CHAPTER XIII

ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ARANEAE—EXTERNAL STRUCTURE— INTERNAL STRUCTURE.

=Order III. Araneae.=

(ARANEIDA, ARANEINA.)

Arachnida breathing by tracheae and “lung-books.” Cephalothorax and pedicellate abdomen, the latter usually soft, and only very rarely showing any traces of segmentation. Two-jointed non-chelate chelicerae, the distal joint bearing the orifice of a poison-gland. The tarsal joint of the male pedipalp develops a sexual organ. The abdomen is furnished with spinning mammillae.

The true Spiders can readily be distinguished from allied Arachnid groups, with which they are often popularly confounded, by the presence of a narrow constriction or “waist” between the cephalothorax and abdomen, and of a group of “spinnerets” or external spinning organs beneath the hind portion of the body. Thus the so-called “Harvest-spider” or “Harvestman” is clearly not a Spider, for there is no constriction of its body into two parts, nor does it possess any spinnerets. It belongs to the Phalangidea. The same considerations will exclude the “Red Spider” of popular nomenclature, which must be referred to the Acarina or Mites.

The Araneae, even as at present known, form a very extensive and widely-distributed order of animals. Compared with certain insect orders, they have received little attention from the collector, and the number of known forms is certain to be very largely increased. They form an extremely compact and natural group, for though, within the order, there is an infinite variety of detail, their uniformity in essential points of structure is remarkable, and they are sharply marked off from the neighbouring groups of Arachnida.

FIG. 172.—Epeira angulata. ♀. ]

It is perhaps unfortunate that the obtrusiveness of particularly unattractive specimens of the race has always caused spiders to be regarded with more or less aversion. This prejudice can hardly fail to be modified by a wider acquaintance with these animals. There are certainly few groups which present points of greater interest in respect to their adaptation to special modes of life and the ingenuity displayed in the construction of their nests and the ensnaring of their prey.

Spiders are wingless, yet they may often be observed travelling through the air. They are air-breathing, yet many are amphibious in their habits, and one species at least spends the greater part of its existence beneath the surface of the water. On land they may be found in all imaginable localities which admit of the existence of that insect life on which they depend for food.

=External Structure.=—The spider’s body consists of two portions, the cephalothorax and the abdomen.

=Cephalothorax.=—Looked at dorsally (Fig. 173), the cephalothorax is generally seen to have a depression near the middle, the “median fovea,” and from this certain lines, the “radial striae,” radiate towards the sides. These depressions indicate the attachment of internal muscles.

The head region or “caput” lies in front of the foremost of the radial striae, and is often clearly marked off from the thorax, and different from it in elevation. It bears the eyes, which, in the great majority of spiders, are eight in number. Many, however, are six-eyed, while in rare cases the number is reduced to four (Tetrablemma, see p. 404), or even to two (Nops, see p. 395). The number, relative size, and particular arrangement of these eyes are of considerable systematic importance. Their disposition varies very greatly, but it is generally possible to regard them as forming two transverse rows, an anterior and a posterior, each possessing a pair of median and a pair of lateral eyes.

FIG. 173.—Diagrammatic dorsal view of a Spider. ch, Chelicera; f, median fovea; n, normal marking; o, ocular area; p, pedipalp; st, stria. (The dotted line should reach the radial marking on the cephalothorax.) ]

In many spiders all the eyes have a dorsal aspect, but in some groups (Attidae, Lycosidae) the prevailing arrangement is to have the anterior eyes directed forwards and the posterior upwards. In other spiders, again, a dorsal view may only show the eyes in profile, all having their axes directed forwards or sideways, or they may be mounted on turrets, and thus command a wide range of view. The rows are described as straight, “procurved” (with the convexity backwards), or “recurved” (with the convexity forwards). Thus, in Fig. 177, the anterior row is slightly, and the posterior row considerably “recurved.”

Sometimes there is a marked difference in the colour of the eyes, two or more being black, while the remainder are pearly white. In other cases they are homogeneous, either of the black or the white type. Simon considers the black eyes to be diurnal and the white nocturnal, but the evidence for this is indirect and not altogether satisfactory. The portion of the caput occupied by the eyes is often alluded to as the “ocular area.” The space between the ocular area and the chelicerae, well shown in Fig. 177, is known as the “clypeus.” It is usually more or less vertical, but in the Aviculariidae (see p. 386) it is horizontal and dorsal.

The under surface of the cephalothorax is protected by the “sternum” or “plastron,” a large plate of variable shape, usually notched at either side for the reception of the legs, and having in front a small plate, generally hinged, but sometimes soldered to it, known as the “labium.” This has no homology with the labium of insects, but is a true sternite, more correctly described as “pars labialis sterni.”

The labium and the maxillary lobes of the palpi more or less conceal the under surface of the caput. The shape of the sternum and of the labium, and the contour and degree of inclination towards one another of the maxillae, are important considerations in the taxonomy of Spiders.

FIG. 174.—Diagrammatic ventral view of a Spider. Cephalothorax—l, Labium; m, maxilla; p, paturon of chelicera; st, sternum; u, unguis of chelicera. Abdomen—a.t, Anal tubercle; c, colulus; ep, epigyne; s, stigma; sp, spinnerets; tr, tracheal opening. ]

The appendages of the cephalothorax, which are the chelicerae or jaws, the pedipalpi or feelers, and the four pairs of ambulatory legs, will be treated separately.

