THE METAMORPHOSES OF THE RESPIRATORY SYSTEM. 47 



Dust is kept out of the spiracles by some fine hairs which are 

 attached to them, and noxious gases and impure air can be shut 



THE RESPIRATORY ORGANS (BRANCHIAE) OF THE AQUATIC LARV.^ OF CloC blOCldata 



AND Ephemera vulgata. 



out by the action of certain muscles that close the tracheae more 

 or less hermetically. The same apparatus enables the insect to 

 retain a large quantity of air in its respirator}' tubes. 



When the larvae and nymphs of insects live in water they 



