In the vertebrated animals, the blood is driven through branching tubes to receptacles of air placed within the chest; the air-channels terminate in blood extremities, and the blood-vessels cover these as a net-work. The mechanical act of respiration merely serves to change the air contained within the air-receptacles. In the insects, this entire process is reversed; the air is carried by branching tubes to receptacles of blood scattered throughout the body; the blood-channels terminate in blood-extremities, and a capillary net-work of air-vessels is spread over these. Now, in the vertebrated creature, the chest is merely the grand air-receptacle into which the blood is sent to be aerated; while in the insect, the chest contains but its own proportional share of the great air-system. In the latter case, therefore, there is a great deal of available space, which would have been, under other circumstances, filled with the respiratory apparatus, but is now left free to be otherwise employed. The thoracic cavity of the insect serves as a stowage for the bulky and powerful muscles that are required to give energy to the legs and wings. The portion of the body that is almost exclusively respiratory in other animals, becomes almost as exclusively motor in insects. It holds in its interior the chief portions of the cords by which the moving levers and membranes are worked, and its outer surface is adorned by those levers and membranes themselves. Both the legs and wings of the insect are attached to the thoracic segment of its body.