=Pedicle.=—The chitinous investment of the narrow stalk which unites the thorax with the abdomen is for the most part thin and flexible, with only slight indurations of various patterns on the dorsal surface, where it is in most cases more or less protected by the forwardly-projecting abdomen. Beneath, it is usually quite membranous, guarded only by a sort of collar formed by the raised border of the anterior portion of the abdomen at the point of insertion. In some Spiders, however (Dysderidae), there is a posterior sternal plate, the “plagula,” closely corresponding with the labium in front, which partly embraces the pedicle. In Hermippus (Zodariidae) the plagula is detached from the sternum, and is succeeded posteriorly by two smaller paired plates.

=Abdomen.=—The abdomen differs remarkably in shape in the different groups of Spiders. In some families the prevailing shape is more or less globular, and in others cylindrical, while it may be diversified to almost any extent by prominences or spines. Ordinarily no sign of segmentation is observable, but in Liphistius it is covered dorsally by seven well-marked chitinous plates.

In most Spiders the integument of the abdomen is uniformly soft and flexible all over, but it is not rare to find portions of it thickened and hardened to form “scuta.” In the Gasteracanthinae and the Phoroncidinae there is a great dorsal scutum armed with spines, while in several families there are species characterised by the possession of a smooth dorsal scutum; and in some a ventral scutum is present.

FIG. 175.—Spider profiles. 1, Poltys ideae; 2, Phoroncidia 7–aculeata; 3, Ariamnes flagellum; 4, Stegosoma testudo; 5, Formicinoides brasiliana. ]

That these scuta are sometimes indicative of an obsolete segmentation would seem likely from the study of the remarkable species, Tetrablemma medioculatum (Fig. 176), described by Pickard-Cambridge, from Ceylon. In addition to large dorsal and ventral scuta, the sides and posterior extremity are guarded by smaller scuta, the disposition of which is well seen in the figure.

FIG. 176.—Tetrablemma medioculatum, much enlarged. =A=, Posterior view; =B=, profile, showing the scuta. (After Cambridge.) ]

The normal smooth abdomen presents dorsally no very striking features. In species of variegated coloration there is very generally noticeable a median dentated band (Fig. 173), the “normal marking” of some writers, which would appear to have some correlation with the underlying dorsal vessel. Beneath the abdomen are to be seen the orifices of the breathing and genital organs, the spinnerets, and the anal aperture upon its tubercle.

The breathing organs are, as will be explained later, of two kinds, lung-books and tracheae. The great majority of Spiders possess only two lung-books, and their transverse, slit-like openings (“stigmata” or “spiracles”) may be seen on either side of the anterior part of the abdomen. Where, as in the Theraphosae, there are four lung-books, the second pair open by similar slits a short distance behind the first. According to Bertkau, pulmonary sacs are entirely lacking in the genus Nops.

The tracheae generally debouch by a single median stigma towards the posterior end of the abdomen, just in front of the spinnerets. This opening clearly results from the fusion of two stigmata, which in some species retain their paired arrangement.

On a level with the openings of the anterior lung-books or pulmonary sacs there is usually observable a slight transverse ridge, the epigastric fold (Fig. 174), and in the centre of this is the genital opening. This is never visible until after the last moult, and in the male is always a simple inconspicuous aperture. This is also the case with the females of some groups (Theraphosae, Filistatidae, Dysderidae, etc.), but in most cases there is a more or less complicated armature, the “epigyne,” the special design of which is of great specific value. In its simplest form it is merely a plate, usually of dark colour, with one or two apertures (Fig. 174, ep), but in some families, notably the Epeiridae, it is more complicated, and is furnished with a hooked median projection, the “ovipositor” (“clavus” of Menge), which is often absurdly like a petrified elephant’s trunk in miniature.

The abdomen also presents on its under surface, usually towards the posterior end or apex, a group of finger-like mammillae or spinnerets. They are normally six in number, two superior (or posterior), two median, and two inferior (or anterior). The number is reduced, in most of the Theraphosae, to four, while a few spiders possess only a single pair of spinnerets. These organs are described more fully on p. 325.

A small papilla, the “colulus” (Fig. 174, c), is often observable, projecting between the anterior spinnerets. The “anal tubercle” (Fig. 174, a.t), on which the vent is situated, terminates the abdomen, and is generally in close juxtaposition with the posterior spinnerets.

=Appendages.=—The cephalothoracic appendages are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen are the mammillae or spinnerets.

=Chelicerae.=—These are two-jointed appendages, articulated immediately below or in front of the clypeus. They are the “mandibles” of many authors, but there is good reason for believing that they are not homologous with the mandibles of Insects. There is little agreement, moreover, with regard to the names given to the two joints of which they consist. The term “falx,” often applied to the basal joint, is much more appropriate to the sickle-like distal joint. Base and fang are tolerably satisfactory, or we may avoid ambiguity by adopting the terms “paturon” and “unguis” suggested by Lyonnet.

