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.