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.
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Fig. 129. Insect's Heart (plan).
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Fig. 130.—Diagrams of Evolution of Pericardial Sac around insect's heart from a number of veins (Lankester).
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Fig. 131.—Position of Insect's Heart, food tube, and nerve chain.
The Circulation.—Near the dorsal surface of the abdomen (Fig. 131) extends the long, slender heart (Fig. 129). The heart has divisions separated by valvelike partitions. The blood comes into each of the heart compartments through a pair of openings. The heart contracts from the rear toward
