40 



THE INTERNAL STRUCTURE OF MUSOA DOMESTICA 



which it opens. A short distance before entering the hypo- 

 pharynx the salivary duct (fig. 3, sal.d.) is provided with a small 

 valve controlled by a pair of fine muscles {s.m.) which serve to 



Fio. 15. 



Fig. 16. 



Fig. 15. 



Fig. 16. 



Transverse section of the lingual salivary gland, showing the fibrillar 

 character of the gland cells, x 220. (Camera lucida drawing.) 



Vertical section of one of the rectal glands, to show its structure. 

 (Camera lucida di'awing.) 



sh. Perforate chitinous sheath, r.gl. Gland cell. tr. Trachea. 



X.56. 



regulate the flow of the salivary secretion. The glands are com- 

 posed of glandular cells (fig. 15), which are convex externally and 

 have a fibrillar appearance in . section. No vacuoles have been 

 found in the cells. 



The raalpighian tubes. 



A pair of malpighian tubes (fig. 14, malp.) arise at the point 

 of junction of the proximal and distal intestines, that is, where 

 the mesenteron joins the proctodaeum. Each malpighian tube 

 is shortly divided at an angle of 180° into two malpighian tubules. 

 The malpighian tubules are very long and convoluted, and inti- 

 mately bound up with the diffuse fat body, so that it is a matter 

 of considerable difficulty to dissect them out entire. They have 

 a moniliform appearance and are of uniform width throughout ; 

 never more than two cells can be seen in section. They are 

 generally yellowish in colour. As in most insects, they are un- 

 doubtedly of an excretory nature, as the contents of the cells 



