INTERNAL ORGANS. 95 



thread has enlarged and here and there developed 

 special vessels, just as we have seen in the aliment- 

 ary canal ; so that at last it would scarcely seem 

 possible that so complicated a system could have 

 arisen from a simple thread. 



Finally, the silk vessels of the caterpillar have 

 entirely vanished before half the pupal stage is 

 past, and the immense material stored in the cel- 

 lular system has become absorbed by the require- 

 ments of the rapidly growing organs until scarcely 

 a trace is left. Thus we see that revolutions have 

 occurred within the body quite as great as those 

 of the external envelope, and these changes have 

 mostly transpired during the earlier stages of pupal 

 life, or in the day or two which precede and fol- 

 low the absolute change from larva to chrysalis. 



There are certain organs of the butterfly which, 

 as we have said, are not mere alterations or out- 

 growths of parts previously exist- 

 ing, but new structures, each of 

 which has its own proper develop- 

 ment ; we will briefly consider the 

 most interesting of these, the wings. 



If we were to open a fully grown 

 caterpillar, we should discover [Fig. 

 80] upon either side of the last two thoracic seg- 

 ments a lustrous, translucent, circular pad, at- 

 tached to the inner walls of the body and direct- 

 ed upward ; these pads closely resemble the flakes 



