The animal faculty of voluntary motion, in almost every case, confers upon the creature the ability to transfer its body from place to place. In some animals, the weight of the body is sustained by immersion in a fluid as dense as itself. It is then carried about with very little expenditure of effort, either by the waving action of vibratile cilia scattered over its external surface, or by the oar-like movement of certain portions of its frame especially adapted to the purpose. In other animals, the weight of the body rests directly upon the ground, and has, therefore, to be lifted from place to place by more powerful mechanical contrivances. In the lowest forms of air-living animals, the body rests upon the ground by numerous points of support; and when it moves, is wriggled along piecemeal, one portion being pushed forward while the rest remains stationary. The mode of progression which the little earthworm adopts, is a familiar illustration of this style of proceeding. In the higher forms of air-living animals, a freer and more commodious kind of movement is provided for. The body itself is raised up from the ground upon pointed columns, which are made to act as levers as well as props. Observe, for instance, the tiger-beetle, as it runs swiftly over the uneven surface of the path in search of its dinner, with its eager antennae thrust out in advance. Those six long and slender legs that bear up the body of the insect, and still keep advancing in regular alternate order, are steadied and worked by cords laid along on the hollows and grooves of their own substance. While some of them uphold the weight of the superincumbent body, the rest are thrown forwards, as fresh and more advanced points of support on to which it may be pulled. The running of the insect is a very ingenious and beautiful adaptation of the principles of mechanism to the purposes of life. But in the insect organisation, a still more surprising display of mechanical skill is made. A comparatively heavy body is not only carried rapidly and conveniently along the surface of the ground, it is also raised entirely up from it at pleasure, and transported through lengthened distances, while resting upon nothing but the thin transparent air. From the top of the central piece--technically termed thoracic--of the insect's body, from which the legs descend, two or more membraneous sails arise, which are able to beat the air by repeated strokes, and to make it, consequently, uphold their own weight, as well as that of the burden connected with them. These lifting and sustaining sails are the insect's wings.