The paturon is a stout joint of more or less cylindrical or conical shape. The unguis (the “crochet” of Simon) is hook-like, and can generally be folded back upon the paturon, which often presents a groove for its reception. The Theraphosid spiders are distinguished from all others by the fact that the plane of action of the chelicerae is vertical and longitudinal. The paturon projects forward in a line parallel with the axis of the body, and its distal end can be raised or depressed, but not moved laterally; while the unguis in action has the point directed downwards, and, at rest, is applied to the under surface of the paturon.

In other spiders the patura hang more or less vertically, and while to some extent mobile in all directions, their principal motion is lateral, and the ungues have their points directed towards each other in action, and are applied to the inner surfaces of the patura in repose. The plane of action in this case is also more or less vertical, but transverse.

FIG. 177.—Front view of Textrix denticulata. × about 10. 1, Caput; 2, eyes; 3, paturon; and 4, unguis of chelicera. ]

The paturon is always extremely hard and strong. In Theraphosae of burrowing habits the distal end is furnished with a group of powerful teeth, the “rastellus.” The groove for the reception of the unguis is often guarded on one side or on both by rows of teeth, the arrangement of which is frequently an important specific character. The inner anterior border is also often furnished with a group of stiff hairs or bristles. This powerful joint is of use in crushing and expressing the fluids of insects pierced by the ungues.

The crescent-shaped unguis is tapering and smooth, except for the presence, on the posterior surface, of one or two feebly dentated ridges. Near its free extremity there is a small orifice leading to the poison reservoir and gland.

In the genus Pholcus (see p. 401) the chelicerae may almost be regarded as chelate, the unguis being met by a spiny projection from the inner anterior border of the paturon.

=Rostrum.=—On examining a spider, even under a dissecting microscope, it will not be easy at first to discover the mouth. Indeed, Lyonnet had almost come to the conclusion that Spiders, like some Myrmelionid larvae, imbibed the juices of their prey by way of the mandibles, before he found the orifice and gave a remarkably accurate description of the adjacent parts.

If a specimen be placed on its back, and the labium raised while the chelicerae are pushed forward, no orifice is visible, but on careful examination it will be found that what appears to be a thick and fleshy labium is, in reality, two organs. The labium is thin and flat, and closely opposed to its upper surface is a somewhat flattened cone. This is the “rostrum,” and when it is separated from the labium the buccal orifice is disclosed. In a few spiders (Archeidae) in which the chelicerae are far removed from the mouth, the rostrum is tolerably conspicuous, but in most it is so hidden as to have escaped the observation of the great majority of observers. Schimkewitsch considers it homologous with the labrum of insects, but Simon thinks that it represents all the insect mouth-parts reduced to an exceedingly simple form. It is more probable that a beak consisting of a simple labrum and labium was a primitive Arachnid characteristic. If the rostrum be removed and its inner (or posterior) surface examined, a lance-shaped chitinous plate, the “palate,” becomes visible. It is furrowed down the middle by a narrow groove, which is converted into a tube for the passage of fluids when the rostrum is opposed to the labium.

FIG. 178.—Pedipalp of Tegenaria domestica ♂, × 5. 1, Coxa; 2, maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus; 8, palpal organ. ]

=Pedipalpi.=—The pedipalpi are extremely leg-like feelers, and are six-jointed, the metatarsal joint of the ambulatory legs being absent. The joints, therefore, are the coxa, trochanter, femur, patella, tibia, and tarsus (Fig. 178).

In the Theraphosae the coxa resembles that of the ambulatory leg, but in other spiders it is furnished, on the inner side, with a blade-like projection, the “maxilla” (Fig. 178). The shape of the maxillae and the degree of their inclination towards the labium are of considerable taxonomic importance. The inner border of the maxilla is furnished with a tuft of hairs, which assist in retaining the juices expressed by the chelicerae, and its anterior border presents a cutting edge with a finely dentated ridge called the “serrula.”

In the female, and in the immature male, the remaining joints differ little from those of the legs, except that the tarsal joint is either clawless or has a single claw, which is generally smooth, and is never much dentated.

At the last moult but one the male pedipalp appears tumid at the end, and after the last moult the tarsus is seen to have developed a remarkable copulatory apparatus, the “palpal organ,” comparatively simple in some families, but in others presenting an extraordinary complexity of structure.

FIG. 179.—Diagram of palpal organ. 1, Tarsus; 2, bulb; 3, receptaculum seminis; 4, its aperture; 5, style; 6, haematodocha; 7, alveolus; 8, tibia. ]

=Palpal Organs.=—Externally the essential parts of the palpal organ are three, the “haematodocha,” the “bulb,” and the “style.” The spines and projections, or “apophyses,” which often accompany the palpal organ proper, are of secondary importance, and in many spiders are entirely absent; nor is their function when present at all clear; but the infinite variety of design which they exhibit, and their singular uniformity in all the males of a species, render them of the utmost value as specific characteristics.

The “haematodocha” is the portion of the palpal organ attached to the tarsus, and often received into an excavation, the “alveolus,” on its under surface. It is a fibro-elastic bag, in its normal collapsed state usually somewhat spirally disposed round the base of the following portion, the “bulb.”

