THE CATERPILLAR AND THE MOTH 



ing compound eyes of the adult. The antennae {Ant), as 

 already noted, have increased greatly in size, and they 

 show evidence of their future multiple segmentation. 

 The upper lip, or labrum (Lm), on the other hand, is much 

 smaller in the propupa than in the caterpillar, and the 

 great biting jaws of the caterpillar are reduced to mere 

 rudiments in the propupa {Md), while the spinneret (Fig. 

 152, Spt) is gone entirely. The labium and the two 

 maxillae are longer and more distinct from each other in 

 the propupa (Fig. 159 H, Lb, Mx) than in the caterpillar, 

 and their parts are somewhat more simplified. The 

 labium bears two prominent palpi {LbP/p). 



The remodeling in the external form of the insect pro- 

 ceeds from particular groups of cells in the epidermis, 

 cells that have remained inactive since the time of the 

 embryo, and which, as a consequence, retain an unused 

 vitality. These groups of regenerative epidermal cells, 

 which are the histoblasts, or imaginal discs, of the body 

 wall, have not been particularly studied in the caterpillar; 

 but in certain other insects they have been found to occur 

 in each segment, typically a pair of them on each side of 

 the back, and a pair on each side of the ventral surface. 

 At the beginning ot metamorphosis, as the larval cuticula 

 separates from the epidermis, the cells of the discs multiply 

 and spread trom their several centers, and the areas newly 

 formed by them take on the contour and structure of the 

 pupa instead of that of the larva. The old cells of the 

 larval epidermis, which have reached the limits of their 

 growing powers and are now in a state of senescence, give 

 way before the advancing ranks of invading cells; their 

 tissues go into dissolution and are absorbed into the body. 

 The new epidermal areas finally meet and unite, and to- 

 gether constitute the body wall of the pupa. 



While the new epidermis is giving external form to the 

 pupa beneath the larval cuticula, its cells are generating 

 a new pupal cuticula. As long as the latter is soft and 

 plastic the cell growth may proceed, but when the cutic- 



[297] 



