INSECTS
While the larval tissues are undergoing dissolution, the adult tissues are being built up from those groups of dormant cells, the histoblasts, that have retained their vitality. Whatever it is that produces histolysis in the defunct larval tissues, it has no effect on the regenerative tissues, which now begin a period of active development, or histogenesis (i.e., tissue building), which results in the completion of the adult organs. In most of the organs the two processes, histolysis and histogenesis, are complemental to each other, the new tissues spreading as the old are dissolved, so that there is never a lack of continuity in the parts undergoing reconstruction. It is only in the muscles, as we have already observed, that the old tissues are destroyed before the new ones are formed.
Because of the high physiological activity (metabolism) going on within the pupa, the blood of the insect at this stage becomes filled with a great quantity of matter resulting from the dissolution of the larval tissues. During the pupal period, the insect takes no food nor does it discharge any waste materials—the substance of the growing tissues is derived from the débris of those degenerating. But the transformation is not all direct. The insect is provided with an organ for converting some of the products of histolysis into proteid compounds that can be utilized by the tissues in histogenesis. This organ is the fat-body (see Chapter IV and Figure 158). During the larval life the cells of the fat body store up large quantities of fat, and in some insects glycogen, both of which energy-forming substances are discharged into the blood at the beginning of the pupal period. And now the fat cells become also active agents in the conversion of histolytic products into proteid bodies, probably by enzymes given off from their nuclei. These proteid bodies are finally also discharged into the blood, where they are absorbed as nutriment by the tissues of the newly-formed organs. At the close of the pupal period, the fat-body itself is often almost entirely consumed or
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