Page:Insects - Their Ways and Means of Living.djvu/360



The development of the internal organs proceeds with- out interruption from the beginning of the propupal period until the adult organs are c.ompleted at the end of the pupal stage. The external parts, however, do hot 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 bas 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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