THE INTERNAL ANATOMY OF INSECTS 113 



sequence of the layers of the hind-intestine is, therefore, as follows: 

 the intima, the epithelium, the basement membrane, the ental circular 

 muscles, the longitudinal muscles, the ecial circular muscles, and the 

 peritoneal membrane. 



The regions of the hind-intestine. — Three distinct regions are 

 commonly recognized in the hind-intestine, these are the small intestine 

 (Fig. 124, k), the large intestine (Fig. 124, /), and the rectum (Fig. 

 124, w). 



The Malpighian vessels.^ — There open into the beginning of the 

 hind-intestine two or more simple or branched tubes (Fig. 124, ;), 

 these are the Malpighian vessels. The number of these vessels varies 

 in different insects but is very constant within groups; there are 

 either two, four, or six of them; but, as a result of branching, there 

 may appear to be one hundred or more. The function of the Mal- 

 pighian vessels has been much discussed ; it was formerly believed to 

 be hepatic, but now it is known that normally it is urinary. 



The Malpighian vessels as silk-glands. — There are certain larvas 

 that in making their cocoons spin the silk used from the anus. These 

 larvas are chiefly found among those in which the passage from the 

 mid-intestine to the hind-intestine is closed. The silk spun from the 

 anus is secreted by the Malpighian vessels. 



Among the larvag in which the Malpighian vessels are known to 

 secrete silk are those of the Myrmeleonidas, Osmylus (Hagen 1852), 

 Sisyra (Anthony '02), Lebia scapularis (Silvestri '05), and the 

 Coccidag (Berlese '96). Berlese states that the Malpighian vessels 

 secrete the woof of the scale of the Coccidas. 



The caecum. — In some insects there is a pouch-like diverticulum 

 of the rectum, this is the ccecum. 



The anus. — The posterior opening of the alimentary canal, the 

 anus, is situated at the caudal end of the abdomen. 



IV. THE RESPIRATORY SYSTEM 



Insects breathe by means of a system of air-tubes, which ramify 

 in all parts of the body and its appendages ; these air-tubes are of two 

 kinds, which are termed irachece and tracheoles, respectively. In 

 adult insects and in most nymphs and larvce, the air is received 

 through openings in the sides of the segments of the body, which are 

 known as spiracles or stigmata. 



Many insects that live in water are furnished with special devices 

 for obtaining air from above the water; but with naiads and a few 



