THE ANIMAL KINGDOM. 127 



their aerial mouths are in some lined with a membrane, 

 which is pierced like a sieve, fitted to arrest the smallest 

 particles of air and act like a verital)le sifter. In others 

 each respiratory opening is obstructed by hairs, which 



73. Aerial Jlouth or Stigma of tlie Common Fly, seen with the Microscope. 



form a kind of tree-like net, intended for the same purpose. 

 Without these providential precautions the aerial tubes 

 of these animals, often as fine as hairs, would be obstructed 

 every instant by the dust in the midst of which they live. 



In the case of insects inhabiting the water, other pre- 

 cautions, not less admirable, prevent the fluid from forcing 

 its way into the air -passages. Sometimes at the en- 

 trance of the respiratory organ there is a door, with five or 

 six leaves of the most ingenious mechanism, which the 

 animal opens or shuts at will. It only opens them Avhen 

 it comes to the surface of a 2)ool to lireathe; when it plunges 

 into the depths the leaves of this little air-door are closely 

 shut, and the pneumatic channels are efficaciously defended 

 against the invasion of the liquid, which would disturb the 

 organization. This is seen in the larva of the common gnat, 

 which swarms in our stagnant waters. 



In the larger animals the respiratory function is per- 

 formed by the aid of a distinct, restricted apparatus, con- 



