CHAPTER XVII. Arachnida Embolobranchiata (_continued_)—Podogona—Phalangidea =

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.

FIG. 228.—Cryptocellus simonis, × 4. (After Hansen and Sörensen.) ]

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.

FIG. 229.—Oligolophus spinosus. (After Pickard-Cambridge.) ]

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.

FIG. 230.—Hood of Metopoctea. (After Simon.) ]

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.

FIG. 231.—Mouth-parts of Phalangium. =A=, =B=, =C=, Gnathobases of pedipalp and first and second legs; ch, chelicera; ep, epistome; lab, labium; m, mouth; ped, pedipalp; pre.ep, pre-epistome; st, sternum, shown by the removal of the anterior part of the genital process, which extends to the dotted line; 1, 2, 3, 4, legs. ]

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.

FIG. 232.—Nervous and respiratory systems of a Phalangid. Nerves black, tracheae white. c.g, Cerebral ganglion; g′, g″, g‴, ganglia supplying viscera; m.n, median abdominal nerve; oe, passage for oesophagus; st, stigma; th.g, thoracic ganglion; tr, main trunk of tracheae. ]

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.

FIG. 233.—Parasiro corsicus, enlarged. (After Simon.) ]

=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.

FIG. 234.—Phalangodes terricola, enlarged. (After Simon.) ]

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.

FIG. 235.—Sclerosoma quadridentatum. (After Pickard-Cambridge.) ]

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.

FIG. 236.—Oligolophus spinosus. (After Pickard-Cambridge.) ]

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.

FIG. 237.—Nemastoma lugubre. ]

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.”

FIG. 238.—Trogulus aquaticus. a, Hood. (After Simon.) ]

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. 230, 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.