CHAPTER 4 



Digestion 



Insects feed upon almost every type of organic substance found in 

 nature : some on plants, others on animals ; some on the sap of plants 

 or the tissue fluids of animals, others on foliage, or flesh, dry timber, 

 or hair and feathers; some on fungi or the live and dead bacteria in 

 the excrement of animals, others on the sterile juices of living forms. 

 Their feeding mechanisms, and the structure and chemistry of their 

 digestive system, present, therefore, the most extraordinary variety, 

 and it becomes singularly difficult to sift out the general principles in 

 the physiology of their nutrition. 



The alimentary canal 



With the exception of the earliest stages of some parasitic insects, 

 which absorb nutriment through the general body surface, all insects 

 take their food into an alimentary canal. This consists always of 

 three parts: the fore-gut, mid-gut and hind-gut. The fore-gut and 

 hind-gut are both lined with cuticle; in the mid-gut the cells are 

 freely exposed; from which it follows that secretion of digestive juices 

 can occur only in the mid-gut, and here, also, at least the greater 

 part of absorption undoubtedly takes place. In many insects the 

 secretion of the mid-gut is supplemented by that from the salivary 

 glands, mixed with the food before it is swallowed. 



Fig. 7 shows some of the commoner types of intestinal system, 

 and the different uses to which their parts are put. In the most primi- 

 tive insects, and many larval forms (Diptera-Nematocera, Lepidop- 

 tera, Tenthredinidae, and many Coleoptera) (Fig. 7, A), the fore-gut 

 or oesophagus has no other function than to conduct the food into 

 the mid-gut; from which it is passed on, often more or less continu- 

 ously, to the hind-gut. But in very many insects (Dermaptera, 

 Orthoptera, Isoptera, Odonata, Hymenoptera, many Coleoptera) 



