THE ANIMAL KINGDOM. 115 



aerial mouths are in some closed with a membrane, which 

 is perforated like a sieve with an immense number of small 

 holes, and so is fitted to arrest the smallest particles floating 

 in the air, and act like a veritable strainer. In others each 

 respiratory opening is obstructed by hairs, which form a 

 kind of net, intended for the same purpose. Without these 



57. Aerial Mouth, or Stigma, of the Common Fly, seen with the microscope. 



providential precautions the air-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 precau- 

 tions, not less admirable, prevent the fluid from forcing its 

 way into the air-passages. Sometimes at the entrance 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 when it comes to the 

 surface of a pool to breathe ; when it plunges into the 

 depths the leaves of this little air-door are closely shut, and 

 the pneumatic channels are effectually 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. 



