434 THE INVERTEBRATA 



the troughs of folds or bottoms of pits into which the mid gut epithelium 

 is thrown. In many insects the surface is increased by the formation 

 of long diverticula, the pyloric caeca, the cells of which are not in any 

 way diiferent from the rest of the epithelium. These vary greatly in 

 number. Though the mid gut epithelium has not an internal chitinous 

 lining there is a curious chitinous tube, free in its cavity, the peritrophic 

 membrane, which is, however, secreted by special cells of the pro- 

 ventricular region (which may be ectodermal). Its function and place 

 in digestion are not understood. 



In certain cases, however, the mid gut is differentiated into func- 

 tional regions. The first part of the mid gut of the tsetse fly, for 

 instance, is concerned with water absorption which reduces the meal 

 of blood to a viscid mass. Digestion of the food takes place in a region 

 behind this, and in the lowest region of the mid gut absorption is 

 effected. These functional regions are histologically distinct. In the 

 cockroach in which no such histo-physiological distinction exists 

 between the several parts of the mid gut, it appears that much di- 

 gestion takes place in the crop to which place the enzymes from other 

 parts may have to pass to meet the food before its further passage 

 backwards. The so-called gizzard has been shown in this case to act 

 not only as a triturating organ, but as a complicated sphincter 

 guarding against the passage of any but the finest particles from the 

 crop to the mid gut. After digestion has proceeded in the crop as 

 the result of salivary and other secretory activity, the food passes 

 through the gizzard, there to be triturated ; and so on to the mid gut to 

 meet the enzymes produced by the walls of this part of the gut. 

 Resorption of the digested food takes place in the mid gut as well as 

 in the hind gut. The hind gut begins where the Malpighian tubules 

 enter the alimentary canal and is usually divided into a small intestine 

 or ileum, a large intestine or colon, in both of which the chitinous lining 

 is sometimes folded and produced into spines, and a short globular 

 rectum. In most insects rectal glands in the form of thickened patches of 

 epithelium occur .These have been shown to absorb water from the faeces 

 and therefore play a most important part in water conservation. 



Though the digestive enzymes of insects in the main belong to the 

 same classes as those of mammals there are many significant differ- 

 ences. An omnivorous insect like the cockroach produces all the 

 classes of enzymes except that represented by pepsin which is peculiar 

 to vertebrates. Then also the enzymes of insects appear to work in a 

 rather more acid medium than do the enzymes of mammals. Finally 

 the specialization in feeding habits in insects is responsible for the 

 absence of enzymes which are not wanted and either the acquisition 

 of enzymes not generally found in the Animal Kingdom or the 

 formation of a symbiotic partnership. 



