16 



APPLIED ENTOMOLOGY 



of these begin to grow inward from the surface of the body after that 

 surface has begun the formation of its chitinous exo-skeleton, and 

 accordingly also have this power, and line the inside of the parts of the 

 canal which they form, with chitin. In that portion of the canal termed 

 the mid-intestine, however, this power does not appear to be present, and 

 the mid-intestine is without this lining. 



FIG. 23. Alimentary canal of a Carnivorous Beetle, ad, anal glands; cd, stomach; 

 ed, hind intestine; in, crop; k, head and mouth parts; oe, oesophagus; pv, pro veil triculus; 

 r, rectum; vm, malpighian tubes. (Modified from Lang's Lehrbuch.) 



The mid-intestine forms the stomach of the adult insect; the 

 fore-intestine forms those parts of the alimentary canal from the mouth 

 to the stomach; and the hind-intestine those from the stomach 

 to the anus. Each of these sections may sometimes have portions 

 differing in structure, producing a greater or lesser number of subdivisions. 

 Thus the fore-intestine, by differences of structure, may sometimes con- 

 sist of a mouth cavity, oesophagus, crop and proventriculus : the stomach 

 may develop side pouches or gastric caeca; and the hind-intestine is often 

 separable by differences of structure into an ileum, colon and rectum. 



Lined as these parts are by chitin which often bears rough, tooth-like 

 projections and spines, some persons have suggested that in insects where 

 these structures are present in the fore-intestine, the food is masticated 

 more thoroughly and mixed with digestive juices before it reaches the 

 stomach. In the stomach digestion is probably completed and absorp- 

 tion at least begun, but the length of the hind-intestine in many insects 



