INSECT METAMORPHOSIS 



groups or islands of cells in the larval tissues. These 

 dormant cell groups are known as imaginal discs, or his- 

 toblasts. {Imaginal is from imago, an image, referring 

 to the adult; histoblast means a tissue bud.) 



When analyzed closely, the apparent "double" struc- 

 ture of the embryo will be found to be only the result 

 of an exaggeration of the usual processes of growth, ac- 

 companied 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 de- 

 velopment, 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 ot 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 dissolu- 

 tion, called histolysis. It can not be explained at the pres- 

 ent 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 histol- 

 ysis takes place whether phagocytes are present or absent. 



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