon, who also considers Blothrus to comprise merely eyeless species of Obisium. In Chthonius the cephalothorax is broadest in front, and the digits of the chelae are straight.

The Obisiinae are found in moss and débris, and under stones. Three species of Obisium, two of Roncus, and four of Chthonius are recorded in England.

The subjoined list of British species of Chernetidea is taken from the monograph of the Rev. O. P. Cambridge, cited above:—

Group I.—Four eyes.

Chthonius orthodactylus, Leach.
„ rayi, L. Koch.
„ tetrachelatus, Preyssler.
„ tenuis, L. Koch.
Obisium muscorum, Leach.
„ sylvaticum, C. L. Koch.
„ maritimum, Leach.

Group II.—Two eyes.

Roncus cambridgii, L. Koch.
„ lubricus, L. Koch.
Chelifer hermannii, Leach.
„ cancroides, Linn.
„ meridianus, L. Koch.
„ subruber, Simon.
„ latreillii, Leach.

Group III.—No eyes.

Chernes nodosus, Schr.
„ insuetus, Camb.
„ cimicoides, Fabr.
„ dubius, Camb.
„ phaleratus, Simon.
Chiridium museorum, Leach.

Chapter XVII. Arachnida Embolobranchiata ( Continued )—Podogona—Phalangidea = Opiliones—Habits—Structure—Classification

Order VII. Podogona (Ricinulei).

Tracheate Arachnids with two-jointed chelate chelicerae and prehensile pedipalpi. The tarsus of the third leg of the male bears a copulatory organ.

In 1838 Guérin-Méneville described an Arachnid from West Africa which he named Cryptostemma westermannii. At rare intervals occasional specimens of allied forms have been taken in the same region until six species of Cryptostemma have been established. In South America, also, two unique examples of very similar creatures are the only known representatives of the two species of the allied genus Cryptocellus. All the examples hitherto found are of fair size (between ⅕ inch and ½ inch in length), and bear some general, though superficial, resemblance to the Trogulidae, which has led to their being placed among the Phalangidea by almost all the Arachnologists who have noticed them. Their claim to this systematic position, however, is extremely doubtful, and Hansen and Sörensen, who have had the opportunity of studying the group much more minutely than previous writers, are of the opinion that they ought to constitute a separate order of Arachnids, more nearly allied to the Pedipalpi than to the Phalangidea. In this place it is only possible to indicate some of their peculiar characteristics. Their integuments are particularly hard and coriaceous. The cephalothorax is united to the abdomen by a rather broad pedicle, but there is also a remarkable coupling apparatus which makes the constriction between cephalothorax and abdomen appear very slight. There is a movable anterior projection of the cephalothorax, the “cucullus.” The two-jointed chelicerae terminate in minute chelae, as also do the five-jointed pedipalps. There are no spiracles on the abdomen, but two are situated on the thorax above the coxae of the third pair of legs. Perhaps the most remarkable fact is that, as in the Araneae, a modified limb is used by the male for the fertilisation of the female; but in this case it is not the tarsus of the pedipalp, but of the third leg of the male, which is specially developed as an intromittent organ.

Ordinal rank is not universally accorded to the group, but whatever its true position, the known forms fall under a single family Cryptostemmatidae, including the two genera Cryptostemma and Cryptocellus.

Order VIII. Phalangidea (Opiliones).

Tracheate Arachnids, with abdomen united to the cephalothorax by its whole breadth. They are oviparous, and undergo no metamorphosis. Abdomen always segmented. A pair of odoriferous glands opening on the thorax. Two simple eyes; three-jointed chelate chelicerae; pedipalpi not chelate. Spinning organs absent.

“Harvesters,” “Harvestmen,” or “Harvest-spiders,” as these animals are popularly called, need never be confounded with true Spiders if the absence of a constriction between the cephalothorax and abdomen be noted. They are more difficult to distinguish from Mites, members of which group have sometimes been described as Phalangids. The Phalangid is, however, generally recognisable by its segmented abdomen, and as a further point of distinction, it may be noted that, whereas the anal orifice is always transverse or circular in Phalangids, it is uniformly longitudinal in the Acarines.

Members of this group vary considerably in habit. The best known forms are exceedingly active, and trust to their speed in endeavouring to escape from danger, at the same time emitting an odorous fluid from two apertures situated just above the coxae of the first pair of legs. These active Harvestmen are only found in the mature state at certain seasons of the year, and are believed, therefore, to live only for a single season. Slow-moving forms, like the Nemastomatidae and the Trogulidae, which live amidst grass and herbage, have a much longer duration of life. In danger they remain perfectly still, and trust to their earthy appearance to escape observation.

They are stated to be extremely thirsty animals, and have been observed drinking from the dewdrops on herbage. It is probably on this account that they are sometimes seen attacking juicy vegetable matter, for without doubt they are essentially carnivorous. The larvae of insects, young spiders, mites, and myriapods are their customary food. It is not requisite that the prey should be alive, but they will not touch anything mouldy.

Notwithstanding their apparently weak mouth-parts, they do not merely suck the juices of their victims, but masticate and swallow solid particles. Cannibalism is frequently observed among them.

The males fight fiercely with one another at the breeding time. The females, with their long extrusible ovipositors, place groups of twenty to forty eggs in small holes in the ground or under stones or bark, unprotected by any form of cocoon. The eggs hatch into fully-formed Phalangids, which are at first white, but attain their coloration after the first moult. They subsequently moult from five to nine times.

The distribution of this group is world-wide, and some of the exotic species are very remarkable in form. Only twenty-four species have as yet been recorded in this country.

External Structure.—In the Phalangidea there is no constriction between the cephalothorax and the abdomen, and in the Ischyropsalidae alone is the distinction between them readily observable. This is due to the partial or complete fusion of the first five segments of the abdomen with the carapace or cephalothoracic shield in most species, these segments being indicated, if at all, merely by faint striae or successive transverse rows of spines or tubercles. In the forms possessing hard integuments (Gonyleptidae, Nemastomatidae, Trogulidae) this fusion results in a dorsal “scutum,” the component parts of which cannot easily be distinguished.

The cephalothorax is often surmounted by a turret—usually grooved dorsally, and beset on its edges with a spiny armature—on the sides of which are the two simple eyes. The position and shape of this turret and the arrangement of its spines are of importance in the classification of the group.

In the Trogulidae the base of the turret gives rise to a remarkable, forwardly-directed, bifurcate structure, furnished with numerous strong tubular bristles. This is called the “hood,” and its hollowed-out under surface forms a chamber, the “camerostome,” in which lie the basal joints of the pedipalpi.

In most European Phalangids the under surface of the cephalothorax is almost entirely concealed by the forwardly-projecting portion of the abdomen bearing the generative opening, and by the gnathobases, not only of the pedipalpi, but of the first and sometimes of the second legs. As in Spiders, however, there is always present a “sternum” and generally a “labium.” The sternum is long and narrow in the Mecostethi, and Cyphophthalmi, but in the Plagiostethi, which include most of the forms found in temperate regions, it is very short and transverse, and is hidden by the abdominal prolongation before mentioned.

