Spiders

by Cecil Warburton

Fig. 12. View, from the inner side, of one of each of the three spinnerets of Epeira. A, anterior; B, median; C, posterior spinneret.

In reality the line is double, emerging from the 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 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 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 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 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.