INSECTS 



therefore, must have, in addition to its mechanisms for 

 bringing food and oxygen to the cells, a means for the re- 

 moval of wastes. 



The supplying of oxygen and the removing of carbon 

 dioxide and some of the excess water are accomplished by 

 respiration. Respiration is primarily the exchange of gases 

 between the cells of the body and the outside air. If an 

 animal is sufficiently small and soft-skinned, the gas ex- 

 change can be made directly by diffusion through the skin. 

 Larger animals, however, must have a device for conveying 

 air into the body where the tissues will have closer access 

 to it. It will be evident, then, that there is not neces- 

 sarily only one way of accomplishing the purposes of 

 respiration. 



Vertebrate animals inhale air into a sac or pair of sacs, 

 called the lungs, through the very thin walls of which the 

 oxygen and carbon dioxide can go into and out of the 

 blood respectively. The blood contains a special oxygen 

 carrier in the red matter, hemoglobin, of its red corpuscles, 

 by means of which the oxygen taken in from the air is 

 transported to the tissues. The carbon dioxide is carried 

 from the tissues partly by the hemoglobin, and partly 

 dissolved in the blood liquid. 



Insects have no lungs, nor have they hemoglobin in their 

 blood, which, as we have seen, is merely the liquid that fills 

 the spaces of the body cavity between the organs. Insects 

 have adopted and perfected a method of getting air dis- 

 tributed through their bodies quite different from that of 

 the vertebrates. They have a system of air tubes, called 

 tracheae (Fig. 70), opening from the exterior by small 

 breathing pores, or spiracles (Sp), along the sides of the 

 body, and branching minutely within the body to all parts 

 of the tissues. By this means the air is conveyed directly 

 to the parts where respiration takes place. There are 

 usually in insects ten pairs of spiracles, two on the sides of 

 the thorax, and eight on the abdomen. The spiracles 

 communicate with a pair of large tracheal trunks lying 



[n 4 ] 



