366 THE TKACHEAL SYSTEM Ol- THE EM AGO. 



air-sacs, which occurs soon after the insect emerges from the 

 pupa, separates the cells of the peritoneal coat, so that they 

 ultimately form a mere stellate network on the outer surface 



tf 



of the intima. It may be best described as a delicate wide- 

 meshed reticulum of stellate cells. 



The air -sacs give off numerous arborescent cylindrical 

 vessels, which are distributed to the various internal parts, on 

 the surface of which they form a delicate network, from which 

 the tracheal capillaries arise. The distribution of their 

 branches is precisely similar to that of the branches of the 

 abdominal arborescent tracheae, which arise directly from the 

 spiracular sacs. 



The intima of the smaller trachea; exhibits no spiral 

 marking, but, like that of the tracheal capillaries, is apparently 

 structureless. 



Intra-Tracheal Valves. So far as I know, no writer has 

 hitherto described valves within the air tubes except at their 

 spiracular orifices, but such valves certainly exist, although 

 they are not easily demonstrated by sections. They are 

 apparently very numerous, and I believe the}' always exist in 

 the narrow channels between the air-sacs. 



These intra-tracheal valves are somewhat like the valves in 

 the veins of Vertebrates, but they have corrugated surfaces and 

 the ruga interlock when the valves are closed ; these ruga; give 

 the valve-flaps a fimbriated appearance in sections (Fig. 48). 



I have on several occasions seen such valves between the 

 great abdominal air-sacs and the paragastric trunks, which 

 permit the air to pass forward, but not backwards ; hence the 

 contraction of the abdomen serves to raise the intra-thoracic air 

 pressure by increasing the quantity of air in the thoracic 

 cavity. 



The movement of the air is also controlled, in some of the 

 trachea-, by infoldings of the intima, which are opened and 

 closed by muscles ; such valves are found in the head, and 

 serve to open and close the great trachea; of the proboscis 

 (see Chapter XI.) 



The valvular arrangements of the intima of the trachea; 



