THE ALIMENTARY CANAL AND ITS GLANDS. 87 



urea. The first, being: a ^as, mixes with the air in the tracheal tubes 

 and so reaches the exterior during exhahition. Much of the water is 

 also o-iven oil" through the tracheal system in the form of vapor which 

 exhales from the spiracles, but, since insects are covered by their 

 hard chitinous shell, it is probable that they do not " sweat."' The 

 compounds of urea, and probably also some water, are separated 

 from the blood by the excretory glands, called Malpighian tubules 

 in insects, which empty their products back into the alimentary 

 canal, whence they are discharged with the fseces from the intestine. 



Digestion is brought about by substances called enzymes Avhich are 

 contained in the various liquids mixed with the food in the alimentary 

 canal. These liquids are secreted by the salivary glands and by the 

 cellular walls of the stomach. 



2. THE SALIVARY GLANDS. 



The opening of the salivary duct on the base of the proboscis has 

 already been described (see pp. 49-51). The true salivary glands, or 

 those corresponding with the salivary glands of other insects, are 

 arranged in two pairs, one situated within the head (figs. 19 and 42, 

 2GI) and the other within the thorax (fig. 42, 3GI). The four ducts 

 unite into one median tube, which enters the base of the labium (fig. 

 19. SalD) and opens upon the upper surface of the ligula (fig. 15 F, 

 and fig. 10, SalDO). The large and conspicuous glands lying within 

 the anterior and upper parts of the head and opening into the 

 pharynx will be described hiter in connection with this organ. They 

 are special pharyngeal glands in no way homologous with the salivary 

 glands of other insects, and are by many supposed to secrete the 

 brood food instead of a digestive liquid like saliva. 



The salivary glands of the head {Si/stem No. 2 of Cheshire, post- 

 cerehral e/Jamh of Bordas) lie against the posterior walls of the 

 cranium. In the worker each consists of a loosely arranged mass of 

 pear-shaped follicles or acini whose individual ducts unite irregu- 

 larly with one another and eventually form a common duct on each 

 side (figs. 19, 42, and 43 F, 2GI). Their two ducts unite with the 

 median duct from the thoracic glands just before the bases of the 

 mesocephalic pillars (fig. 19). In the drone these glands have a 

 quite different appearance from those of the female, each consisting 

 of a compact mass of very small follicles connected by minute ducts 

 and flattened against the posterior walls of the head (fig. 43 B and C, 

 2GI). A large lobe of this gland in the drone extends forward on 

 each side against the face, between the compound eye and the clypeus 

 (fig. 10 cIy/Z), thus occupying the position of the large mandibular 

 ghiiid in the worker (A, hldOl) and in the queen (B, IMdGl). 

 There is also a prominent triangular mass of glandular cells in the 

 drone situated just above the ocelli (fig. 10 C, 2GI) which has been 



