142 



ANIMAL PHYSIOLOGY. 



[LESSON 40. 



FIG. 235. 



simple in their structure. The salivary glands, however, in some of 

 them are greatly developed ; thus, in the caterpillar of the Goat 

 Moth (Cossus ligniperda], where the creature is found in the interior 

 of the willow tree, for three years, constantly feeding on the heart 



of the wood, large reservoir 

 bags are added to the glands 

 to hold a supply of this im- 

 portant secretion. The nu- 

 trimental organs of this in- 

 sect are shown in Fig. 235. 

 The drawing was made 

 from a preparation, and to 

 display all the parts the 

 salivary glands had to be 

 divided ; they were placed, 

 therefore, on either side 

 the nutrimental canal. 



638. The ramified tu- 

 bular portion (a) at the 

 lower part, is the secreting 

 tube, or gland proper. 

 They coalesce, and form 

 one evacuating duct (5), 

 which transmits the secre- 

 tion to c, a large collecting 

 or reservoir bag. At the 

 summit of this bag is another tube (d) ; these combine to form one, 

 and through it the secretion of these glands, and another pair, not 

 present in the preparation, is delivered to, not the mouth, but to a 

 little bony tube connected with the lowef lip (Fig. 236), called the 

 spinneret, found only in those larvae which prepare a web for their 

 pupa change. 



639. The elands not present are of great length, extending from 

 one end of the tody to the other, and almost filiform ; they are called 

 silk vessels, and are supposed to originate this material. And this 

 is true ; but the saliva is always combined with it, as necessarily it 

 must be when the four glands have only a common outlet. 



640. When a Caterpillar is about to change into a Chrysalis, the 

 salivary glands are painfully distended with a secretion that will no 

 more be used for its legitimate purpose. 



641. It is therefore poured out through the spinneret (in the 



Nutrimental organs, Cossus ligniperda. 



