How they perform the primary functions 



Fig. 19. Two water-bugs: a skater walking on the surface film, and a true 



water-bug just below the surface. The complete air-bubble shown is 



diagrammatic. In practice the air is carried under the fore-wings, or 



beneath the body 



tubes called tracheae. These are found in all insects except some of the 

 most primitive wingless ones at the one extreme, and some of the most 

 highly specialised internal parasites at the other. 



A trachea is a flexible tube supported by an internal stiffener in the 

 form either of a spiral, or a series of separate rings, called taenidia : 

 much the same as the flexible hose of a vacuimi-cleaner. Big tracheal 

 trunks nm along and across the body, and branch into smaller and 

 smaller tracheae, then finally into tracheoles, which end among, and even 

 within, the cells of the various organs (Fig. 17). 



In most insects, some at least of the tracheal trunks have an opening 

 to the exterior, which is known as ^ spiracle, or stigma. The most com- 

 plete set of spiracles ever seen consists of two pairs on the thorax, and 

 eight pairs on the abdomen, but in many insects, and especially in 

 larvae, some of the spiracles that can be seen have no opening. 



The Hning of the tracheae is of the same nature as the outer skin, or 

 integument, of the insect, as if the branching tubes were merely part of 

 the skin that had been turned in. The cavity of the tracheal system is 

 therefore technically still outside the body-surface, and the tracheae 

 themselves are no more than ducts for carrying air. They do not play 



35 



