The =food tube= (Fig. 127) begins at the mouth, which usually contains salivary glands (4, Fig. 127). What is the color of the grasshopper’s saliva? The food tube expands first into a _croplike_ enlargement; next to this is the _stomach_ (6, Fig. 127), which resembles the gizzard in birds, as its inner wall is furnished with chitinous teeth (_b_, Fig. 114). These reduce the food fragments that were imperfectly broken up by the biting jaws before swallowing. _Glands_ comparable to the liver of higher animals open into the food tube where the stomach joins the small intestine. At the junction of the small and large intestine (9) are a number of _fine tubes_ (8) which correspond to kidneys and empty their secretion into the large intestine. [Illustration: FIG. 127.--VISCERA OF GRASSHOPPER. Key in text. Compare with Fig. 114.] [Illustration: FIG. 128.--AIR TUBES OF INSECT.] The =breathing organs= of the insects are peculiar to them (see Fig. 128). They consist of tubes which are kept open by having in their walls continuous spirals of horny material called _chitin_. Most noticeable are the two large membranous tubes filled with air and situated on each side of the body. Do these tubes extend through the thorax? (Fig. 128.) The air reaches these two main tubes by a number of pairs of short windpipes, or _tracheas_, which begin at openings (_spiracles_). In which division are the spiracles most numerous? (Fig. 128.) Which division is without spiracles? Could an insect be drowned, _i.e._ smothered, by holding its body under water? Could it be drowned by immersing all of it but its head? The motion of the air through the breathing tubes is caused by a bellowslike _motion of the abdomen_. This is readily observed in grasshoppers, beetles, and wasps. As each ring slips into the ring in front of it, the abdomen is shortened, and the impure air, laden with carbon dioxid, is forced out. As the rings slip out, the abdomen is extended and the fresh air comes in, bringing oxygen. [Illustration: FIG. 129.--INSECT’S HEART (plan).] [Illustration: FIG. 130.--DIAGRAMS OF EVOLUTION OF PERICARDIAL SAC around insect’s heart from a number of veins (Lankester).] [Illustration: FIG. 131.--POSITION OF INSECT’S HEART, food tube, and nerve chain.]