608 EVOLUTION OF ANIMALS Part V 



mayfly nymphs. The blood cells dally along the muscles, are moved toward 

 the foot, then drift slowly back to the body and turn toward the heart. 



Release of Energy — Breathing and Respiration. Skin was the original 

 respiratory organ of all multicellular animals but the skin of insects is covered 

 with cuticle. In them its place is taken by a tubular ventilating system through 

 which air is brought in and out by the muscular action of breathing (Fig. 

 30.16). The tracheal tubes open to the outside through spiracles. The structure 

 of their walls is similar to that of the body wall and they originate by ingrowths 

 of it during embryonic development. Tracheae carry oxygen directly to the 

 cells and bring carbon dioxide away. Their walls are permeable to gases espe- 



FiG. 30.16. Diagram of the tracheal system of the grasshopper by which oxygen 

 is carried directly to the tissues. It finally reaches them through tracheoles, the 

 minute ends of the tracheae not shown here. The main tracheae and air sacs of 

 one side are shown with the digestive tube removed. A, main air sac; O, trachea 

 surrounding the compound eye; E, inner surface of ear surrounded by trachea; 

 S, abdominal air sacs; numbers indicate spiracles, the external openings of the 

 system. (Courtesy, Matheson: Entomology. Ithaca, N.Y., Comstock Publishing 

 Co., 1944.) 



cially to carbon dioxide. The spiracles are opened and closed by valves that 

 control the flow of air and evaporation. The chitinous lining of the tracheae is 

 strengthened by spiral bands (taenidia) that with the aid of a microscope can 

 be unwound like the spring of a curtain roller. Tracheae divide again and 

 again, until they terminate in exceedingly minute tracheoles. Clusters of these, 

 clearly visible with the great magnification of the electron microscope, extend 

 from the tracheae to the cells of the body and end blindly within them or on 

 their surfaces (Fig. 30.17). Tracheoles are the main functional part of the 

 tracheal system. When oxygen is under high pressure in the tracheoles it 

 passes into the cells where the pressure is lower; substances in the cell com- 

 bine with the oxygen, energy is set free, and carbon dioxide diffuses into the 

 tracheoles. 



Tracheae are frequently enlarged into air sacs and muscles squeeze and re- 

 lease these like bellows thus aiding the intake and expulsion of air through the 

 spiracles. Air sacs also lighten the body and must make it easier to jump and 

 fly. There are also air sacs in birds. 



By-products of Metabolism — Excretion. The function of an excretory 



