66 The Form of Insects 



The fore-gut and hind-gut, lined with the same hard 

 substance which covers the outside of the body, are 

 to be regarded as inpushings of the outer skin. 

 Their extent is much greater than that of the 

 stomach, and such a food-canal, formed for the most 

 part by the outer skin turned inwards, is characteristic 

 of the whole group of Arthropods. 



Functions of its Parts. — The food of the Cock- 

 roach, having been passed from the mouth and gullet 

 into the crop, is subjected there to the action of the 

 spittle, which changes starch into glucose. The 

 glucose is absorbed in the crop and passes through 

 its walls into the blood-spaces. The gizzard has 

 often been regarded as a crushing organ, as might 

 readily be supposed from its strong internal teeth. 

 But it is now generally believed to act only as a 

 strainer ; its circular muscles, by their contraction, 

 draw the teeth closely together, and prevent the food 

 particles from passing down into the stomach until 

 digestion is completed. The crop is the seat of the 

 digestive process ; an acid fluid secreted by the blind 

 tubes is passed up thither through the gizzard, and 

 acts on the food, absorbing fat and converting albu- 

 minoids into peptones. After digestion the walls of 

 the gizzard relax and the food is allowed to pass on 

 into the stomach, through whose walls its nutritious 

 substances are diffused to be absorbed by the blood, 

 while the waste material travels on through the 

 intestine to be expelled at the vent (i, 40). 



Modification of the Food-canal. — The various 

 parts of the food-canal, just described, are to be 

 recognised in the great majority of insects. The crop 

 and gizzard of Beetles are very similar to those of the 

 cockroach, but the stomach is, throughout its length, 

 beset with numerous blind tubes, while the small 

 intestine is, in some cases, of excessive length. In 



