Insects and their world 



Fig. 24. The aquatic nymph of a dragon-fly, which has its tracheal gills 



concealed in the rectum, the final section of the intestine. Other members 



of the Order Odonata, the damselflies, have three external gills at the 



tip of the abdomen 



more than a mere reservoir of a limited supply of oxygen. Oxygen is 

 constantly diffusing to and fro between the trapped air and the 

 surrounding water, and left to itself would reach a state of equilibrium. 

 As oxygen is used up in the respiration of the insect, more diffuses out 

 of the water to take its place. The bubble thus acts as a kind of lung, 

 extracting oxygen from the water and supplying it to the insect. A limit 

 is set to this process, not by shortage of oxygen, but by loss of nitrogen, 



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