INTEKNAL ANATOMY. 371 



rior part of the body, gradually enlarging in the seventh 

 segment, thence extending forward to the fourth segment, 

 where it bends on itself and leads backward and upward to 

 near the middle of the eighth segment. Here it reaches its 

 maximum diameter. It bends downward and gradually 

 tapers again to a thread-like duct, which extends forward, 

 and, after a more or less irregular course, discharges at the 

 spinneret on the under side of the labium. 



In the full-grown larva (Fig. 1, sg) these glands are dis- 

 tended with the silk-forming fluid ; in those which have spun 

 their cocoon (Fig. 2) they have contracted to less than 

 one-half of their original size. One of these glands is 

 shown in Fig. 7, and its extreme posterior tip in Fig. 8. 



When the caterpillar is ready to pupate (Fig. 2), the 

 alimentary canal is emptied of its contents, the anterior 

 stomach (as) shrinks to less than one-half of the diameter 

 of the posterior stomach (ps), the latter having also 

 shrunken to about one-half of its original diameter. The 

 intestine and rectum undergo a corresponding diminution. 

 In the pupal stage (Fig. 5) the anterior stomach grows still 

 narrower, the posterior stomach and rectum shorten, while 

 the intestine, although less than one-fourth of the diameter 

 of the larva, elongates to nearly twice its original length. 

 In the imago (Fig. 6) the digestive system is atrophied, the 

 anterior part being represented by a slender thread-like 

 remnant of the oesophagus and anterior stomach, while the 

 posterior stomach has been transformed into the small 

 shrunken stomach (Fig. 6), at the hinder end of which may 

 be found the remains of the malpighian vessels. A slender 

 intestinal tube connects the stomach with the rectum (r), 

 which lies in the upper posterior part of the abdominal 

 cavity, and is usually filled with a thick cream or salmon 

 colored fluid. 



It is interesting to note that the digestive system of the 

 male imago does not appear to have undergone so marked 

 a degeneration as that of the female. The stomach is much 

 better developed, and the relation of the different parts can 

 be more clearly demonstrated. 



