64 INSECT TRANSFORMATION 



arise as buds or thickenings of the outer body- wall (Fig. 34, a, b). 

 These, by growth and inpushing of the adjacent skin, come to 

 lie in pouches (Fig. 34, c, d), and as they do not project 

 externally beyond the skin no cuticle is formed over them. 

 Only towards the end of the last larval stages are they pushed 

 out of their pouches, and then the new cuticle must of necessity 

 clothe them so that in the pupal stage they will be outwardly 

 visible. Further, it is found that the other characteristic 

 structures of the butterfly first revealed in the pupa the long 

 feelers, maxillae and legs, for example are similarly developed 

 in the caterpillar from buds arising in inwardly-directed 

 pouches. As all these are rudiments of structures in prepara- 

 tion for the perfect winged insect (or imago) they are known 

 generally as imaginal discs or imaginal buds. Rapid growth 

 of these takes place during the last larval stage, and as the body 

 of the pupa is thus built up, many structures of the caterpillar 

 are broken down, their cellular elements dissolved as they are 

 no longer needed for the insect's life. 



It is well known that the pupa of a butterfly takes no food 

 and remains passive ; if stimulated, only a twitching of those 

 few abdominal segments capable of movement on each other 

 can be observed. The pupal stage marks a definite period 

 of quiescence in the life-history, which is indeed essential 

 because the divergence between the larva and imago necessi- 

 tates such dissolution and reconstruction for which, as we 

 have seen, provision is made by the destruction (histolysis) 

 of much larval tissue and the formation of the perfected 

 organs from the imaginal discs. This process is continued 

 through the pupal stage, the cuticular structure of the butterfly 

 scales, hairs, spines being formed beneath the pupal cuticle. 

 The emergence of the imago from this cuticle is essentially 

 like that of the developed grasshopper or dragon-fly. Through 

 the dorsal, longitudinal slit, the head, thorax, wings, legs and 

 abdomen are successively withdrawn ; the new cuticle then 

 sets and hardens ; the wings, small and crumpled immediately 

 after emergence, expand and stiffen ; and the aerial life, for 

 which, as has been seen, preparation began long before in 

 the hidden growth of the caterpillar's imaginal buds, is at 

 length attained. 



The life-history of the butterfly, compared with that of the 



