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Spiders

<h2>CHAPTER XIII</h2> [Pg 110] THE SPINNING APPARATUS, AND THE FEET Seeing that the possession of spinnerets is a characteristic of all spiders, and that a great deal of the interest attaching to their life-history arises from their spinning operations, any account of the group, however brief, would be incomplete without some attempt to describe these remarkable organs. Among the spiders to which the attention of the reader has been directed, some have been highly accomplished spinners, constructing complicated snares, retreats and egg-cocoons, while in the case of others the spinning work is very meagre and employed chiefly for the protection of the eggs. As might be expected, the organs attain a very much higher development in some spiders than in others, and the most complex of all are those of the Epeiridae, the constructors of the circular snare. Now in the first place it is rather striking that the spiders with the most conspicuous spinnerets are by no means the most able spinners. The “bird-eating” spiders are a case in point, for they spin very little, yet two of their spinnerets are much more obvious than anything Epeira has to show, for they protrude behind the body and strike the eye at the first[Pg 111] glance. Indeed excessive length has nothing to do with complexity but is found wherever a wide sweep is necessary in laying down the threads—as we saw in the case of Agelena, when constructing its sheet-web. Roughly speaking, the spinnerets are very mobile finger-like projections, generally situated under the hind end of the abdomen and, bearing more or less numerous tubes from which the silken threads proceed. The usual number of spinnerets is six, but there is a pretty wide range, one group of spiders having only two, while a few possess eight. The spinnerets, then, are only the bearers of the actual tubes which emit the silk. The distribution of the tubes themselves is different in the different kinds of spiders, but it is usually possible to distinguish two kinds. There are generally present a large number of very fine cylindrical tubes or “spools” and a few conical tubes of much larger base, which are called spigots. Each of these orifices, whether on spool or spigot, is connected by a fine tube with a separate silk gland, or organ for manufacturing silk, situated within the spider’s abdomen. Epeira has about 600 of such glands, each with its own terminal spool or spigot, and the large number of these tubes has given rise to a misconception that is very widely spread—namely that the spider’s line, fine as it is, is “woven” of hundreds of threads of very much finer silk. This is not so, as we shall presently see. [Pg 112] Though Epeira has some 600 silk-glands, it has only five different kinds of gland, manufacturing silk of different properties. No other family of spiders has so many, though two other kinds of gland have been found in less elaborate spinners. Within the spider the silk is fluid but it solidifies on meeting the air, each thread hardening as it emerges though still continuous with the fluid contents of the gland, so that the drawing out of a silken thread is just like the operation so familiar with the glue-pot, or with spun glass, except that the hardening is not due to cooling but to exposure to the air. This general description will, it is hoped, make an account of the organs in Epeira more comprehensible. The spinnerets of Epeira are so small and inconspicuous that their disposition is not very easy to make out. When not in use they form a tiny cone under the tip of the abdomen, and only four are visible, their free ends being so brought together as entirely to conceal a small central pair. There are really, then, three pairs of spinnerets which we may call at once the anterior, median and posterior pairs, though when at rest only the anteriors and posteriors can be seen. If the spider is observed with a pocket-lens as it crawls about in a glass tube it will be noticed that the spinnerets are capable of great mobility. Their ends can be separated or brought together, or they may be made to rub against each[Pg 113] other or against the sides of the tube. The anteriors and posteriors, moreover, are two-jointed though the medians consist only of a single joint. So much can be seen without any great magnification, but the microscope will be necessary if a complete understanding of their mechanism is to be arrived at. What it reveals will now be briefly described, and will, it is hoped, be made tolerably clear by the accompanying figures which are simplified by the omission of a large number of bristles which tend to hide the essential structure, and by a great reduction in the number of “spools,” though the spigots are all indicated. The anterior spinneret (that nearest the head end of the animal) is a sort of cone, divided into a large basal joint and a small terminal joint. The latter bears on its inner side a single spigot (fig. 12 a) and is crowned with a battery of spools, about a hundred in number. The median spinneret has three spigots, two at the tip and one on the inner side (fig. 12 b), and about a hundred spools, mostly on its inner surface. The posterior spinneret is divided very obliquely into two joints, so that the terminal joint extends much lower down on the inner than on the outer side. It has five spigots in groups of three and two, and again there are about a hundred spools. Now the point that I wish to make clear is that[Pg 114] there is no interweaving of the output of these various spools and spigots. At the moment of emission the threads are adhesive, and can be made to stick to the glass or to one another, but they are not in any sense either fused or interwoven. For ordinary operations the brunt of the work is borne by the spigots marked a in the figure, sometimes reinforced by silk from the spigots on the median spinnerets marked b, the functions of all the other spools and spigots being special and occasional. For instance, when Epeira is laying down a foundation line, this is what happens. The spider sits down, so to speak, on a twig, separating its spinnerets and rubbing them on the surface. As it raises its abdomen a multitude of little threads are seen merging into what appears to be a single line. <div class="figcenter"> <ANTIMG src="images/i_125.png" width-obs="500" height-obs="191" alt="Fig. 12. View, from the inner side, of one of each of the three spinnerets." /> Fig. 12. View, from the inner side, of one of each of the three spinnerets of Epeira. A, anterior; B, median; C, posterior spinneret. </div> In reality the line is double, emerging from the[Pg 115] spigots (a) on the anterior spinnerets, and it can easily be separated into two—and two only—any where along its length. The multitudinous spools have emitted short lengths of silk to anchor the foundation line at its commencement, but they are then closed and have no share in the