366 THE TRACHEAL SYSTEM OF THE IMAGO. 
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 
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 trachez, which arise directly from the 
spiracular sacs. 
The intima of the smaller trachez 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 they 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 rugz interlock when the valves are closed ; these rug 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 
trachee, 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 trachez of the proboscis 
(see Chapter XI.) 
The valvular arrangements of the intima of the trachez 
