INSECTS 



slowly while in the tent, and then goes out for more when 

 the crop is empty. It is quite easy to tell by feeling one 

 of these caterpillars whether it is hungry or not. The 

 empty, contracted crop is a small bag contained in the 

 first three segments of the body (Fig. 156 A, Cr) ; but the 

 full crop stretches out to a long cylinder like a sausage, 

 rilling the first six segments of the body (B, Cr), its rear 

 end sunken into the stomach, and its front end pressed 

 against the back of the head. 



The fresh food in the crop consists of a solt, pulpy mass 

 of leaf fragments. As this is passed into the stomach, 

 the crop contracts and the stomach expands, and the 

 caterpillar's center of gravity is shifted backward with the 

 food burden. As the stomach becomes empty there ac- 

 cumulates in it a dark-brown liquid, and it becomes in- 

 flated with bubbles of gas. When the caterpillar goes to 

 its meals both crop and stomach are sometimes empty, 

 but usually the stomach still contains some food besides 

 an abundance of the brown liquid and numerous gas 

 bubbles. The refuse that accumulates in the middle sec- 

 tion of the intestine is subjected to pressure by the mus- 

 cles of the intestinal wall, and is here molded into a pellet 

 which retains the imprint of the constrictions and pouches 

 of this part of the intestine and looks like a small mulberry 

 when passed on into the rectum and finally extruded from 

 the body. 



The alimentary canal is a tube made of a single layer 

 of cells extending through the body; but its outer surface, 

 that toward the body cavity, is covered by a muscle layer 

 of lengthwise and crosswise fibers, which cause the move- 

 ment of the food through the canal. The gullet and crop 

 and the intestine are lined internally with a thin cuticula 

 continuous with that covering the surface of the body, 

 and these linings are shed with the body cuticula every 

 time the caterpillar molts. 



The Malpighian tubules (Figs. 154, 156 A, Mai), being 

 the kidneys of insects, are excretory organs that remove 



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