The bulb is generally the most conspicuous portion of the organ, and is a sub-globular sac with firm, though often semi-transparent, integument. Its base rests upon the haematodocha, and its apex is produced, often spirally, to a point which bears the seminal orifice. This external opening leads into a coiled tube within the bulb, ending in a blind sac, the “receptaculum seminis,” which projects into the haematodocha; and it is the aperture by which the sperm both enters and leaves the organ. How the sperm is conveyed to the receptaculum was long a matter for speculation, after the belief in a direct communication between the generative glands and the pedipalpi had been abandoned. The process has been actually observed in the case of a few spiders, which have been seen to deposit their sperm on a small web woven for the purpose, and then, inserting the styles of their palpal organs into the fluid, to suck it up into the receptacula seminis. This is probably the usual method of procedure, though it may be true, as some have asserted, that the palp is sometimes applied directly to the genital orifice.

The receptaculum and its tube being thus charged with sperm, it is the function of the haematodocha to eject it by exerting pressure on its base. For this purpose the haematodocha is in communication with the cavity of the tarsus, from which, in copulation, it receives a great flow of blood, and becomes greatly distended. Bertkau believes that he has detected very minute pores (meatus sanguinis) communicating between the haematodocha and the receptaculum, and allowing some of the blood-plasma from the former to mingle with the semen, but this appears to be very doubtful.

The =Legs= are uniformly eight in number, and are seven-jointed, the joints, counting from the body, being the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. In a few cases, through the presence of false articulations, i.e. rings of softer chitin, this number appears to be exceeded. Some of the Palpimanidae (see p. 398) were at first thought to have only six joints on their anterior legs, but the tarsus is present, though very small.

In the case of most spiders, the legs take a general fore and aft direction, the first pair being directed forwards, the second forwards or laterally, and the third and fourth backwards. In the large group of “Crab-spiders” (Thomisidae), and in many of the Sparassinae, all the legs have a more or less lateral direction, and the spider moves with equal ease forwards, backwards, or sideways. The legs are usually more or less thickly clothed with hairs, but in some genera the clothing is so sparse that they appear glossy, while in others they have a positively shaggy appearance. Stouter hairs or “bristles” are often present, and some of the joints are also often furnished with “spines,” which in many cases are erectile.

The tarsi of all spiders are furnished with terminal claws, usually three in number, though in some families (Drassidae, Thomisidae, etc.) there are only two. The two principal claws are paired and usually dentated, though the number of their teeth may be unequal. The third claw, when present, is always smaller, median, and inferior.

FIG. 180.—Spider tarsi. 1, Tarsus of Epeira showing three claws and supplemental serrate hairs (a); 2, tarsus of a Thomisid Spider, with two claws; 3, 3a, lateral and dorsal view of tarsus of an Attid Spider, showing scopula at b. ]

In many spiders of climbing habits the place of the third claw is taken by a remarkable tuft of club-like hairs termed a “scopula” (Fig. 180, b), by means of which they are able to cling to smooth surfaces where claws would be able to obtain no hold. In some species there is a special false articulation—the “onychium”—at the end of the tarsus to bear the claws.

In the Cribellatae the metatarsus is always furnished with a comb-like organ, the “calamistrum,” correlated with an extra spinning apparatus, the “cribellum,” but this will be dealt with when we reach the systematic portion of the subject.

The general direction taken by the legs, the comparative length of the different joints, their armature of hairs, bristles, and spines, and the number and conformation of the tarsal claws, are points of great importance in the classification of Spiders.

Under considerable magnification the legs of all Spiders exhibit a number of minute organs, arranged with absolute uniformity throughout the Araneae, and known as the “lyriform organs.” They consist of little parallel ridges of thickened chitin, the slit between them being covered by thinner chitin. They are eleven on each leg, and are distributed near the distal extremities of each of the first six joints. Their function is unknown, though some authors consider them to be organs of hearing.

FIG. 181.—Spinnerets of Epeira diademata. =A=, Ventral view of Epeira; =B=, spinnerets magnified; =C=, profile. ]

The =Spinnerets= are normally six in number, and, except in rare instances, are placed beneath the abdomen, near its apex and immediately in front of the anal tubercle. Their arrangement varies greatly, but they can generally be recognised as comprising three pairs, a posterior (or superior) pair, a median pair, and an anterior (or inferior) pair.

In nearly all the Theraphosae the anterior pair are absent, while the posterior spinnerets are largely developed. In the Palpimanidae only the anterior spinnerets are present. When all six are found, the usual arrangement is in the form of a rosette, the median spinnerets being hidden by the others in repose, but this disposition is widely departed from. In Hahnia (Agelenidae), for instance, they are ranged in a transverse row at the end of the abdomen, the posterior spinnerets occupying the extremities of the row, and the median ones the centre.

These spinnerets are highly mobile appendages, and additional play is given to their action by the presence of articulations, much resembling the “false” joints sometimes found on the legs, on the posterior and anterior pairs. They are always at least bi-articulate, and sometimes present three or four joints. They are movable turrets on which are mounted the “fusulae” or projections where the tubes from the spinning glands open. These are often very numerous, especially in the orb-weaving spiders, where the spinning powers are most highly developed. They consist of two portions, a cylindrical or conical basal part, succeeded by a very fine, generally tapering tube.