The anterior wall of the mouth is formed by a beak-like plate, the “epistome,” the basal portion of which is covered externally by a second plate, for which Simon proposes the name “pre-epistome.” In some Phalangids there are three little chitinous plates, one median and two lateral, on the clypeus, between the anterior border of the carapace and the insertion of the chelicerae. They are best seen in Nemastoma.

The abdomen always presents evidences of segmentation, though there is a difference of opinion as to the number of segments of which it is composed. This is due to the already mentioned partial or complete fusion of the anterior segments with the cephalothorax. From the admirable researches of Hansen and Sörensen it seems likely that the normal number of abdominal segments is ten. Ventrally, the abdomen is produced forward into a “sternal process” which is capped by a genital plate, hardly distinguishable in the Phalangidae, but readily visible in the other families, which surrounds and masks the unpaired genital orifice. Two stigmata or breathing pores are situated on the sides of the first ventral plate, which these authors consider to be composed of two fused sternites.

As in other Arachnids there are six pairs of appendages articulated to the cephalothorax. They are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs.

The chelicerae are three-jointed and chelate, the second joint having its inner portion produced into an apophysis to which the final joint is apposed. In certain forms (Gonyleptidae, Ischyropsalis) the chelicerae are remarkably long, and may considerably exceed the total length of the trunk.

The pedipalpi are six-jointed, possessing coxa, trochanter, femur, patella, tibia, and tarsus. They are leg-like and are never chelate, but in some forms terminate in a single movable claw. The coxal joints are provided with maxillary plates.

The legs are normally seven-jointed, as in Spiders, the penultimate joint being the metatarsus. The tarsus is always multi-articulate, the number of its joints being variable. It bears terminally one or two simple claws. “False articulations” (where the parts are not inserted one into the other, but are only marked off by a membranous ring) are of frequent occurrence in the legs of these creatures. The first legs, like the pedipalps, bear maxillary plates, as do also the second in most Phalangids. The maxillae of the second legs are, however, entirely absent in Nemastoma, and rudimentary in the Gonyleptidae and the Ischyropsalidae. The coxae of the legs are all largely developed, but are not capable of free motion, being soldered to, and practically forming part of, the cephalothoracic floor. In some forms they are only separated from one another by slight grooves. The extreme length of the legs, and their hard and brittle nature, are characteristic features of the Phalangids, though in some species (Trogulidae) they are comparatively short. The first pair of legs are always the shortest, and the second the longest.

The sexual organs of Phalangids are ordinarily concealed, and the sexes can only be distinguished by certain very variable secondary characters, the males being usually smaller of body and longer of leg than the females, besides being more distinctly coloured and being armed with more numerous and longer spines. Sometimes the male chelicerae are highly characteristic.

Phalangids are usually destitute of spinning organs, but such have been discovered, in a rudimentary state, in the Cyphophthalmi, which are said to spin slight webs.

Internal Structure.—In Phalangium the mouth leads upwards into a membranous pharynx, wider than that of Spiders, but narrowing into an oesophagus which passes between the cerebral and thoracic ganglionic nerve-masses. It then turns backwards over the thoracic ganglion, being slightly dilated at that point. Immediately afterwards it dilates into a flask-like gastric sac which occupies almost the whole width of the abdomen, and proceeds straight to the anus. Viewed from above, the shape of this sac is entirely concealed by the large number of caeca (thirty) to which it gives rise dorsally and laterally. The two largest of these caeca extend, parallel to each other, over the whole of the abdominal portion of the gastric sac, and are flanked by four lateral pairs of smaller caeca, while there is a cluster of small caeca covering the anterior and narrower portion of the flask-like stomach.

The large hepatic mass so conspicuous on opening dorsally the abdomen of a Spider is here entirely absent, but its functions are believed to be performed by certain wrinkled, tubular, longitudinally parallel bodies, about seven in number, closely applied to the under surface of the flask.

The masticating portions of the maxillae of the pedipalpi and the first pair of legs are hollow distensible sacs, often seen in a swollen condition in specimens kept in spirits. They are furnished, on the inner surface, with a horny ridge.

Owing to the fixity of the coxae of the legs, their maxillary plates are incapable of much lateral motion, but are rubbed against each other vertically.

Beyond the fact that the heart is a dorsal tube lying along the anterior two-thirds of the alimentary canal, and divided by constrictions into three well-marked and equal portions, little is known of the blood-system of these animals. It is probably essentially like that of Spiders, but the presence of a pericardial sac has not yet been established, nor has the course of the blood-vessels been described in detail.

As in other Arachnids, the principal ganglionic nerve-masses closely embrace the oesophagus. Immediately anterior to it, forming a conical mass with its base on the oesophagus, is the cerebral ganglion, while just behind it is the transverse portion of the large thoracic nerve-centre. In Phalangium opilio, according to Tulk, a median nerve is given off from the apex of the cerebral mass (the paired nature of which is apparent) and bifurcates to the two eyes. Two lateral nerves proceed to certain organs near the origin of the second pair of legs, which were thought by the old writers to be lateral eyes, but which are now known to be glands for the manufacture of the odorous fluid which these animals can exude.

The thoracic ganglion expands, on either side of the oesophagus, into a mass which extends nearly as far forward as the apex of the cerebral ganglion. These lateral masses give off nerves to the appendages. From the back of the transverse portion proceed three nerves. The median nerve passes above the generative organs, and soon branches into two nerves which presently swell out to form ganglia of considerable size, beyond which they soon join again and give off an anastomosing network of nerve-fibres. The lateral nerves immediately branch. The outer branch dilates into a ganglion which supplies the external part of the generative organ. The inner branch, which is longer, also forms a ganglion the nerves from which are chiefly distributed to the under surface of the alimentary canal.

The respiratory organs consist of two large tracheal tubes with numerous branches, having their external openings or “stigmata” near the base of the fourth pair of legs. The two main tubes are directed forwards, and are mainly concerned with supplying the largely developed muscles of the legs. The distribution of branches to the abdomen is comparatively feeble. The particular arrangement of tubes in P. opilio, according to Tulk, may be seen in the accompanying figure. There are a pair of coxal glands, of excretory function, opening in the neighbourhood of the coxae of the third pair of legs.

The Phalangidea are remarkable among Arachnids in the possession of large protrusible external organs of generation. The ovipositor of the female may be as long as the whole body of the animal, and the intromittent organ of the male is of almost equal length. The pedipalpi take no part in the fertilisation of the female, which is accomplished directly.

