INSECTS 



77 



«JL AY* A.W. wK* 



Fig. 130. — Diagrams of Evolution 

 of Pericardial Sac around in- 

 sect's heart from a number of veins 

 (Lankester). 



Fig. 129. 



Insect's 



Heart 



(plan). 



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. 



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 



Fig. 131. — Position of Insect's Heart, 

 food tube, and nerve chain. 



