How they perform the primary functions 



any part in the intake of oxygen into the tissues of the body, and should 

 not be thought of as a substitute for a lung : the human equivalent is the 

 windpipe, which is also called the trachea. The transfer of oxygen from 

 the air to the tissues takes place in the tracheoles, the finest branches, 

 which have specially thin walls, and which are in intimate contact with 

 the cells of the body. 



As the tissues withdraw oxygen from the tracheoles, more oxygen 

 diffuses along the tracheae to replace it. Diffusion alone is fast enough 

 to supply the needs of small insects, or of bigger ones as long as they 

 remain inactive, e.g. in the pupal stage. In active adult insects the air 

 in the tracheae is changed by respiratory movements, particularly of the 

 abdominal segments. If you watch a living insect you can often see the 

 abdomen pulsating gently in this way. Many insects have air sacs, thin- 

 walled balloon-hke structures, which increase the volume of air on the 

 move inside the insect, and so have an effea Hke bellows. It is calculated 

 that an insect may renew at one stroke a higher proportion of the air in 

 its body than we can by means of our lungs ; on the other hand no insect 

 can ever be bigger than a small mammal, because there is a Hmit to the 

 distance that air can be forced through narrow tracheae. 



The spiracles themselves can generally be partially or completely 

 closed. If different spiracles open and close at different moments during 

 a respiratory cycle, they act as valves, causing the air to move through 

 the tracheal trunks in one direction only, instead of merely moving to 



/ 



Fig 21 A water-beetle breaking the surface of the water with Its antenna 



(arrow), and so allowing air to flow into the bubble which it carries on the 



lower (ventral) surface of its body. After Hrbacek, 1950 



37 