The protrusible organs are concealed under the forwardly-projecting anterior segment of the abdomen beneath, the genital orifice being thus in many cases quite near the head region. The internal sexual organs are not very complex. The ovary re-enters upon itself, forming a ring, and from the point of re-entry a tube proceeds towards the centre of the ring, dilating to form an ovisac. It then narrows, turns forward, dilates once more into a second ovisac, from which the oviduct proceeds to the base of the ovipositor. This is a flattened organ, grooved on its upper surface and bifid at its extremity. The testis of the male is a single sac-like gland, from either end of which proceeds a vas deferens, which, after several convolutions, unite into a sperm-sac which opens at the base of the penis.

Partial hermaphroditism is a very frequent phenomenon among the Phalangids, the testis often producing ova as well as spermatozoa.

Though the males fight fiercely at the breeding time, the animals for the most part live peacefully together. Henking found that the eggs of Liobunum, which were about half a millimetre in diameter, were laid during October and hatched out in the following April.

Classification.—The Order Phalangidea is divided into three Sub-orders: 1, Cyphophthalmi; 2, Mecostethi; 3, Plagiostethi.

Sub-Order 1. Cyphophthalmi

Phalangids with dorsal and ventral scutum, only the last abdominal segment remaining free. Eyes two or absent. Maxillary lobe on coxae of first pair of legs rudimentary. Sternum long and narrow. Anterior segment of abdomen not projecting ventrally beyond the coxae of the fourth pair. Odoriferous glands open on prominences.

In 1875 Stecker published a description of a remarkable creature which he said he had found in Bohemia, and which he named Gibocellum sudeticum. Among other points it possessed four eyes and four spinning mammillae, and it differed so much from other Cyphophthalmi as to necessitate the foundation of a family, Gibocellidae, for its reception. No one else appears to have seen the animal, or any of Stecker’s preparations of it, and Hansen and Sörensen adduce grave reasons for believing that it never existed at all. If this species is to be disallowed, the Cyphophthalmi all fall into a single family.

Fam. Sironidae.—These somewhat Mite-like Phalangids are rarely met with, partly, no doubt, because of their retiring habits and small size, the known forms ranging from 6 mm. to less than 2 mm. in length. Of the seven genera which have been established, Stylocellus numbers eight species from Borneo and Sumatra, and Pettalus two species from Ceylon. Ogovia, Miopsalis, and Purcellia have one species each, from South Africa, Further India, and the Cape, respectively. The only European forms are the two species of Siro (France and Austria), and Parasiro corsicus. No species has yet been found in England.

Sub-Order 2. Mecostethi.

(LANIATORES).

Sternum long and narrow. Dorsal scutum leaving at least the last three segments free. Openings of odoriferous glands not on prominences. The fourth pair of legs usually long and powerful. One terminal claw on each of the first two pairs of legs; two on the last two pairs.

The Mecostethi are essentially tropical forms, though a few representatives are found in the caves of Southern Europe. One family (Phalangodidae) has its headquarters in the hot regions of the Old World, while the other two (Cosmetidae, Gonyleptidae) are confined to Central and South America.

Fam. 1. Phalangodidae.—Body piriform or triangular, broadest behind. Last ventral segment of abdomen much the largest. Very narrow sternum. Eye-turret near anterior border of cephalothorax. Chelicerae narrow at base. Pedipalpi long and strong. Maxillary plates on first pair of legs rudimentary. No stigmata visible.

The only European forms of this family belong to the genus Phalangodes. They all avoid the light, and are usually found in caves. Simon records six species found in France. A North American species, P. armata, is entirely destitute of eyes.

The family has representatives in Australia and in tropical Africa and Asia. Mermerus, Epidanus, Maracaudus, and Sitalces are some of the exotic genera.

The other two families of this Sub-order—Fam. 2, Cosmetidae; Fam. 3, Gonyleptidae—include a large number of species, some of considerable size (up to an inch in length of body), found in Central and South America.

Sub-Order 3. Plagiostethi. (Palpatores.)

First abdominal segment produced forward ventrally to the level of the first pair of legs, bringing the mouth and the genital opening very near together. Sternum consequently much reduced. Pedipalpi thin, with terminal claw absent or rudimentary. Terminal claws of the legs single.

The Plagiostethi include most of the Harvestmen of temperate regions, the most familiar examples of these creatures belonging to the large family Phalangidae, and being much more in evidence than the slow-moving and ground-living forms included in the other families.

Fam. 1. Phalangiidae.—Eye-turret always far removed from anterior border of cephalothorax. Second pair of legs with well-marked maxillary lobes. Legs similar, without the false joint called “trochantin.” Multiarticulate tarsi. Simple pedipalpi, with tarsus much longer than tibia, and possessing terminal claw. Some have soft, some coriaceous integuments.

The Phalangidae fall naturally into two groups or sub-families, named by Simon Sclerosomatinae and Phalangiinae. The first group consists of more or less coriaceous forms living among moss and herbage. They are not very numerous, there being only about twelve known European species divided among the three genera, Sclerosoma, Mastobunus, and Astrobunus.

Two species of Sclerosoma are found in England, S. quadridentatum occurring not uncommonly among moss or under stones in various parts of the country. Its back is studded with wart-like tubercles, which give it a characteristic appearance.

The Phalangiinae are soft-bodied Harvestmen, always with long legs, which in the genus Liobunum attain an inordinate length. There are nine European genera, Liobunum, Prosalpia, Gyas, Oligolophus, Acantholophus, Phalangium, Dasylobus, Platybunus, and Megabunus, comprising in all about fifty species. Five of these genera are represented in England.

The familiar Phalangids, with small, almost spherical bodies and ridiculously long legs, belong to the genus Liobunum, L. rotundum being the common species. It is mature in autumn, when it may be seen scampering at a great pace among the herbage. It very readily parts with its limbs, and Pickard-Cambridge relates that he once “saw one running with very fair speed and facility, having lost all but two legs, an anterior one on one side and a posterior one on the other.”

The Harvestmen so frequently seen on walls belong, as a rule, to the genus Phalangium. The best known example is Phalangium opilio (the P. cornutum of Linnaeus), the male of which possesses a remarkable development of the chelicerae.

The genus Oligolophus is well represented in this country, nine species having been recorded. They do not differ greatly from Phalangium, but have, as a rule, more massive bodies, and rather stout, though tolerably long legs. The largest English Harvestman, not rare under stones at Cambridge, is O. spinosus, whose body measures half an inch in length. O. agrestis is perhaps the commonest British Phalangid, and is abundant in woods and among herbage, and on low trees.

Platybunus has two, and Megabunus one British representative. They are of small size, and are to be sought for among heather or dead leaves in spring or early summer.

Fam. 2. Ischyropsalidae.—Coriaceous Phalangids, with eye-turret far removed from anterior border of cephalothorax. Maxillary lobes of second pair of legs rudimentary, in the form of tubercles. Legs similar, without “trochantin.” Multiarticulate tarsi. Tarsus of pedipalp without claw, and shorter than metatarsus. Pedipalps long and horizontal.

This family includes a small number of large or moderate-sized Phalangids, which are found occasionally in thick moss, or in caves, in mountainous regions of the south of Europe, and belong to the genera Ischyropsalis and Sabacon. There is a North American genus, Taracus.