In some spiders the fusulae are all much alike, but usually a few very much larger than the rest are noticeable under the microscope, and these are often alluded to as “spigots.” The smaller ones are also divisible into two kinds, a few short conical fusulae being noticeable amongst the much more numerous cylindrical tubes. We shall treat of the functions of the various fusulae later (see pp. 335 and 349).

Simon remarks that though the battery of fusulae is most complicated in those spiders which possess the greatest spinning powers, it is by no means among them that extremely long spinnerets are developed. The posterior spinnerets of some of the Hersiliidae are of great length, but these spiders spin very little except in forming their egg-cocoons.

FIG. 182.—=A=, Spinnerets of Amaurobius similis ♀. Much enlarged. a, Anus; cr, cribellum; i.s, inferior spinneret; m.s, median spinneret; s.s, superior spinneret. =B=, Part of the 4th leg of the same Spider, showing the calamistrum (ca) on the metatarsus. ]

In addition to the six spinnerets, and just in front of them, there is to be found in some spiders an extra spinning organ in the form of a double sieve-like plate, the “cribellum.” This is always correlated with a comb of curved bristles on the metatarsi of the fourth pair of legs, the “calamistrum.” Such importance is assigned to these organs by Simon, that the Araneae Veraeare divided by him according to whether they are present or absent, into CRIBELLATAE and ECRIBELLATAE. This is probably an exaggerated view of the importance of these organs, and the spiders possessing them certainly do not seem to form a natural group.

=Stridulating Organs.=—When Arthropod animals are capable of producing a sound, the result is nearly always obtained by “stridulation,” that is, by the friction of two rough surfaces against each other. The surfaces which are modified for this purpose form what is called a “stridulating organ.” Such organs have been found in three very distinct Spider families, the Theridiidae, the Sicariidae, and the Aviculariidae. Hitherto they have only been observed in three positions—either between the thorax and abdomen, or between the chelicerae and the pedipalpi, or between the pedipalpi and the first legs.

In the Sicariidae and the Aviculariidae, the sounds have been distinctly heard and described. Those produced by the Theridiidae would appear to be inaudible to human ears.

FIG. 183.—Stridulating apparatus of Steatoda bipunctata, ♂. Much enlarged. =A=, Ridged and toothed abdominal socket; =B=, striated area on the cephalothorax; =C=, profile of the Spider, × 5. ]

Westring was the first to discover (1843) a stridulating organ in the small Theridiid spider Asagena phalerata. The abdomen, where the pedicle enters it, gives off a chitinous collar, which projects over the cephalothorax, and has the inner surface of the dorsal part finely toothed. When the abdomen is raised and depressed, these teeth scrape against a number of fine striae on the back of the posterior part of the cephalothorax. A similar organ has been since found in various allied spiders, of which the commonest English species is Steatoda bipunctata. In this group it is generally possessed by the male alone, being merely rudimentary, if present at all, in the female.

In 1880 Campbell observed that in some of the Theridiid Spiders of the genus Lephthyphantes, the outer surface of the chelicera and the inner surface of the femur of the pedipalp were finely striated at the point, where they were rubbed together when the palps were agitated, but though the appropriate motion was frequently given, he could hear no sound.

FIG. 184.—Chilobrachys stridulans in stridulating attitude. After Wood-Mason. Natural size. ]

Meanwhile the noise produced by a large Theraphosid spider in Assam (Chilobrachys stridulans) had attracted attention, and its stridulating apparatus was described in 1875 by Wood-Mason. The sound resembled that obtained by “drawing the back of a knife along the edge of a strong comb.”

Subsequently certain Sicariid spiders of a genus confined to the southern hemisphere were heard to produce a sound like the buzzing of a bee by the agitation of their palps, and both sexes were found to possess a very perfect stridulating organ, consisting of a row of short teeth on the femur of the pedipalp, and a striated area on the paturon of the chelicera.

Pocock has recently discovered that all the large kinds of Theraphosidae in the countries between India and New Zealand are, like Chilobrachys, provided with a stridulating organ. In these spiders also it is between the palp and the chelicera, and consists of a row of teeth or spines constituting a “pecten,” and a series of vibratile spines or “lyra,” but whereas in Chilobrachys and its near relations the lyra is on the palp and the pecten on the paturon, in other spiders the positions are reversed. The lyra is a very remarkable organ, consisting of club-shaped, often feathery bristles or spines, which lie parallel to the surface to which they are attached, and which is slightly excavated for their reception.

Lastly, many African Theraphosids possess a similar organ, not between the palp and the chelicera, but between the palp and the first leg.

