INTERNAL ORGANS. 91 



throughout that period ; there is no sign what- 

 ever of any approaching change, so long as the 

 caterpillar continues to feed ; but when it has 

 attained maturity in this stage, the entire con- 

 tents of the alimentary canal are voided, and from 

 this moment the form and relations of the parts 

 begin to change and new parts arise ; first, the 

 whole canal becomes more slender, a natural re- 

 sult of its emptiness ; then the oesophagus and 

 intestine lengthen, at the expense of the stomach, 

 which tapers at either end ; next, the oesophagus 

 swells -just at the entrance of the stomach, and 

 the colon, which has retained a globular form 

 while the intestine grew slender, begins to turn a 

 little from a direct course ; the malpighian vessels, 

 too, formerly attached to the middle of the intes- 

 tine, often begin to approach the extremity of the 

 stomach ; and when they have finally reached it, 

 perhaps only a week after the caterpillar has 

 changed to chrysalis, the little swelling at the end 

 of the oesophagus has grown to a bladder with a 

 minute orifice, and is destined to be at least half 

 as large as the stomach itself upon which it lies ; 

 at the same time, the colon has grown asym- 

 metrical and developed a coecum upon one side. 

 After this, the intestine begins to lengthen and 

 grow tortuous, until, at last, the alimentary tract is 

 often twice as long as the body. The little blad- 

 der at the end of the oesophagus has been consid- 



