Spiders are usually classed according to their difference of color, whether black, brown, yellow, &c., or sometimes by the number and arrangement of their eyes: of these organs some possess no fewer than ten, others eight, and others again six[143].
[143] Porter’s “Treatise on the Silk Manufacture,” p. 168.
Some species of spiders are known to possess the power of not merely forming a web, but also of spinning, for the protection of their eggs, a bag somewhat similar in form and substance to the cocoon of the silk-worm. The apparatus by which they construct their ingenious fabrics, is much more complicated than that which is common to the various species of caterpillars. Caterpillars have only two reservoirs for the materials of their silk; but the spider spins minute fibres from fine papillæ, or small nipples placed in the hinder part of its body. These papillæ serve the office of so many wire-drawing machines, from which the silken threadlets are ejected. Spiders, according to the dissections of M. Treviranus, have four principal vessels, two larger and two smaller, with a number of minute ones at their base. Several small tubes branch towards the reservoirs, for carrying to them, no doubt, a supply of the secreted material. Swammerdam describes them as twisted into many coils of an agate color[144]. We do not find them coiled, but nearly straight, and of a deep yellow color. From these, when broken, threads can be drawn out like those spun by the spider, though we cannot draw them so fine by many degrees.
[144] Hill’s Swammerdam, part i. p. 23.
From these little flasks or bags of gum, situated near the apex of the abdomen, and not at the mouth as in caterpillars, a tube originates, and terminates in the external spinnerets, which may be seen by the naked eye in the form of five little teats surrounded by a small circle, as represented in Fig. 8. [Plate IV.]; this figure shows the garden spider (Epeira diadema) suspended by a thread proceeding from its spinneret.
We have seen that the thread of the silk-worm is composed of two filaments united, but the spider’s thread would appear, from the first view of its five spinnerets, to be quintuple, and in some species which have six teats, so many times more. It is not safe, however, in our interpretations of nature to proceed upon conjecture, however plausible, nor to take anything for granted which we have not actually seen; since our inferences in such cases are almost certain to be erroneous. If Aristotle, for example, had ever looked narrowly at a spider when spinning, he could not have fancied, as he does, that the materials which it uses are nothing but wool stripped from its body. On looking, then, with a strong magnifying glass, at the teat-shaped spinnerets of a spider, we perceive them studded with regular rows of minute bristle-like points, about a thousand to each teat, making in all from five to six thousand. These are minute tubes which we may appropriately term spinnerules, as each is connected with the internal reservoirs, and emits a thread of inconceivable fineness. Fig. 9. represents this wonderful apparatus as it appears in the microscope.
We do not recollect that naturalists have ventured to assign any cause for this very remarkable multiplicity of the spinnerules of spiders, so different from the simple spinneret of caterpillars. To us it appears an admirable provision for their mode of life. Caterpillars neither require such strong materials, nor that their thread should dry as quickly. It is well known in our manufactures, particularly in rope-spinning, that in cords of equal thickness, those which are composed of many smaller ones united are stronger than those spun at once. In the instance of the spider’s thread, this principle must hold still more strikingly, inasmuch as it is composed of fluid materials that require to be dried rapidly, and this drying must be greatly facilitated by exposing so many to the air separately before their union, which is effected at about the tenth of an inch from the spinnerets. In Fig. 10. [Plate IV]. each of the threads shown is represented to contain one hundred minute threads, the whole forming only one of the spider’s common threads. In the figure the threads are, of course, greatly magnified, so that, for the small space represented, the lines are shown as parallel. The threadlets, or filaments as they come from the papillæ, are too fine to be counted with any degree of accuracy, but it is evident that very many are sent forth from each of the larger papillæ. This fact tends to explain the power possessed by the spider of producing threads having different degrees of tenuity. By applying more or less of these papillæ against the place whence it begins its web, the spider joins into one thread the almost imperceptible individual filaments which it draws from its body; the size of this thread being dependent on the number of nipples employed, and regulated by that instinct which teaches the creature to make choice of the degree of exility most appropriate to the work wherein it is about to engage.
Réaumur relates that he has often counted as many as seventy or eighty fibres through a microscope, and perceived that there were yet infinitely more than he could reckon; so that he believed himself to be far within the limit of truth in computing that the tip of each of the five papillæ furnished 1000 separate fibres: thus supposing that one slender filament of a spider’s web is made up of 5000 fibres!
Leeuwenhoeck, in one of his extraordinary microscopical observations on a young spider, not bigger than a grain of sand, upon enumerating the threadlets in one of its threads, calculated that it would require four millions of them to be as thick as a hair of his head!
Another important advantage derived by the spider from the multiplicity of its threadlets is, that the thread affords a much more secure attachment to a wall, a branch of a tree, or any other object, than if it were simple; for, upon pressing the spinneret against the object, as spiders always do when they fix a thread, the spinnerules are extended over an area of some diameter, from every hair’s breadth of which a strand, as rope-makers term it, is extended to compound the main cord. Fig. 11. [Plate IV]. exhibits, magnified, this ingenious contrivance. Those who may be curious to examine it, will see it best when the line is attached to any black object, for the threads, being whitish, are, in otherwise, not so easily perceived.