FEEDING AND BREATHING 31 



gut, its inner sheet of cells formed by inpushing of the 

 outer skin, and therefore lined with a chitinous sheet that 

 is an extension of the cuticle. The hind-gut is readily 

 divisible into several regions. The front portion is usually 

 a cylindrical tube of narrower diameter than the stomach, 

 and a section immediately behind the stomach may be 

 distinguished — in a bee (Fig. 10, il), for example — as a small 

 intestine or ileum. From the extreme front end of this 

 are given off the elongate Malpighian or excretory tubes 

 (Fig. 10, Mt), already briefly mentioned (p. 7), whose 

 form and function will be discussed more fully later (pp. 38- 

 39). The food-canal widens behind into the large in- 

 testine or colon (co). In these regions the cellular layer 

 (epithelium) is regular and distinct, its inner surface pro- 

 tected by a delicate but definite chitinous Hning, while the 

 muscular coat is relatively thick, the contractions of its 

 circularly arranged fibres driving on the contained food 

 still undergoing digestion. The terminal portion of the 

 hind- gut is the rectum, usually a capacious, bladder- like 

 organ (Fig. 10, r) with relatively thin walls, often thrown 

 into longitudinal folds that form ridges and furrov/s. Here 

 the epithelial layer is largely degenerate, the cell boundaries 

 being indistinct except in certain regularly arranged blind 

 tubes or pouches — the so-called rectal glands — whose lining 

 cells are large and columnar. The secretion of these glands 

 is apparently not digestive but auxiliary to the ejection of 

 the unusable residue of the food. 



Despite the cuticular lining of the hind-gut there seems 

 to be no doubt that food-absorption takes place through 

 its walls. S. Metalnikoff (1896) states that the cuticle of the 

 cockroach's large intestine is porous and that the epithelium 

 cells absorb particles of iron administered in the insect's 

 food. In hive-bees the rectum often contains a quantity 

 of pollen-grains, which constitute the source of the nitro- 

 genous and fatty food-supply of these insects ; only on 

 reaching the rectum is the digestion of much of the pollen 

 concluded, so the high probability of the absorption of 

 the digestive products there may be inferred. 



