116 THE UNIVERSE. 



In the larger animals the respiratory function is per- 

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

 fined to one region of the body. In the insects it has a 

 much larger field of action. The air diffuses itself every- 

 where, and after having bathed the internal organs by 

 means of particular vessels, the trachea, which are easily 

 distinguished by their pearly tint, it reaches the extreme 

 terminations of the feet and the antennas. For this purpose 

 these are provided with a most remarkable structure. They 

 are composed of a fine cartilaginous lamina, rolled in like 



58. Larva of the Common Gnat, Culex pipiens (Linn.), seen with the microscope. 



the metallic thread in an elastic brace. This arrangement 

 serves to keep their walls constantly separated, and to fa- 

 cilitate the free circulation of air through their impercepti- 

 ble canals. 



Every person has seen, and with some disgust too, a 

 white larva with a long tail, which lives in the filthy, stag- 

 nant waters of our courts and roads, and which is vulgarly 

 called the rat-tailed maggot. When I was young this crea- 

 ture inspired in me the same repugnance that it does in 

 most people ; but since I have examined it by the aid of a 

 lens, and studied its habits, repugnance has given way to 



