INSECT METAMORPHOSIS
groups or islands of cells in the larval tissues. These dormant cell groups are known as imaginal discs, or histoblasts. (Imaginal is from imago, an image, referring to the adult; histoblast means a tissue bud.)
When analyzed closely, the apparent "double" structure of the embryo will be round to be only the result of an exaggeration of the usual processes of growth, accompanied by an acceleration in certain tissues and a retardation in others. In general, wherever an adult organ is represented by an organ in the larva, even though the latter is greatly reduced, the cells that are to give this organ its adult form do not begin to develop until the larval growth is completed. But if an organ is lacking in the larval stage, the regenerative cells may start to develop at an earlier period—even in the embryo in a few cases. Hence, the remodeling of a larval organ in the pupal stage is only a completion of that organ's normal development, and the production of a "new" organ is only the deferred development of one that has been suppressed during the larval period.
The special organs or forms of organs that the larva has built up for its own purposes necessarily become useless when the larval life has been completed. Such organs, therefore, must be destroyed if they can not be directly made over into corresponding adult organs. Their tissues consequently undergo a process of dissolution, called histolysis. It can not be explained at the present time what causes histolysis, or why it begins at a certain time and in particular tissues, but histolysis is only one of the physiological processes that depend probably on the action of enzymes. In some insects a part of the degenerating tissues of the larva is devoured during the pupal stage by ameboid cells of the blood, known as phagocytes. It was once supposed that the phagocytes are the active agents of the destruction of the larval tissues, but this now seems improbable, since histolysis takes place whether phagocytes are present or absent.
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