INSECTS 



The development of the internal organs proceeds with- 

 out interruption from the beginning of the propupal period 

 until the adult organs are completed at the end of the pupal 

 stage. The external parts, however, do not make a con- 

 tinuous growth. After reaching a certain stage of de- 

 velopment, the form of the body wall and of the append- 

 ages is fixed by the hardening of the new cuticula on 

 their outer surfaces. In this stage, therefore, they must 

 remain, and the half-mature form attained is that char- 

 acteristic of the pupa. The final development of the body 

 wall and the appendages of the adult is accomplished by 

 a second separation of the epidermis from the cuticula, 

 which allows the cellular layers, now protected by the 

 pupal cuticula, to go through a second period of growth 

 during the pupal stage. This pupal period of growth at 

 last results in the perfection of the external characters 

 of the adult, which are in turn fixed by the formation of 

 the adult cuticula. In the meantime, the new muscles 

 that are to be retained have become anchored at their 

 ends into the new cuticula, and the mechanism of the 

 adult insect is ready for action. The perfect insect, 

 cramped within the pupal shell, has now only to await 

 the proper time for its emergence. 



Through the whole period of metamorphosis, the insect 

 must depend on its internal resources for food materials. 

 Oxygen it can obtain by the usual method, for its respira- 

 tory system remains functional; but in the matter of 

 food it is in a state of complete blockade. The pupa has 

 two sources of nourishment: first, the food reserves stored 

 in the cells of the fat-body; second, the materials resulting 

 from the breaking down of the larval tissues, which are 

 scattered in the blood and eventually absorbed. 



The fat cells, at the beginning of metamorphosis in some 

 insects, give up most of their stored fat and glycogen; and 

 they now become filled with small granules of proteid 

 matter. The proteid granules are probably elaborated in 

 the fat cells from the absorbed detritus of the larval 



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