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125 ANATOMY AND PHYSIOLOGY OF INSECTS.

thus traced the progress of insects from the time of their birth till they arrive at their full maturity, and likewise noticed the most remarkable features in their external conformation, it now becomes requisite to advert to their internal structure, and give a comprehensive view of the complex system of organs by which life is maintained. In these a very close analogy, for the most part, exists to the vital system of the higher animals; but in some respects curious and most important differences prevail. Viewing the animal kingdom as forming a progressive series, from the most simple to the most complex organism, insects may be regarded as occupying nearly the centre of the scale; for, though some properties might entitle them to rank higher, there are others in which they are so deficient, as completely to counterbalance that consideration. In muscular vigour, for example, and the complication of the digestive canal, they are almost equal to the vertebrata, while the circulating system is so imperfect, that it was, till lately, a matter of doubt whether it deserved the name. From these considerations, it may be inferred that the systems of organs are neither very complex nor very simple, and that in these respects, there is some degree of inequality among them.