Page:Insects - Their Ways and Means of Living.djvu/355

 CATERPII.LAR AND THE MOTH

ing compound eves of the adult. The antennae (.4nt), as already noted, Jaave increased greatly in size, and they show evidence of their future multiple segmentation. The upper lip, or labrum (Lin), 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, 5'pt) 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 (LbPlp). 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 vitalitv. These groups of regenerative epidermal cells, which'are the histoblasts, or imaginal discs, of the body wall, have hot been particularly studied in the caterpillar; but in certain other insects they have been round 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 of metamorphosis, as the larval cuticula separates from the epidermis, the cells of the discs multiply and spread from their several centers, and the areas newly formed by them takeon the contour and structure of the pupa ins{ead 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 tisues go into dissolution and are absorbed into the body. The new epidermal areas finally meet and unite, and to- gether cons-titute the body wallof 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 sort and plastic the cell growth may proceed, but when the cutic-

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INSECTS