ANATOMY OF INSECTS INTERNAL 31 



Stomach. The stomach (ventriculus) is usually a simple tube 

 somewhat larger in diameter than the esophagus or intestine, but 

 of variable size and strength. As the food passes into the stomach 

 it is acted upon by the secretions of the cczcal tubes (gastric c&ca) 

 which are glandular pouches, or tubes, opening into the anterior 

 end of the stomach. Their number, size, and shape are quite vari- 

 able, and they secrete a weak acid which emulsifies fats and con- 

 verts albuminoids into peptones. The stomach is not lined with 

 chitin, as is the rest of the alimentary tract, but is glandular and 

 secretes a neutral or alkaline fluid which aids in the further diges- 

 tion of the food. The chief function of the stomach, however, is 

 to absorb the digested food and pass it into circulation. 



FIG. 35. Digestive canal of a carabid beetle 



l>, esophagus ; c, crop ; d, proventriculus ; f, stomach with its caeca ; g, posterior portion 

 of stomach ; //, intestine ; /, two pairs of Malpighian tubes ; k, rectum ; /, anal glands. 



(After Dufour) 



Intestine. The food passes from the stomach into the intestine 

 through a pyloric valve which prevents its passage backward. The 

 intestine is divided into three fairly distinct parts, the ileum, colon, 

 and rectum. The length and size of these parts varies greatly ac- 

 cording to the food of the insect, the ileum often being considerably 

 coiled. In the ileum the digested food materials are absorbed and 

 passed into the blood circulation ; the colon, which is often absent, 

 contains undigested matter and waste products ; while the rectum 

 has thick, muscular walls and expels the feces through the anus, 

 which opens through the last segment of the abdomen. 



Malpighian tubes. Opening into the intestine, just back of the 

 stomach, are several small, slender tubes, variable in number, in 

 which uric acid is found, and which are considered to be excretory 

 organs similar in function to the kidneys of higher animals. 



