Among the Butterflies, for example, there are many which, in their pupal stage of existence, are attached to upright stems. They are fixed to the stem by a few threads at the tail, answering to the feet of the tree-climber, while the body is kept in position by a stout silken thread passed loosely round it.
The illustration represents the pupa of the common Swallow-tailed Butterfly, while in the centre is the same insect in the perfect state as it appears when resting. It really seems as if the ancient habit of the pupa had been remembered by the perfect insect, the long ends of the hinder wings taking the place of the pupal tail, and the legs that of the belt.
The Wheel.
Yet another aid to locomotion is found in the Wheel, a contrivance for diminishing friction.
When man first learnt that heavier weights could be dragged than carried, he simply placed them on flat boards to which ropes were attached. The next step was necessarily the invention of the sledge, the burden resting on two parallel runners, the ends of which were slightly curved so as to prevent them from hitching against any small obstruction. In some countries—such, for example, as in Esquimaux-land—the sledge is the only vehicle practicable, and even Europeans, when they visit that country, are fain to adopt the sledge if they would live.
But, in more temperate zones, the Wheel is paramount. In its earlier stages the wheel was a very simple business. It was simply a section of, a tree-trunk, dubbed roughly round, and with a hole in the centre, through which the axle passed. Such wheels are still in existence in many parts of Europe; and, owing to the want of regularity of outline in the circumference, and the utter absence of grease, the wheels keep up a continuous shriek, almost deafening to those who are unused to it, but perfectly unheeded by those who own or drive the vehicle.
The next improvement was to make the circumference of the wheel as perfectly circular as the art of man could devise, and, instead of having the wheel solid, to fill up its interior with spokes, thus gaining lightness and strength at the same time.
Of all locomotive wheels, I suppose that the modern Bicycle affords the best example. The driving wheel is larger than the hind wheel of an ordinary coach, and yet the spokes are not nearly so thick as the porcupine quill with which this account is written.
If we look at the ancient sculptures and paintings of Egypt and Assyria, as preserved in the British Museum, we shall see that either kind of wheel was used according to the work which it had to do. The solid, uneven, squeaking, wooden wheel was devoted to agriculture, while the light, spoked wheel was sacred either to warfare or hunting.