Fam. 3. Nemastomatidae.—Coriaceous Phalangids, with cephalothorax fused with the first five segments of the abdomen, forming a scutum. Eye-turret near anterior border. No maxillary lobe on second coxae. Similar legs, without “trochantin.” Multiarticulate tarsi. Tarsus of pedipalp without claw, and shorter than metatarsus.

There is but one genus, Nemastoma, in this family, and the members of it are, as a rule, rather small and dark Phalangids, which live under stones or in moss or débris, and are found in the mature state at all seasons of the year. There are about twenty European species, but only two of these, N. lugubre and N. chrysomelas, have as yet been found in Britain. N. lugubre is a very common animal, and though it does not obtrude itself upon public notice, its little black body with two pearly white spots must be a familiar object to all insect collectors who have occasion to search under stones or among moss in damp places. Its legs are short and stout, but those of N. chrysomelas, which is a brighter coloured Harvestman with spots of dull gold colour, are long and slender.

Fam. 4. Trogulidae.—Coriaceous and very hard integument. Anterior part of cephalothorax produced into a bifurcate “hood.” Often a “trochantin.”

The Trogulidae are very slow-moving Phalangids of moderate or large size (a sixth to half an inch in body), found under stones or in damp moss and débris. They are Mite-like in general appearance, and may readily be distinguished from all other Harvestmen by the presence of the “hood” (Fig. , p. 442), the hollowed-out under surface of which forms a chamber, called by Simon the “camerostome,” in which lie the basal portions of the pedipalps.

Only a single immature specimen has been found in England, belonging probably to the species Trogulus tricarinatus. It was found in Dorsetshire. Some members of the family are not uncommon in various regions of the Continent. There are four genera, Dicranolasma, Anelasmocephalus, Calathocratus, and Trogulus. Two other genera, Amopaum and Metopoctea, have been established, but the former is probably the young of Dicranolasma and the latter of Trogulus.

According to the monograph on the British Phalangidea by the Rev. O. Pickard-Cambridge, cited above, the following species have been recorded in this country. They all fall under the sub-order Plagiostethi:—

BRITISH PHALANGIDEA.

Phalangiidae.

Sclerosoma quadridentatum, Cuvier.
„ romanum, L. Koch.
Liobunum rotundum, Latr.
„ blackwallii, Meade.
Phalangium opilio, Linn.
„ parietinum, De Geer.
„ saxatile, C. L. Koch.
„ minutum, Meade.
Platybunus corniger, Meade.
„ triangularis, Herbst.
Megabunus insignis, Meade.
Oligolophus morio, Fabr.
„ alpinus, Herbst.
„ cinerascens, C. L. Koch.
„ agrestis, Meade.
„ tridens, C. L. Koch.
„ palpinalis, Herbst.
„ ephippiatus, C. L. Koch.
„ spinosus, Bosc.

Nemastomatidae.

Nemastoma lugubre, Müller.
„ chrysomelas, Hermann.

Trogulidae.

Anelasmocephalus cambridgii, Westwood.
Trogulus tricarinatus, Linn.

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. ). 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. ). 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. ), 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. ), 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. ) 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. ) 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. ) 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. ), 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. ) 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.

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. ), 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.

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. , 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.

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.

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.

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. , 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.

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.

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.

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.

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.

(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

BY

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

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

Chapter XIX. Tardigrada

Occurrence—Ecdysis—Structure—Development—Affinities—Biology—Desiccation—Parasites—Systematic

The animals dealt with in this chapter lead obscure lives, remote from the world, and few but the specialist have any first-hand acquaintance with them. Structurally they are thought to show affinities with the Arachnida, but their connexion with this Phylum is at best a remote one.

Tardigrades are amongst the most minute multicellular animals which exist, and their small size—averaging from ⅓ to 1 mm. in length—and retiring habits render them very inconspicuous, so that as a rule they are overlooked; yet Max Schultze asserts that without any doubt they are the most widely distributed of all segmented animals. They are found amongst moss, etc., growing in gutters, on roofs, trees or in ditches, and in such numbers that Schultze states that almost any piece of moss the size of a pea will, if closely examined, yield some members of this group, but they are very difficult to see. The genus Macrobiotus especially affects the roots of moss growing on stones and old walls. M. macronyx lives entirely in fresh water, and Lydella dujardini and Echiniscoides sigismundi are marine; all other species are practically terrestrial, though inhabiting very damp places.

In searching amongst the heather of the Scotch moors for the ova and embryos of the Nematodes which infest the alimentary canal of the grouse, I have recently adopted a method not, as far as I am aware, in use before, and one which in every case has yielded a good supply of Tardigrades otherwise so difficult to find. The method is to soak the heather in water for some hours and then thoroughly shake it, or to shake it gently in a rocking machine for some hours. The sediment is allowed to settle, and is then removed with a pipette and placed in a centrifugaliser. A few turns of the handle are sufficient to concentrate at the bottom of the test-tubes a perfectly amazing amount of cryptozoic animal life, and amongst other forms I have never failed to find Tardigrades.

Many Tardigrades are very transparent; their cells are large, and arranged in a beautifully symmetrical manner; and since those of them that live in moss, and at times undergo desiccation, are readily thrown into a perfectly motionless state, during which they may be examined at leisure, it is not surprising that these little creatures have been a favourite object for histological research. One way to produce the above-mentioned stillness is partly to asphyxiate the animals by placing them in water which has been boiled, and covering the surface of the water with a film of oil.

The whole body is enclosed in a thin transparent cuticle, which must be pierced by a needle if it be desired to stain the tissues of the interior. As a rule the cuticle is of the same thickness all over the body, but in the genus Echiniscus the cuticle of the dorsal surface is arranged in thickened plates, and these plates are finely granulated. From time to time the cuticle is cast, and this is a lengthy process, so that it is not unusual to find a Tardigrade ensheathed in two cuticles, the outer of which is being rubbed off. The Macrobioti lay their eggs in their cast cuticle (Fig. ). The end of each of the eight legs bears forked claws of cuticular origin. The legs are not jointed except in the genus Lydella, where two divisions are apparent.

Within the cuticle is the epidermis, a single layer of cells arranged in regular longitudinal and transverse rows along the upper and under surface, where the cells are as uniformly arranged and as rectangular as bricks. The cells on the sides of the body are polygonal, and not in such definite rows. The nuclei show the same diagrammatic symmetry as the cells which contain them, and lie in the same relative position in neighbouring cells. In a few places, such as the end of each limb and around the mouth and arms, the cells of the epidermis are heaped up and form a clump or ridge. In some genera a deposit of pigment in the epidermis, which increases as the animal grows old, obscures the internal structures. It is generally brown, black, or red in colour.

