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 (BRANCHIE#) OF THE AQUATIC LARV OF Cloe bzoculata 
AND L£phemera vulgata. 
out by the action of certain muscles that close the trachee more 
or less hermetically... The same apparatus enables the insect to 
retain a large quantity of air in its respiratory tubes. 
When the larve and nymphs of insects live in water they 
