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 com- 

 plemental to each other, the new tissues spreading as the 

 old are dissolved, so that there is never a lack of con- 

 tinuity 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 re- 

 sulting 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 debris of those degen- 

 erating. 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 



[260] 