The cuticle and epidermis enclose a space in which the various internal organs lie. This space is traversed by numerous symmetrically disposed muscle-fibres, and contains a clear fluid—the blood—which everywhere bathes these organs. This fluid evaporates when desiccation takes place, and is soon replaced after rain; it forms no coagulum when reagents are added to it, and it probably differs but little from water. Floating in it are numerous corpuscles, whose number increases with age. In well-fed Tardigrades the corpuscles are packed with food-reserves, often of the same colour—green or brown—as the contents of the stomach, which soon disappear when the little creatures are starved.

The alimentary canal begins with an oral cavity, which is in many species surrounded by chitinous rings. The number of these rings and their general arrangement are of systematic importance. The oral cavity opens behind into a fine tube lined with chitin, very characteristic of the Tardigrada, which has been termed the mouth-tube. By its side, converging anteriorly, lie the two chitinous teeth, which may open ventrally into the mouth-tube, as in Macrobiotus hufelandi and Doyeria simplex, or may open directly into the oral cavity, as in Echiniscus, Milnesium, and some species of Macrobiotus. In some of the last named the tips of the teeth are hardened by a calcareous deposit. The hinder end of each stylet or tooth is supported by a second chitinous tooth-bearer, and the movement of each is controlled by three muscles, one of which, running forwards to the mouth, helps to protrude the tooth, whilst the other two running upwards and downwards to the sheath of the pharynx, direct in what plane the tooth shall be moved.

The mouth-tube passes suddenly into the muscular sucking pharynx, which is pierced by a continuation of its chitinous tube. Roughly speaking, the pharynx is spherical; the great thickness of its walls is due to radially arranged muscles which run from the chitinous tube to a surrounding membrane. When the muscles contract, the lumen of the tube is enlarged, and food, for the most part liquid, is sucked in. Two large glands, composed of cells with conspicuous nuclei, but with ill-defined cell outlines, pour their contents into the mouth in close proximity to the exit of the teeth. The secretion of the glands—often termed salivary glands—is said in many cases to be poisonous.

The pharynx may be followed by a distinct oesophagus, or it may pass almost immediately into the stomach, which consists of a layer of six-sided cells arranged in very definite rows. In fully-fed specimens these cells project into the lumen with a well-rounded contour. Posteriorly the stomach contracts and passes into the narrow rectum, which receives anteriorly the products of the excretory canals and the reproductive organs, and thus forms a cloaca. Its transversely placed orifice lies between the last pair of legs. The food of Tardigrades is mainly the sap of mosses and other humble plants, the cell-walls of which are pierced by the teeth of the little creatures.

The organs to which an excretory function has been attributed are a pair of lateral caeca, which vary much in size according as the possessor is well or ill nourished. They recall the Malpighian tubules of such Mites as Tyroglyphus. Nothing comparable in structure to nephridia or to coxal glands has been found.

The muscles show a beautiful symmetry. There are ventral, dorsal, and lateral bundles, and others that move the limbs and teeth, but the reader must be referred to the works of Basse, Doyère, and Plate for the details of their arrangement. The muscle-fibres are smooth.

The nervous system consists of a brain or supra-oesophageal ganglion, whose structure was first elucidated by Plate, and a ventral chain of four ganglia. Anteriorly the brain is rounded, and gives off a nerve to the skin; posteriorly each half divides into two lobes, an inner and an outer. The latter bears the eye-spot when this is present. Just below this eye a slender nerve passes straight to the first ventral ganglion. The brain is continued round the oral cavity as a thick nerve-ring, the ventral part of which forms the sub-oesophageal ganglion, united by two longitudinal commissures to the first ventral ganglion. Thus the brain has two channels of communication between it and the ventral nerve-cord on each side, one by means of the slender nerve above mentioned, and one through the sub-oesophageal ganglion. The ventral chain is composed of four ganglia connected together by widely divaricated commissures. Each ganglion gives off three pairs of nerves, two to the ventral musculature, and one to the dorsal. The terminations of these nerves in the muscles are very clearly seen in these transparent little creatures, though there is still much dispute as to their exact nature.

The older writers considered the Tardigrada as hermaphrodites, but Plate and others have conclusively shown that they are bisexual, at any rate in the genus Macrobiotus. The males are, however, much rarer than the females. The reproductive organs of both sexes are alike. Both ovary and testis are unpaired structures opening into the intestine, and each is provided with a dorsal accessory gland placed near its orifice. In the ovary many of the eggs are not destined to be fertilised, but serve as nourishment for the more successful ova which survive.

No special circulatory or respiratory organs exist, and, as in many other simple organisms, there is no connective tissue.

The segmentation of the egg in M. macronyx is total and equal, according to the observations of von Erlanger. A blastula, followed by a gastrula, is formed. The blastopore closes, but later the anus appears at the same spot. There are four pairs of mesodermic diverticula which give rise to the coelom and the chief muscles. The reproductive organs arise as an unpaired diverticulum of the alimentary canal, which also gives origin to the Malpighian tubules. The development is thus very primitive and simple, and affords no evidence of degeneration.

With regard to their position in the animal kingdom, writers on the Tardigrada are by no means agreed. O. F. Müller placed them with the Mites; Schultze and Ehrenberg near the Crustacea; Dujardin and Doyère with the Rotifers near the Annelids; and von Graff with the Myzostomidae and the Pentastomida. Plate regards them as the lowest of all air-breathing Arthropods, but he carefully guards himself against the view that they retain the structure of the original Tracheates from which later forms have been derived. He looks upon Tardigrades as a side twig of the great Tracheate branch, but a twig which arises nearer the base of the branch than any other existing forms. These animals seem certainly to belong to the Arthropod phylum, inasmuch as they are segmented, have feet ending in claws, Malpighian tubules, and an entire absence of cilia. The second and third of these features indicate a relationship with the Tracheate groups; on the other hand there is an absence of paired sensory appendages, and of mouth-parts. Von Erlanger has pointed out that the Malpighian tubules, arising as they do from the mid-gut, are not homologous with the Malpighian tubules of most Tracheates, and he is inclined to place this group at the base or near the base of the whole Arthropod phylum. They, however, show little resemblance to any of the more primitive Crustacea. The matter must remain to a large extent a matter of opinion, but there can be no doubt that the Tardigrades show more marked affinities to the Arthropods than to any other group of the animal kingdom.

Biology.—Spallanzani, who published in the year 1776 his Opuscules de physique animale et végétale, was the first satisfactorily to describe the phenomena of the desiccation of Tardigrades, though the subject of the desiccation of Rotifers, Nematodes, and Infusoria had attracted much notice, since Leeuwenhoek had first drawn attention to it at the very beginning of the century. In its natural state and in a damp atmosphere Tardigrades live and move and have their being like other animals, but if the surroundings dry up, or if one be isolated on a microscopic slide and slowly allowed to dry, its movements cease, its body shrinks, its skin becomes wrinkled, and at length it takes on the appearance of a much weathered grain of sand in which no parts are distinguishable. In this state, in which it may remain for years, its only vital action must be respiration, and this must be reduced to a minimum. When water is added it slowly revives, the body swells, fills out, the legs project, and gradually it assumes its former plump appearance. For a time it remains still, and is then in a very favourable condition for observation, but soon it begins to move and resumes its ordinary life which has been so curiously interrupted.

