Spiders

by Cecil Warburton

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.

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 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 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.