Now will appear a number of organs closely packed together, and mostly stretching along the entire length of the creature. In order to assist the inquirer, I here present a plan or chart of the interior of the caterpillar when thus opened. It must be understood that the drawing is not meant to represent the particular anatomy of any one species, but to give a general view, by means of which the anatomical details of any caterpillar may be recognised. And in order to give greater distinctness, only one of each organ is seen, though with the exception of the intestinal canal, there is a double set of each organ, one on each side.
Running in a straight line from head to tail is seen the digestive apparatus, consisting of throat, stomach, and intestines, with their modifications; and this apparatus is marked A A in the cut.
INTERIOR OF CATERPILLAR.
On the surface of the digestive apparatus, and straight along its centre, lies the nervous system, represented by tiny white threads dotted at regular distances by rather larger spots of the same substance. If the nerve is examined closely, it will be seen to be composed of two very slender threads, lying closely against each other, but easily separable: in which state they are shown. And the little knobs are called “ganglia,” each forming a nervous centre, from which smaller nerves radiate to the different portions of the body.
As for brains, the caterpillar dispenses with them almost entirely; and instead of wearing one large brain in the head, is furnished with a row of lesser brains, or ganglia, extending through its whole length. This is the reason why caterpillars are so tenacious of life. If a man loses his head, he dies immediately; but an insect is not nearly so fastidious, and continues to live for a long time without any head at all. Indeed, there are some insects, which, if beheaded, die, not so much on account of the head, but of the stomach: for, having then no mouth, they cannot eat, and so die of hunger. And some insects there are which positively live longer if decapitated than if left in possession of their head.
On the right hand may be seen a curiously twisted organ, marked C, swelling to a considerable size in the middle, and diminishing to a mere thread at each end. This is one of the vessels that contain the silk, or rather the substance which becomes silk when it is spun.
If this organ be cut open in the middle, it will be seen filled with a gummy substance of curious texture, partly brittle and partly tough. From this substance silk is spun, by passing up the tube, through the thread-like portion, and so at last into a tiny tube, called the spinneret, which opens from the mouth, and wherefrom it issues in a fine thread.
There are two of these silk-making organs, and both unite in the spinneret. Consequently, if silk is examined in the microscope, the double thread can clearly be made out, both threads adhering to each other, but still distinguishable. If the threads lie parallel to each other, the silk is good; if not so, it is of an inferior quality, and liable to snap.
Most caterpillars possess this silk-factory, but some have it much more largely developed than others—the silk-worm, for instance. It is of considerable size in the larva which we are examining, because the Woolly Bear has to spin for itself a silken hammock in which to swing while it is in the sleep of its pupal state. Just before it begins to spin, the organ is of very large size, and distended with the liquid silk; but after the hammock is completed, the organ diminishes to a mere thread, and is soon altogether absorbed.