ALIMENTARY SYSTEM 



233 



beginning seven or eight club-shaped pyloric caeca. The gizzard 

 projects funnel-wise into the mid-gut. The hind-gut is coiled 

 and divided into a narrow ileum, a wider colon, and a wide 

 rectum, which has six internal ridges. At the beginning of the 

 hind-gut are attached a number of long, fine Malpighian tubules 

 the epithelium of which is excretory. 



DIGESTION AND EXCRETION 



The food is cut up by the mandibles 

 and maxillae, moistened with saliva, and 

 pushed by maxillae and labium into the 

 mouth ; it is held up for a time in the 

 crop, where it is acted upon by the saliva, 

 which digests only starch, and by the 

 mid-gut secretion which leaks forward 

 and digests fat. Most of the crop digestion 

 is by yeasts and bacteria which are 

 subsequently themselves digested by 

 their host. The food is then admitted, 

 little by little, into the gizzard and there 

 broken up fine by the teeth and strained 

 by the bristles as it passes into the mid- 

 gut. The juice secreted here digests all 

 classes of food stuffs : it is secreted by 

 the break-up of epithelial cells, which are 

 replaced from reserve cells. The delicate, 

 uncuticlate epithelium is protected from hard particles not, 

 like that of backboned animals, by the secretion of mucus 

 (p. 509), but by a very delicate chitinous envelope, the peri- 

 trophic membrane, which is secreted by the epithehum but 

 adheres to it only around the entrance from the gizzard. This 

 membrane is permeable both to digestive enzymes and to digested 

 food. It is in the mid-gut that absorption mainly takes place. 

 The pyloric caeca are mere extensions of the mid-gut and do not 

 differ from it in function. In the hind-gut water is absorbed both 

 from the f^ces and from the urine excreted by the Malpighian 

 tubules. Nitrogen is excreted as uric acid. In most insects some 

 is got rid of by the Malpighian tubules and some laid up in the 

 fat body (see p. 235), but the cockroach appears not to eliminate 

 nitrogen in the urine. 



Fig. 169. — The heart and 

 neighbouring structures 

 of a cockroach ; some- 

 what diagrammatic. 



a.tn., Areas marked by dotted lines 

 to show the position of alary 

 muscles below the fatty body ; 

 f.b., fatty body ; ht., heart 

 tr., tracheae. 



