STRUCTURE AND DEVELOPMENT OF INSECTS. 



17 



the middle of the back, above the alimentary canal, is a 

 long tube popularly called the heart (Fig. 9, ]i\ Fig. 10, 

 dr). This heart is composed of a number of chambers 

 each of which is funished with side valves for admitting 

 blood from the body-cavity. The blood coming into the 

 heart from the body- cavity is propelled forward toward 

 the head, where it again flows into the body-cavity. Thus 

 various currents of blood are maintained throusfhout the 



Fig. 9. — Ideal section through au iu.sect. a, ahmentaiy canal; h, 

 heart; n, nerve-cord; s, stigmata: /, tracheal tubes; I, legs; w, 

 wings. (From Riverside Nat. History.) 



body, but other than the heart there is no system of blood- 

 vessels, the blood merely filling the body- cavity around 

 and through the various organs and tissues. Constantly 

 flowing around the respiratory tubes or tracheae, the blood 

 is quickly and thoroughly purified, though the exact 

 manner in which this is done is not definitely known. 

 The respiratory system has absolutely no connection with 

 the mouth or pharynx (Fig. 10, ^j^), as have the lungs of 

 the higher animals, and if an insect is to be suffocated, it 

 must be done by closing the spiracles. It is in this way 



