Page:Insects - Their Ways and Means of Living.djvu/348

 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 hot. The empty, contracted crop is a small bag contained in the first three segments of the body (Fig. 56 A, Cr); but the full crop stretches out to a long cylinder like a sausage, filling the first six segments of the body fB, 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 sort, pulpy mass of leaf ffagments. 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- cunulates 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 usua]]y 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 fina!ly extruded from the body. The alimentary canal is a tube made of a single layer of cells extending through the bodv; but its outer surface, that toward the body cavity, is covered by a muscle layer of lengthwise and crosswise fibers, which cause the more- 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 rime the caterpillar molts. The Malpighian tubules (Figs. 54, 56 A, Mal), being the kidneys of insects, are excretory organs that remove

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