STRUCTURES OF SPIDERS.
Modern naturalists do not rank spiders among insects, because they have no antennæ, and no division between the head and the shoulders. They breathe by leaf-shaped gills, situated under the belly, instead of spiracles in the sides; have a heart connected with these; have eight legs instead of six; and eight fixed eyes. But as spiders are popularly considered insects, it will sufficiently suit our purpose to introduce them here as such.
The apparatus by which spiders construct their ingenious fabrics is much more complicated than that which we have described as common to the various species of caterpillars. Caterpillars have only two reservoirs for the materials of their silk; but 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 colour.[EJ] We do not find them coiled, but nearly straight, and of a deep-yellow colour. 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.
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 larger spiders, in the form of five little teats surrounded by a circle, as represented in the following figure.
Garden Spider (Epeira diadema), suspended by a thread proceeding from its spinneret.
We have seen that the silken thread of a caterpillar is composed of two united within the tube of the spinneret, 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. In the following figure, this wonderful apparatus is represented as it appears in the microscope.
Spinnerets of a Spider magnified to show the Spinnerules.