All Tardigrades have not this peculiar power of revivification—anabiosis, Preyer calls it—it is confined to those species which live amongst moss, and the process of desiccation must be slow and, according to Lance, the animal must be protected as much as possible from direct contact with the air.

According to Plate, the Tardigrada are free from parasitic Metazoa, which indeed could hardly find room in their minute bodies. They are, however, freely attacked by Bacteria and other lowly vegetable organisms, and these seem to flourish in the blood without apparently producing any deleterious effects on the host. Plate also records the occurrence of certain enigmatical spherical bodies which were found in the blood or more usually in the cells of the stomach. These bodies generally appeared when the Tardigrades were kept in the same unchanged water for some weeks. Nothing certain is known as to their nature or origin.

Systematic.—A good deal of work has recently been done by Mr. James Murray on the Polar Tardigrades and on the Tardigrades of Scotland, many of which have been collected by the staff of the Lake Survey. Over forty species have been described from North Britain.

The following table of Classification is based on that drawn up by Plate:—

Table of Genera.

I. The claws of the legs are simple, without a second hook. If there are several on the same foot they are alike in structure and size.

A. The legs are short and broad, each with at least two claws.

2–4 claws Gen. 1. ECHINISCUS, C. Sch. (Fig. ).

7–9 claws Sub-gen. 1a. ECHINISCOIDES, Plate.

B. The legs are long and slender; each bears only one small claw.

Gen. 2. LYDELLA, Doy.

II. The claws of the legs are all or partly two- or three-hooked. Frequently they are of different lengths.

A. There are no processes or palps around the mouth.

I. The muscular sucking pharynx follows closely on the mouth-tube.

α. The oral armature consists on each side of a stout tooth and a transversely placed support.

Gen. 3. MACROBIOTUS, C. Sch. (Fig. ).

β. The oral armature consists on each side of a stylet-like tooth without support.

Gen. 4. DOYERIA, Plate.

II. The mouth-tube is separated from the muscular sucking pharynx by a short oesophagus.

Gen. 5. DIPHASCON, Plate (Fig. ).

B. Six short processes or palps surround the mouth, and two others are placed a little farther back.

Gen. 6. MILNESIUM, Doy.

1. Genus ECHINISCUS (= EURYDIUM, Doy.).—The dorsal cuticle is thick, and divided into a varying number of shields, which bear thread- or spike-like projections. The anterior end forms a proboscis-like extension of the body. Two red eye-spots. There are many species, and the number has increased so rapidly in the last few years that specialists are talking of splitting up the genus. E. arctomys, Ehrb.; E. mutabilis, Murray; E. islandicus, Richters; E. gladiator, Murray; E. wendti, Richters; E. reticulatus, Murray; E. oihonnae, Richters; E. granulatus, Doy.; E. spitzbergensis, Scourfield; E. quadrispinosus, Richters; and E. muscicola, Plate, are all British. More than one-half of these species are also Arctic, and E. arctomys is in addition Antarctic. In fact, the group is a very cosmopolitan one. The genus is also widely distributed vertically, specimens being found in cities on the sea level and on mountains up to a height of over 11,000 feet.

1a. Sub-genus ECHINISCOIDES differs from the preceding in the number of the claws, the want of definition in the dorsal plates, and in being marine. The single species E. sigismundi, M. Sch., is found amongst algae in the North Sea (Ostend and Heligoland).

2. Genus LYDELLA.—The long, thin legs of this genus have two segments, and in other respects approach the Arthropod limb. Marine. Plate suggests the name L. dujardini for the single species known.

3. Genus MACROBIOTUS has a pigmented epidermis, but eye-spots may be present or absent. The eggs are laid one at a time, or many leave the body at once. They are either quite free or enclosed in a cast-off cuticle. The genus is divided into many species and shows signs of disruption. They mostly live amongst moss; but M. macronyx, Doy., is said to live in fresh water. The following species are recorded from North Britain: M. oberhäuseri, Doy.; M. hufelandi, Schultze; M. zetlandicus, Murray; M. intermedius, Plate; M. angusti, Murray; M. annulatus, Murray; M. tuberculatus, Plate; M. sattleri, Richters; M. papillifer, Murray; M. coronifer, Richters; M. crenulatus, Richters; M. harmsworthi, Murray; M. orcadensis, Murray; M. islandicus, Richters; M. dispar, Murray; M. ambiguus, Murray; M. pullari, Murray; M. hastatus, Murray; M. dubius, Murray; M. echinogenitus, Richters; M. ornatus, Richters; M. macronyx?, Doy.

4. Genus DOYERIA.—The teeth of this genus have no support, and the large salivary glands of the foregoing genus are absent; in other respects Doyeria, with the single species Doyeria simplex, Plate, resembles Macrobiotus, and is usually to be found in consort with M. hufelandi, C. Sch.

5. Genus DIPHASCON resembles M. oberhäuseri, Doy., but an oesophagus separates the mouth-tube from the sucking pharynx, and the oral armature is weak. The following species are British, the first named being very cosmopolitan, being found at both Poles, in Chili, Europe, and Asia: D. chilenense, Plate; D. scoticum, Murray; D. bullatum, Murray; D. angustatum, Murray; D. oculatum, Murray; D. alpinum, Murray; D. spitzbergense, Murray.

6. Genus MILNESIUM has a soft oral armature, and the teeth open straight into the mouth. A lens can usually be distinguished in the eyes. Two species have been described, M. tardigradum, Doy., British, and M. alpigenum, Ehrb. Bruce and Richters consider that these two species are identical.

Chapter XX. Pentastomida

Occurrence—Economic Importance—Structure—Development and Life-History—Systematic

Pentastomids are unpleasant-looking, fluke-like or worm-like animals, which pass their adult lives in the nasal cavities, frontal sinuses, and lungs of flesh-eating animals, such as the Carnivora, Crocodiles, and Snakes; more rarely in Lizards, Birds, or Fishes. From these retreats their eggs or larvae are sneezed out or coughed up, or in some other way expelled from the body of their primary host, and then if they are eaten, as they may well be if they fall on grass, by some vegetable-feeding or omnivorous animal, they undergo a further development. If uneaten the eggs die. When once in the stomach of the second host, the egg-shell is dissolved and a larva emerges (Fig. , p. 494), which bores through the stomach-wall and comes to rest in a cyst in some of the neighbouring viscera. Here, with occasional wanderings which may prove fatal to the host, it matures, and should the second host be eaten by one of the first, the encysted form escapes, makes its way to the nasal chambers or lungs, and attaching itself by means of its two pairs of hooks, comes to rest on some surface capable of affording nutriment. Having once taken up its position the female seldom moves, but the males, which are smaller than the females, are more active. They move about in search of a mate. Further, should the host die, both sexes, after the manner of parasites, attempt to leave the body. Like most animals who live entirely in the dark they develop no pigment, and have a whitish, blanched appearance.

