SLIME SLUGS, MYRIAPODS AND INSECTS 139 



through the spiracles. When the muscles are relaxed and the 

 pressure is removed the elastic- walled tracheae open again and 

 are filled with fresh air which rushes in through the open 

 spiracles. One can readily see this alternate contraction and 

 expansion, or respiratory movement, of the body in a live 

 grasshopper. 



The respiratory system of insects is, as we have learned from 

 its condition in the caterpillar, very different from that of the 

 vertebrate animals. There is no breathing through nostrils or 

 mouth on the head; there are no lungs; there is no taking up 

 and carrying of oxygen by the blood. The air that enters an 

 insect's body through the spiracles is carried to every smallest 

 part of it by the tracheal tubes. Similarly these tubes take 

 up from the tissues and cells of the body waste carbon dioxide 

 and carry it outside the body. The blood has nothing to do 

 with respiration in insects. It only gets what air it needs for 

 itself. 



Circulatory System. The blood of insects is better called 

 blood lymph because it is always a mixture of blood and 

 lymph. There is no elaborate system of arteries and veins, 

 but only a single main longitudinal vessel which lies just under 

 the body-wall of the middle of the back, and is sometimes called 

 heart, but more often, simply, dorsal vessel. To see this organ 

 in a caterpillar it is necessary to cut one open longitudinally 

 along the middle of the underside and to take out carefully 

 all the fat tissue and the alimentary canal. Then there may 

 be seen running along the inner surface of the body-wall of 

 the back a delicate membranous flattened tube which^ is 

 composed of a number of successive chambers separated from 

 each other by delicate valves and provided also with small 

 lateral openings also furnished with valves. In the thin walls 

 there are delicate muscle fibers, so that the vessel can contract 

 and expand as these muscles are contracted and relaxed. 

 This pulsation, combined with the arrangement of the valves, 

 allows the blood lymph, which everywhere else in the body is 

 not confined but flows freely among the body organs, to enter 

 the dorsal vessel through the lateral openings and be forced 