Various suggestions have been hazarded as to the use of these organs, but they partake largely of the nature of conjecture, especially in connexion with the doubt as to the possession of a true auditory organ by the Araneae. They may be summarised as follows. The Theridiid spiders are among those which show most indication of auditory powers, and the stridulating organs, being practically confined to the male, may have a sexual significance. Chilobrachys stridulates when attacked, assuming at the same time a “terrifying attitude,” and its stridulating organ may serve the purpose attributed to the rattle of the rattlesnake, and warn its enemies that it is best let alone. If this be the case, there is no need that it should itself hear the sound, and, indeed, there is no evidence that the Aviculariidae possess the power of hearing. In the inoffensive stridulating Sicariid spiders the sounds could hardly serve this purpose, and the presence of the organ in both sexes, and in immature examples, precludes the idea that its function is to utter a sexual call. Instead of trying to escape when disturbed, the spider starts stridulating, and Pocock suggests that the similarity of the sound produced to the buzzing of a bee may be calculated to induce its enemies to leave it in peace.

=Internal Anatomy.=

=Alimentary System.=—The alimentary canal of the Spider is divided into three regions, the “stomodaeum,” the mid-gut or “mesenteron,” and the hind-gut or “proctodaeum.”

=The Stomodaeum= consists of the pharynx, the oesophagus, and the sucking stomach. As we have said, the mouth is to be found between the rostrum and the labium. It opens into the pharynx, the anterior wall of which is formed by a chitinous plate on the inner surface of the rostrum, sometimes called the palate. As the inner surfaces of the rostrum and labium are practically flat, the cavity of the pharynx would be obliterated when they are pressed together, were it not for a groove running down the centre of the palate, which the apposed labium converts into a tube, up which the fluids of the prey are sucked. In the Theraphosidae there is a corresponding groove on the inner surface of the labium.

At the top of the pharynx, which is nearly perpendicular, the canal continues backwards and upwards as a narrow tube, the oesophagus, passing right through the nerve-mass, which embraces it closely on all sides, to the sucking stomach. At the commencement of the oesophagus is the opening of a gland, probably salivary, which is situated in the rostrum.

FIG. 185.—Diagram showing the anatomy of the cephalothorax of a Spider. The right alimentary diverticulum has been removed. a, Aorta; c, left diverticulum with secondary caeca; e, endosternite; oes, oesophagus, descending to the mouth; s, sucking stomach; sh, dorsal shield of sucking stomach. ]

We now reach the sucking stomach, which occupies the centre of the cephalothorax. It is placed directly over a skeletal plate, the “endosternite” (Fig. 185, e), to which its lower surface is connected by powerful muscles, while its upper wall is protected by a hard plate or “buckler,” which is similarly attached to the roof of the cephalothorax in the region of the “fovea media.” The walls of the stomach are not themselves muscular, but by the contraction of the muscles above mentioned its cavity is enlarged, and fluids from the pharynx are pumped up into it.

The canal thus far is lined by chitin, like the exterior of the body, and forms a sort of complicated mouth-apparatus.

The =Mesenteron= lies partly in the cephalothorax and partly in the abdomen. The thoracic portion, shortly behind the sucking stomach, sends forward on either side a large branch or “diverticulum,” from each of which five secondary branches or “caeca” are given off (Fig. 185). Of these the anterior pair sometimes join, thus forming a complete ring; but usually, though adjacent, they remain distinct. The other four pairs of caeca curve downwards, protruding into the coxae of the legs, where they often terminate, but sometimes (Epeira) they continue their curve until they meet, though they never fuse, under the nerve-mass. Behind the origin of the diverticula the mesenteron continues as a widish tube, and shortly passes through the pedicle and enters the abdomen, where, curving slightly upwards, it proceeds along the middle line till it ends in the proctodaeum.

In the abdomen it is surrounded by a large gland, the so-called liver, and is dilated at one spot (Fig. 186) to receive the ducts from this gland. The fluid elaborated by this large abdominal gland has been shown to have more affinity with pancreatic juice than with bile.

The =Proctodaeum= consists of a short rectum, from the dorsal side of which protrudes a large sac, the “stercoral pocket.” At its origin, the rectum receives the openings of two lateral tubes which reach it after ramifying in the substance of the liver. These have been called “Malpighian tubules,” but their function is unknown. Loman has shown that they open into the mid-gut and not into the rectum, and there is reason to believe that true Malpighian tubules homologous to those of Insecta are absent in Arachnida, where their place seems to be taken by the coxal glands, which are considered to be the true excretory organs. In most spiders they open near the third coxae. Like the stomodaeum, the proctodaeum has a chitinous lining.

=Vascular System.=—The earlier investigations on the circulation of the blood in Spiders were made by direct observations of the movements of the blood corpuscles through the more or less transparent integuments of the newly-hatched young. Claparède’s results were arrived at by this method. It is invaluable for demonstrating roughly the course taken by the blood, but in these immature spiders the blood-system has not attained its full complexity, and other methods of research have shown the spider to possess a much more elaborate vascular system than was at first suspected.