The only species of Pentastomid which has any economic importance is Linguatula taenioides of Lamarck, which is found in the nose of the dog, and much more rarely in the same position in the horse, mule, goat, sheep, and man. It is a comparatively rare parasite, but occurred in about 10 per cent of the 630 dogs in which it was sought at the laboratory of Alfort, near Paris, and in 5 out of 60 dogs examined at Toulouse. The symptoms caused by the presence of these parasites are not usually very severe, though cases have been recorded where they have caused asphyxia. The larval stages occur in the rabbit, sheep, ox, deer, guinea-pig, hare, rat, horse, camel, and man, and by their wandering through the tissues may set up peritonitis and other troubles.

As in the Cestoda, which they so closely resemble in their life-history, the nomenclature of the Pentastomids has been complicated by their double life. For long the larval form of L. taenioides was known by different names in different hosts, e.g. Pentastoma denticulatum, Rud., when found in the goat, P. serratum, Fröhlich, when found in the hare, P. emarginatum when found in the guinea-pig, and so on. In the systematic section of this article some of the species mentioned are known in the adult state, some in the larval, and in only a few has the life-history been fully worked out.

Structure.—The body of a Pentastomid is usually white, though in the living condition it may be tinged red by the colour of the blood upon which it lives. The anterior end, which bears the mouth and the hooks (Fig. ), has no rings; this has been termed the cephalothorax. The rest of the body, sometimes called the abdomen, is ringed, and each annulus is divided into an anterior half dotted with the pores of certain epidermal glands and a hinder part of the ring in which these are absent.

On the ventral surface of the cephalothorax, in the middle line, lies the mouth, elevated on an oral papilla, and on each side of the mouth are a pair of hooks whose bases are sunk in pits. The hooks can be protruded from the pits, and serve as organs of attachment. Their shape has some systematic value.

There are a pair of peculiar papillae which bear the openings of the “hook-glands,” lying just in front of the pairs of hooks, and other smaller papillae are arranged in pairs on the cephalothorax and anterior annuli. The entire body is covered by a cuticle which is tucked in at the several orifices. This is secreted by a continuous layer of ectoderm cells. Some of these subcuticular cells are aggregated together to form very definite glands opening through the cuticle by pores which have somewhat unfortunately received the name of stigmata. Spencer attributes to these glands a general excretory function. There is, however, a very special pair of glands, the hook-glands, which extend almost from one end to the other of the body; anteriorly these two lateral glands unite and form the head-gland (Fig. ). From this on each side three ducts pass, one of which opens to the surface on the primary papilla; the other two ducts open at the base of the two hooks which lie on each side of the mouth. Leuckart has suggested that these important glands secrete some fluid like the irritating saliva of a Mosquito which induces an increased flow of blood to the place where it is of use to the parasite. Spencer, however, regards the secretion as having, like the secretion of the so-called salivary cells of the Leech, a retarding action on the coagulation of the blood of the host.

The muscles of Pentastomids are striated. There is a circular layer within the subcuticular cells, and within this a longitudinal layer and an oblique layer which runs across the body-cavity from the dorso-lateral surface to the mid-ventral line, a primitive arrangement which recalls the similar division of the body-cavity into three chambers in Peripatus and in many Chaetopods. Besides these there are certain muscles which move the hooks and other structures.

The mouth opens into a pharynx which runs upwards and then backwards to open into the oesophagus (Fig. ). Certain muscles attached to these parts enlarge their cavities, and thus give rise to a sucking action by whose force the blood of the host is taken into the alimentary canal. The oesophagus opens by a funnel-shaped valve into the capacious stomach or mid-gut, which stretches through the body to end in a short rectum or hind-gut. The anus is terminal.

There appears to be no trace of circulatory or respiratory organs, whilst the function usually exercised by the nephridia or Malpighian tubules or by coxal glands, of removing waste nitrogenous matter, seems, according to Spencer, to be transferred to the skin-glands.

The nervous system is aggregated into a large ventral ganglion which lies behind the oesophagus. It gives off a narrow band devoid of ganglion-cells, which encircles that tube. It also gives off eight nerves supplying various parts, and is continued backward as a ninth pair of prolongations which, running along the ventral surface, reach almost to the end of the body (Fig. ). The only sense-organs known are certain paired papillae on the head, which is the portion that most closely comes in contact with the tissues of the host.

Pentastomids are bisexual. The males are as a rule much less numerous and considerably smaller than the females, although the number of annuli may be greater.

The ovary consists of a single tube closed behind. This is supported by a median mesentery. Anteriorly the ovary passes into a right and left oviduct, which, traversing the large hook-gland, encircle the alimentary canal and the two posterior nerves (Fig. ). They then unite, and at their point of union they receive the ducts of the two spermathecae, usually found packed with spermatozoa. Having received the orifices of the spermatheca, the united oviducts are continued backward as the uterus, a highly-coiled tube in which the fertilised eggs are stored. These are very numerous; Leuckart estimated that a single female may contain half a million eggs. The uterus opens to the exterior in the mid-ventral line a short distance—in P. teretiusculus on the last ring but seven—in front of the terminal anus. In L. taenioides the eggs begin to be laid in the mucus of the nose some six months after the parasite has taken up its position.

The testis is a single tube occupying in the male a position similar to that of the ovary in the female. Anteriorly it opens into two vesiculae seminales, which, like the oviducts, pierce the hook-glands and encircle the alimentary canal (Fig. ). Each vesicula passes into a vas deferens with a cuticular lining. Each vas deferens also receives the orifice of a muscular caecal ejaculatory duct, which, crowded with mature spermatozoa, stretches back through the body. Anteriorly the vas deferens passes into a cirrus-bulb, which is joined by a cirrus-sac on one side and a dilator-rod sac on the other, structures containing organs that assist in introducing the spermatozoa into the female. The two tubes then unite, and having received a dorsally-placed accessory gland, open to the exterior by a median aperture placed ventrally a little way behind the mouth.

Life-history.—The egg undergoes a large portion of its development within the body of the mother. In Linguatula taenioides, which lives in the nasal cavities of the dog, the eggs pass away with the nasal excretions. If these, scattered about in the grass, etc., be eaten by a rabbit, the egg-shell is dissolved in the stomach of the second host and a small larva is set free. In Porocephalus proboscideus and others, which inhabit the lungs of snakes, the eggs pass along the alimentary canal and leave the body with the faeces. They also must be eaten by a second host if development is to proceed.

The larva which emerges when the egg-shell is dissolved has a rounded body provided with two pairs of hooked appendages, and a tail which is more or less prominent in different species (Figs. , 260). Each appendage bears a claw, and is strengthened by a supporting rod or skeleton. Anteriorly the head bears a boring apparatus of several chitinous stylets. The various internal organs are in this stage already formed, though in a somewhat rudimentary state, and it is doubtful if the anus has yet appeared.

