ANATOMY AND PHYSIOLOGY 



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from returning into the stomach. This valve may operate by means of 

 a sphincter, or constricting, muscle, or may, as in Collembola (Fig. 146), 

 consist of a backward-projecting circular ridge, or lip, which closes 

 upon pressure from behind. 



In its primitive condition the hind intestine is a simple tube (Fig. 

 146). Usually, however, it presents two or even three specialized 

 regions, namely and in order, ileum, colon and rectum (Fig. 147). The 

 hind intestine varies greatly in length and is frequently so long as to be 

 thrown into convolutions (Fig. 152). The ileum is short and stout in 

 grasshoppers (Fig. 147) ; long, slender and convoluted in many carniv- 



FIG. 152. Digestive system of Belos- 

 toma. c, cascum; t, ileum; w, mid intestine; 

 mt, Malpighian tubes; r, salivary reservoir; 

 s, salivary gland. After LOCY, from the 

 American Naturalist. 



FIG. 153. Wall of mid intestine of silk- 

 worm, transverse section, b, basement mem- 

 brane; c, circular muscle; i, intima; /, longi- 

 tudinal muscle; n, n, nuclei of epithelial 

 cells; s, secretory cell. 



orous beetles; and quite short in caterpillars and most other larvae; its 

 function is absorption. The colon, often absent, is evident in Orthop- 

 tera and Lepidoptera and may bear (Benacus, Dytiscus, Silphidae, 

 Lepidoptera) a conspicuous caecal appendage (Figs. 150, 152) of doubt- 

 ful function, though possibly a reservoir for excretions. The colon 

 contains indigestible matter and the waste products of digestion, includ- 

 ing the excretions of the Malpighian tubes. The rectum (Fig. 147) is 

 thick-walled, strongly muscular and often folded internally. Its 

 office is to expel excrementitious matter, consisting largely of the indi- 

 gestible substances chitin, cellulose and chlorophyll. The rectum 

 terminates in the anus, which opens through the last segment of the 

 abdomen, always above the 