The tubular heart lies along the middle line in the anterior two-thirds of the abdomen, sometimes close up against the dorsal wall, but occasionally at some little distance from it, buried in the substance of the liver. It is a muscular tube with three pairs of lateral openings or “ostia,” each furnished with a simple valve which allows the entrance, but prevents the exit, of the blood. It is contained in a bag, the “pericardium,” into which the ostia open. Both heart and pericardium are kept in place by a complicated system of connective tissue strands, by which they are anchored to the dorsal wall of the abdomen. Eight arteries leave the heart, the principal one, or “aorta,” plunging downward and passing through the pedicle to supply the cephalothorax. Besides this, there is a caudal artery at the posterior end, and three pairs of abdominal arteries, which proceed from the under surface of the heart, and the ramifications of which supply, in a very complete manner, the various organs of the abdomen. The heart is not divided up into compartments. The anterior aorta passes through the pedicle, above the intestine, and presently forks into two main branches, which run along either side of the sucking stomach, near the front of which they bend suddenly downwards and end in a “patte d’oie,” as Causard expresses it—a bundle of arteries which proceed to the limbs (Fig. 185). Where the downward curve begins, a considerable artery, the mandibulo-cephalic, runs forward to supply the chelicerae and the head region. We have omitted certain minor branches from the main trunks which supply the thoracic muscles. The nerve-mass receives fine vessels from the “patte d’oie.”

FIG. 186.—Diagram of a Spider, Epeira diademata, showing the arrangement of the internal organs, × about 8. 1, Mouth; 2, sucking stomach; 3, ducts of liver; 4, so-called Malpighian tubules; 5, stercoral pocket; 6, anus; 7, dorsal muscle of sucking stomach; 8, caecal prolongation of stomach; 9, cerebral ganglion giving off nerves to eyes; 10, sub-oesophageal ganglionic mass; 11, heart with three lateral openings or ostia; 12, lung-sac; 13, ovary; 14, acinate and pyriform silk-glands; 15, tubuliform silk-gland; 16, ampulliform silk-gland; 17, aggregate or dendriform silk-glands; 18, spinnerets or mammillae; 19, distal joint of chelicera; 20, poison-gland; 21, eye; 22, pericardium; 23, vessel bringing blood from lung-sac to pericardium; 24, artery. ]

There are no capillaries, but the blood is delivered into the tissues and finds its way, by irregular spaces or “lacunae,” into certain main venous channels or “sinuses.” There are three such in the cephalothorax, one median and the others lateral, considerably dilated in front, in the region of the eyes, and connected by transverse passages. By these the blood is brought back through the pedicle to the lung-books. In the abdomen also there are three main sinuses, two parallel to one another near the lower surface, and one beneath the pericardium. These likewise bring the blood to the lung-books, whence it is conducted finally by pulmonary veins (Fig. 186) back to the pericardial chamber, and thus, by the ostia, to the heart.

The Spider’s blood is colourless, and the majority of the corpuscles are “amoeboid,” or capable of changing their shape.

=Generative System.=—The internal generative organs present no great complexity, consisting, in the male, of a pair of testes lying beneath the liver, and connected by convoluted tubes, the “vasa deferentia,” with a simple aperture under the abdomen, between the anterior stigmata.

The ovaries are hollow sacs with short oviducts which presently dilate to form chambers called “spermathecae,” which open to the exterior by distinct ducts, thus forming a double orifice, fortified by an external structure already alluded to as the “epigyne.” The eggs project from the outer surface of the ovary like beads, connected with the gland by narrow stalks, and it was not at first clear how they found their way into the interior cavity, but it has been ascertained that, when ripe, they pass through these stalks, the empty capsules never presenting any external rupture.

The palpal organs have already been described. The spermatozoa, when received by them, are not perfectly elaborated, but are contained in little globular packets known as “spermatophores.”

=Nervous System.=—The Spider’s central nervous system is entirely concentrated in the cephalothorax, near its floor, and presents the appearance of a single mass, penetrated by the oesophagus. It may, however, be divided into a pre-oesophageal portion or brain, and a post-oesophageal or thoracic portion.

The brain supplies nerves to the eyes and chelicerae, while from the thoracic mass nerves proceed to the other appendages, and through the pedicle to the abdomen. The walls of the oesophagus are closely invested on all sides by the nerve-sheath or neurilemma.

=Sense-Organs.=—Spiders possess the senses of sight, smell, and touch. Whether or not they have a true auditory sense is still a matter of doubt. Since sounds are conveyed by vibrations of the air, it is never very easy to determine whether responses to sounds produced near the animal experimented upon are proofs of the existence of an auditory organ, or whether they are only perceived through the ordinary channels of touch. In any case, the organs of hearing and of smell have not yet been located in the Spider. M‘Cook considers various hairs scattered over the body of the spider to be olfactory, but from Gaskell’s researches upon allied Arachnid groups it would seem that the true smelling organ is to be sought for in the rostrum.

=Eyes.=—Spiders possess from two to eight simple eyes, the external appearance and arrangement of which have already been briefly explained. They are sessile and immovable, though often so placed as to command a view in several directions. In structure they are essentially like the ocelli of Insects. Externally there is a lens, succeeded by a mass of transparent cells, behind which is a layer of pigment. Then come the rods and cones of the retina, to which the optic nerve is distributed. A comparison of this with the arrangement in the Vertebrate eye will show a reversal of the positions of the retina and the pigment-layer. The lens is part of the outside covering of the animal, and is cast at the time of moulting, when the spider is temporarily blind. It is stated, however, that the eyes do not all moult simultaneously. There is often a considerable difference between the various eyes of the same spider, especially with regard to the convexity of the lens and the number of rods and cones.

