Fig. 127.—Viscera of Grasshopper. Key in text. Compare with Fig. [114].
Fig. 128.—Air Tubes of Insect.
The food tube (Fig. [127]) begins at the mouth, which usually bears salivary glands (4, Fig. [127], which represents internal organs of the grasshopper). The food tube expands first into a croplike enlargement; next to this is an organ (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 the large intestine (9) are a number of fine tubes (8) which correspond to kidneys and empty their secretion into the large intestine.
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 dioxide, is forced out. As the rings slip out, the abdomen is extended and the fresh air comes in, bringing oxygen.
Fig. 129.—Insect’s Heart (plan).
Fig. 130.—Diagrams of Evolution of Pericardial Sac around insect’s heart from a number of veins (Lankester).