ever-lengthening line as the spider lets itself drop or crawls away to attach it to a new spot. One of their uses, then, is to anchor the main lines from the spigots to external objects, but they have another function not less important. Everybody has seen a garden-spider trussing up a captured fly. It is held in the jaws and front legs and slowly revolved while with its hind legs the spider draws out bands of silk from the spinnerets and swathes it like a mummy. No silken rope, this, of fused or interwoven threads, but a broad band, every strand of which is separate and distinct and proceeds from a different spool. Two or three hundred fine threads wound simultaneously round the insect form a much more effectual winding sheet than would a single cord composed of them all. So far we have accounted for the spools, and for one pair of spigots—those on the anterior spinnerets. The lower spigot (b) on the middle spinneret often assists in laying down a foundation line when extra strength is required. In that case the line is fourfold, and can easily be split into four along its whole length, the threads from the middle spinnerets being[Pg 116] rather finer than those from the anterior, but composed of the same kind of silk. There remain seven pairs of spigots whose function has still to be explained, two on the middle and five on the posterior spinnerets. The three which are clustered together on the posterior spinneret do not form silk at all, that is, the material they emit does not harden on exposure to the air but remains fluid and adhesive. When the spider is spinning the “viscid spiral” of its web it is from these spigots that the sticky matter oozes, enveloping the true silken lines and presently resolving itself into little globules in the manner already described. The remaining spigots—two on the middle and two on the posterior spinnerets are employed only in spinning the egg-cocoon, and the silk they produce is unlike that used in making the snare, being much stronger and less elastic, and—in the case of the garden-spider—of a yellow colour. In the occasional attempts which have been made to substitute spiders for silkworms as commercial silk producers, it is only this cocoon silk that has given any considerable results, the produce of the other glands being far too frail for profitable use. Such attempts, however, have always failed, principally for a reason quite unconnected with the particular nature of the silk, namely, the difficulty of keeping the spiders in captivity. It is a simple matter to supply dozens of[Pg 117] silkworms in the same box with mulberry leaves, but spiders require separate compartments or they will fight and devour each other, and the provision of suitable food for them is such a troublesome matter that it has proved quite impracticable on a commercial scale. We have incidentally seen that there are quite a number of different operations in which the spinning apparatus takes part. There is the line which most spiders lay down as they wander, and which secures them from the danger of a fall if they lose their footing; there is the snare for catching prey, the nest or retreat, and the egg-cocoon, and in addition, silk from the spinnerets may be used to enwrap and paralyse captured insects, or to assist the young spider to migrate. Since the Epeiridae perform all these operations, and are, moreover, the most finished of snare-makers, it does not surprise us to find in them the highest development of the silk glands and the most complete battery of spools and spigots on the spinnerets. Many spiders, as we know, make no snare at all, and in the case of some, very little spinning is attempted beyond the manufacture of a rather rudimentary covering for the eggs. Naturally a less complex spinning apparatus is required, and we accordingly find that jumping spiders, for instance, have only about fifty silk-glands comprising three different kinds of gland, while the glands found in[Pg 118] such of the large Aviculariidae as have been examined have been all alike. There is in some spiders a spinning organ, not to be found in Epeira, which deserves a passing notice. It does not take the place of spinnerets, of which the usual three pairs are present, but it is situated in front of them, and only occurs in the female of the species. Its peculiarity is that the silk does not emerge from projecting spools; but through fine holes in a sieve-like plate, called a cribellum, which is flush with the surface of the abdomen. It has no mobility, therefore, and the threads from it have to be combed out and distributed by the spider’s hind leg. For the better accomplishment of this purpose there is a special comb of stiff hairs or bristles, called a calamistrum, on each of the fourth pair of legs. The web of these spiders is not unlike that of Agelena, but of a rather finer texture, and it can be seen, on magnification, to consist of an irregular ground-work over which have been spread wavy bands of excessively fine silk, combed out from the orifices of the cribellum glands. Some of these cribellate-spiders, of the genus Amaurobius, are not uncommon in our cellars and out-houses; their bodies are of stouter build and their legs much shorter than those of the common house-spider. We have no space for anything approaching a full description of the anatomy of spiders, but there is one[Pg 119] other point of structure of which the reader has been promised some account. Attention was directed to the fact that while some spiders are helpless on smooth perpendicular surfaces unless they have lines to cling to, others can run with ease upon the walls or even the ceiling, of a room. <div class="figcenter"> <ANTIMG src="images/i_130.png" width-obs="400" height-obs="151" alt="Fig. 13. Foot of Jumping Spider (on left), foot of Garden Spider (on right)." /> Fig. 13. Foot of Jumping Spider (on left), foot of Garden Spider (on right). </div> The last joint or tarsus of the spider’s leg is very different in the two cases. It always terminates in claws—either two or three—so that any species can make some show of climbing where the surface is rough and there is anything to cling to, but to obtain a hold on a polished surface it needs a special contrivance. This takes the form of a pad of curiously modified hairs, called a scopula. The hairs are club-shaped, narrow at their stalk and swelling towards the tip, and their clinging power seems to be due to a viscid secretion. The foot of any jumping spider will show this structure well. Epeira has no scopula, and its climbing is always laborious unless it has a thread[Pg 120] to cling to, but it is supreme as a rope walker, treading daintily on the most delicate threads, mounting a line “hand over hand” with great agility, and manipulating the silk in its various spinning operations with unerring skill and facility. <hr class="chap" /> <div style="break-after:column;"></div>

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