Though most spiders possess eight eyes, the number is sometimes smaller, and in some groups of eight-eyed spiders two of the eyes are sometimes so reduced and degenerate as to be practically rudimentary. As might be expected, Cave-spiders (e.g. Anthrobia mammouthia) may be entirely sightless.

=Touch.=—The sense of touch would appear to be extremely well developed in some spiders, and there is reason for believing that the Orb-weavers, at all events, depend far more upon it than upon that of sight.

Among the hairs which are distributed over the spider’s body and limbs, several different forms may be distinguished, and some of them are undoubtedly very delicate sense-organs of probably tactile function.

=Spinning Glands.=—Spiders vary greatly in their spinning powers. Some only use their silk for spinning a cocoon to protect their eggs, while others employ it to make snares and retreats, to bind up their prey, and to anchor themselves to spots to which they may wish to return, and whence they “drag at each remove a lengthening chain.”

All these functions are performed by the silk-glands of the Orb-weavers, and hence it is with them that the organs have attained their greatest perfection. We may conveniently take the case of the common large Garden-spider, Epeira diademata. The glands occupy the entire floor of the abdomen. They have been very thoroughly investigated by Apstein, and may be divided into five kinds.

FIG. 187.—Spinning glands. =A=, Aciniform; =B=, tubuliform; =C=, piriform gland. ]

On either side of the abdomen there are two large “ampullaceal” glands debouching on “spigots,” one on the anterior, and one on the middle spinneret; there are three large “aggregate” glands which all terminate on spigots on the posterior spinneret; and three “tubuliform” glands, two of which have their orifices on the posterior, and one on the middle spinneret. Thus, in the entire abdomen there are sixteen large glands, terminating in the large fusulae known as spigots. In addition to this there are about 200 “piriform” glands whose openings are on the short conical fusulae of the posterior and anterior spinnerets, and about 400 “aciniform” glands which debouch, by cylindrical fusulae, on the middle and posterior spinnerets. Thus there are, in all, about 600 glands with their separate fusulae in the case of Epeira diademata.

The great number of orifices from which silk may be emitted has given rise to the widespread belief that, fine as the Spider’s line is, it is woven of hundreds of strands. This is an entire misconception, as we shall have occasion to show when we deal with the various spinning operations.

A few families are, as has already been stated, characterised by the possession of an extra spinning organ, the cribellum, and the orifices on this sieve-like plate lead to a large number of small glands, the “cribellum glands.”

=Respiratory Organs.=—Spiders possess two kinds of breathing organs, very different in form, though essentially much alike. They are called respectively “lung-books” and “tracheae.” The Theraphosae (and Hypochilus) have four lung-books, while all other spiders, except Nops, have two. Tracheae appear to be present almost universally, but they have not been found in the Pholcidae.

The pulmonary stigmata lead into chambers which extend forwards, and which are practically filled with horizontal shelves, so to speak, attached at the front and sides, but having their posterior edges free. These shelves are the leaves of the lung-book. Each leaf is hollow, and its cavity is continuous, anteriorly and laterally, with the blood-sinus into which the blood from the various parts of the Spider’s body is poured.

The minute structure of the leaf is curious. Its under surface is covered with smooth chitin, but from its upper surface rise vast numbers of minute chitinous points whose summits are connected to form a kind of trellis-work. The roof and floor of the flattened chamber within are connected at intervals by columns. The pulmonary chamber usually contains from fifteen to twenty of these leaves, and the two chambers are always connected internally between the stigmata.

The tracheae are either two or four (Dysderidae, Oonopidae, Filistatidae) in number, and their stigmata may be separate or fused in the middle line. Each consists of a large trunk, projecting forwards, and giving off tufts of small tubes which lose themselves among the organs of the abdomen, but do not ramify. In the tracheae of Argyroneta a lateral tuft is given off immediately after leaving the stigma, and another tuft proceeds from the anterior end. Histologically the main trunk of the trachea is precisely like the general chamber of the pulmonary sac, and differs greatly from the trachea of an insect.

=Cephalothoracic Glands.=—In addition to the generative glands and the so-called “liver” which occupy so large a portion of the abdomen, there are, in Spiders, certain glandular organs situated in the cephalothorax which call for some notice. These are the coxal glands and the poison-glands.

The COXAL GLANDS are two elongated brownish-yellow bodies, situated beneath the lateral diverticula of the stomach, and between it and the endosternite. They present four slight protuberances which project a short distance into the coxae of the legs. The glands appear to be ductless, but their function is thought to be excretory. They were first observed in the Theraphosae.

All Spiders possess a pair of POISON-GLANDS, connected by a narrow duct with a small opening near the extremity of the fang of the chelicerae. The glands are sac-like bodies, usually situated in the cephalothorax, but sometimes partially (Clubiona) or even entirely (Mygale) in the patura, or basal joints of the chelicerae. Each sac has a thin outer layer of spirally-arranged muscular and connective tissue fibres, and a deep inner epithelial layer of glandular cells. The cavity of the gland acts as a reservoir for the fluid it secretes. The virulence of the poison secreted by these glands has been the subject of much discussion, and the most diverse opinions have been held with regard to it. The matter is again referred to on p. 360.