Physiologists tell us that if in the human frame the nerves, the muscles, and the veins and arteries could be separated from one another, while retaining their own relative positions, each would be found to possess the perfect human form. In other words, there would be the nerve-man, the muscle-man, and the blood-vessel-man, as well as the bone-man which supplies the framework of the whole. In the same way we may speak of the tracheal, or breathing-tube insect; for the two main tubes and the endless ramifications of their branches, if they could be detached from the surrounding tissues while themselves suffering no displacement, would exhibit to us the form of the insect from which they were taken, and that so exactly that in many cases we should almost be able to recognize the species. In the smaller branches of these air-vessels considerable variety is to be found. Some retain their tubular character to their very termination. Others assume a curious beaded form, dilating at short intervals into small chambers; while yet others abruptly resolve themselves into sac-like reservoirs, in which a comparatively large quantity of air is stored up. From the larger vessels are thrown off vast numbers of exceedingly delicate filaments, so small that a very powerful microscope is necessary in order to detect them, which float loosely in the blood, and furnish it with the constant supply of oxygen necessary for its purification. Now, we may well ask ourselves how it is that these tubes, which are of almost inconceivable delicacy, should remain open during the various movements of which the flexible body is capable. Why is it, for instance, that the air-supply of the lower leg is not cut off when the limb is bent at the knee-joint? or from the head, when that important part of the frame is tucked away beneath the body? How does the Earwig contrive to breathe while folding its wings by the aid of its tail-forceps? or many of the Cocktail-beetles when curled up in their peculiar attitude of repose? The answer to these questions is simple enough, and may be discovered by a glance at one of the most familiar of our own inventions—the flexible gas-tube. This preserves its tubular form no matter to what degree it may be bent or twisted, for coiled closely within it is a spiral wire, which obliges the interior of the pipe to retain its diameter almost unaltered alike when straight or curved. And as with this, so with the tracheæ of the insect, whose walls are formed of a double layer, the one lying inside the other, while between the two, and surrounding the inner, is coiled a fine but very strong elastic thread, whose convolutions allow the vessel to be bent in any required direction without losing its cylindrical form. By the exercise of a little care the anatomist can often unwind an inch or two of this spiral thread from a single branch of the tracheæ of a tolerably large insect, so closely is it coiled, and so elastic its character.