333 



MANUAL OF ZOOLOGY. 



From this an intestine of variable length proceeds, its terminal 

 portion, or rectum, opening into a dilatation which is common 

 to the ducts of the generative organs, and is termed the 



"cloaca." The oesophagus is 

 furnished with salivary glands 

 of varying size and complexity, 

 which open into the cavity of 

 the mouth. Besides the pro- 

 per salivary glands, the larvae 

 of Insects very usually possess 

 a pair of silk-glands, which 

 discharge their secretion by a 

 single duct, furnished with a 

 spinneret, and developed upon 

 the labium. Rarely (as in 

 Myrmeleo\ there are silk-glands 

 opening in the abdominal re- 

 gion. Those silk-glands which 

 open into the mouth are to be 

 regarded as modified salivary 

 glands, and they are almost in- 

 variably confined to the larvae. 

 No true liver is present, but 

 the stomach is lined by secret- 

 ing cells, which appear to exer- 

 cise an hepatic function. Be- 

 hind the pyloric aperture of the 



Fig. 177- - Digestive system of a Beetle stomach, with Very few CXCCp- 

 (Carabus auratus). a (Esophagus; b . ' , , -, /. 



Crop ; c Gizzard ; d Chylific stomach ; e tlOnS, IS a Variable number of 



| I An P lfgiands beS;/InteStine; ^ C10aCa; C6Cal Convoluted tubes (fig. 



177, e), which open into the 



intestine, and are called the "Malpighian tubes." These vessels 

 are now generally regarded as discharging a renal function, 

 and as corresponding with the kidneys of the higher animals. 

 There are no absorbent vessels, and tlie products of digestion 

 simply transude through the walls of the alimentary canal into 

 the sinuses or irregular cavities which exist between the abdo- 

 minal organs. The apparatus of digestion does not differ 

 essentially from the above in any of the Insects ; but the ali- 

 mentary canal is, generally speaking, considerably lengthened 

 in the herbivorous species. 



There is no regular and definite course of the circulation in 

 the Insects. The propulsive organ of the circulation is a long 

 contractile cavity, situated in the back and termed the " dorsal 

 vessel" (fig. 178, h). This is composed of a number of sacs 