By means of its boring apparatus, and aided by its hooked limbs, the larva now works its way through the stomach-walls of its second host, and comes to rest in the liver or in some other viscus. Its presence in the tissues of its second host causes the formation of a cyst, and within this the larva rests and develops. In man, at least, the cysts often undergo a calcareous degeneration, and Virchow states “dass beim Menschen das Pentastomum am häufigsten von allen Entozoen zu Verwechselungen mit echten Tuberkeln Veranlassungen giebt.” The larva moults several times, and loses its limbs, which seem to have no connexion with the paired hooks in the adult (Fig. ). The internal organs slowly assume the form they possess in the adult. The larva is at first quite smooth, but as it grows the annulations make their appearance, arising in the middle and spreading forward and backward (Fig. ). In this encysted condition the larva remains coiled up for some months, according to Leuckart; six in the case of L. taenioides, and a somewhat shorter period, according to Stiles, in the case of P. proboscideus.

The frequency of what used to be called Pentastoma denticulatum (= the larval form of L. taenioides) in the body of man depends on the familiarity of man with dogs. Klebs and Zaeslin found one larva in 900 and two in 1914 autopsies. Laenger found the larva fifteen times in about 400 dissections, once in the mesentery, seven times in the liver, and seven times in the wall of the intestine. After remaining encysted for some time it may escape, and begins wandering through the tissues, aided by its hooks and annulations, a proceeding not unaccompanied by danger to its host. Should the latter be eaten by some carnivorous animal, the larva makes its way into the nasal cavities or sinuses, or into the lungs of the flesh-eating creature, and there after another ecdysis it becomes adult. If, however, the second host escapes this fate, the larvae re-encyst themselves, and then if swallowed they are said to bore through the intestine of the flesh-eater, and so make their way to their adult abode.

Systematic.—The Pentastomida are a group much modified by parasitism, which has so deeply moulded their structure as to obscure to a great extent their origin and affinities. The larva, with its clawed limbs, recalls the Tardigrades and certain Mites, e.g. Phytoptus, where only two pairs of limbs persist, and where the abdomen is elongated and forms a large proportion of the body. The resemblances to a single and somewhat aberrant genus must not, however, be pressed too far. The striated muscles, the ring-like nature of the reproductive organs and their ducts, perhaps even the disproportion both in size and number of the females to the males, are also characters common to many Arachnids.

The Pentastomida include three genera, Linguatula, Fröhlich, Porocephalus, Humboldt, and Reighardia, Ward. The first two were regarded by Leuckart as but sub-genera, but Railliet and Hoyle have raised them to the rank of genera. They are characterised as follows:—

Linguatula, body flattened, but dorsal surface arched; the edges of the fluke-like body crenelated; the body-cavity extends as diverticula into the edges of the body.

Porocephalus, body cylindrical, with no diverticula of the body-cavity.

Reighardia, devoid of annulations, transparent, with poorly developed hooks and a mouth-armature.

The following is a list of the species with their primary and secondary or larval hosts:—

i. Linguatula pusilla, Diesing, found in the intestine of the fresh-water fish Acara, a South American genus of the Cichlidae. This is possibly the immature form of L. subtriquetra.

ii. L. recurvata, Diesing, found in the frontal sinuses and the trachea of Felis onca.

iii. L. subtriquetra, Diesing, found in the throat of Caiman latirostris and C. sclerops, perhaps the mature form of L. pusilla.

iv. L. taenioides, Lamarck, found in the frontal sinuses and nasal chambers of the dog and ounce, and in the nasal cavities of the wolf, fox, goat, horse, mule, sheep, and man, and in the trachea of the ounce. The immature form has been found in or on the liver of the cat, guinea-pig, and horse; in the lungs of the ox, cat, guinea-pig, porcupine, hare, and rabbit; in the liver and connective tissue of the small intestine of man; and in the mesenteric glands of the ox, camel, goat, sheep, antelope, fallow-deer, and mouse.

v. Porocephalus annulatus, Baird, found in the lungs of the Egyptian cobra, Naja haje; the immature form is thought to live encapsuled in a species of Porphyrio and in the Numidian Crane.

vi. P. aonycis, Macalister, from the lungs of an Indian otter taken in the Indus.

vii. P. armillatus, Wyman, found in the adult state in the lungs of certain African pythons, and in the lion; in the larval form it occurs encysted in the abdomen of the Aard-wolf, the mandril, and man—usually in negroes. Its migrations in the body of its second host sometimes cause fatal results.

viii. P. bifurcatus, Diesing, found in the body-cavity of certain snakes, and in the lungs of boa-constrictors and the legless lizard, Amphisbaena alba. Possibly an immature form.

ix. P. clavatus, Lohrmann, found in the lungs of the Monitor lizard.

x. P. crocidura, Parona, found in the peritoneum of the “musk-rat” Crocidura in Burmah. Probably a larval form.

xi. P. crotali, Humboldt, found in the lungs, body-cavity, kidneys, spleen, and mesentery of many snakes and lizards, and of the lion and leopard. The immature forms occur in the liver and abdominal cavity of species of opossum, armadillo, mouse, raccoon, bat, and marmoset.

xii. P. geckonis, Dujardin, found in the lungs of a Siamese gecko.

xiii. P. gracilis, Diesing, found free in the body-cavity or encapsuled on the viscera and mesenteries of South American fishes, snakes, and lizards,

xiv. P. heterodontis; Leuckart, found encapsuled in the abdominal muscles and mesentery of a species of Heterodon.

xv. P. indicus, v. Linst., found in the trachea and lungs of Gavialis gangeticus.

xvi. P. lari, Mégnin, found in the air-sacs of the Burgomaster or Glaucous gull, Larus glaucus of the Polar seas.

xvii. P. megacephalus, Baird, found embedded in the flesh of the head of an Indian crocodile, C. palustris, the “Mugger.” Probably a larval form.

xviii. P. megastomus, Diesing, found in the lungs of a fresh-water tortoise, Hydraspis geoffroyana.

xix. P. moniliformis, Diesing, found in the lungs of pythons.

xx. P. najae sputatricis, Leuckart, found encapsuled in the abdominal muscles and peritoneum of the cobra, Naja tripudians. Probably a larval form.

xxi. P. oxycephalus, Diesing, found in the lungs of crocodiles and alligators.

xxii. P. platycephalus, Lohrmann, habitat unknown.

xxiii. P. subuliferus, Leuckart, in the lungs of the cobra Naja haje.

xxiv. P. teretiusculus, Baird, found in the lungs and mouth of certain Australian snakes.

xxv. P. tortus, Shipley, found in the body-cavity of a snake, Dipsadomorphus irregularis, taken in New Britain.

xxvi. Reighardia, sp., Ward, found in the air-sacs of Bonaparte’s gull and the common North American tern.