It will thus be seen that each expansion of the respiratory muscles causes the air to rush to every part of the body, the entire bulk of the blood being consequently aerated at each respiration. This fact is a most important one, for, as it is not necessary that the blood should be brought to a definite centre, as in the higher animals, before it can be re-vivified, and then despatched through another series of vessels upon its errand of invigorating the frame, the necessity for a circulatory system is almost wholly at an end, and a large amount of weight consequently dispensed with. Insects have neither veins nor arteries, one principal vessel running along the back, and the blood passing slowly through this, and flowing between the various organs of the body until it again enters it at the opposite extremity to that from which it emerged. Nor is this all. With ourselves, as with the higher animals in general, nearly one-half of the blood, the venous, is always effete and useless, requiring to pass through the lungs before it can again be rendered fit for service. When this is vivified and pumped back by the heart into the system, that which was before arterial becomes venous in its turn; and so on. But not in the case of the insects. The whole bulk of their blood is arterial, if we may use the expression in speaking of animals which do not possess a vascular system. In other words, being incessantly vivified throughout the body, owing to the comprehensive character of the respiratory apparatus, no portion of it becomes at any time effete from the exhaustion of the contained oxygen. Blood so thoroughly and continually aerated, therefore, can practically perform double work, and need be far less in volume than in beings whose circulation is conducted upon different principles. The tracheal structure, consequently, while itself detracting from rather than adding to the substance of the body, permits of the abolition, not only of lungs, but also of veins and arteries and of a considerable proportion of the blood, so that the weight of the insect is reduced to the least possible degree.