12 SCHOOL ENTOMOLOGY 



and expansion in the heart by means of which the blood 

 is forced forward through the aorta and a sluggish cir- 

 culation is kept up. The blood bathes all the tissues of 

 the body and carries food to them. This is its sole func- 

 tion except that it may take up some of the waste prod- 

 ucts. These are, in turn, taken from the blood by the 

 kidney-tubules and carried out of the body through the 

 intestine. The blood does not carry oxygen to the tissues 

 as it does in the vertebrates. For this reason the slow 

 circulation found in insects suffices, even for those forms 

 that are most active, where it would not do for the active 

 vertebrates. 



13. Respiratory System. Respiration is the function 

 of insects which is most different from the same function 

 in other animals. The respiratory system consists of 

 tubes, opening through holes in the sides of the thoracic 

 and abdominal segments, and branching and subdividing 

 into tubes that ramify throughout the body after the 

 fashion of capillaries. These carry the oxygen to all the 

 tissues and from them carry off the carbon-dioxide and 

 other gaseous wastes. The breathing tubes are called 

 trachea, and their external openings spiracles. There 

 are generally large tracheal tubes along each side of the 

 body and branches are given off in each segment. The 

 small tracheal tubes end in thin walled sacks in the tis- 

 sues and through these the gases are exchanged by os- 

 mosis. The blood receives only such oxygen as is re- 

 quired for its own purification. The air is forced from the 

 tracheae by muscular contraction. The tracheal tubes 

 themselves are lined with elastic coiled threads and when 

 the pressure from this contraction is relieved the elasticity 

 of the walls causes the tubes to regain their normal shape, 

 thus drawing more ah* into the body. 



