STRUCTURE AND CLASSIFICATION. 



37 



When this supply is exhausted of oxygen, the insect rises to the 

 surface, allows the drop of tainted air to escape, and at the same 

 time takes in another bubble of pure air. It is then supplied for 

 another period, varying in length in the different species. Cer- 

 tain other forms have the body covered with very fine hair, so 

 dense that the water cannot penetrate it, and these carry a thin 

 film of air that surrounds them entirely while they are in the 

 water ; they really do not get wet at all. This air supply must 

 usually be renewed from time to time ; but occasionally oxygen 

 is absorbed from the water itself, and the filmy covering is thus 

 kept constantly purified. Yet other insects have their surface 

 so absolutely smooth and polished that the water cannot touch 

 it, and here also a slight film of air may be carried about. There 

 are other devices to the same end, which it is needless to special- 

 ize, and we pass to those insects that never come to the surface 

 at all, and yet secure a sufficient supply of oxygen. These crea- 

 tures are mostly larvae, and in them we note the development of 

 structures resembling the gills of fishes in function. That is, in 

 certain leaf-like or thread-like appendages there are a great num- 

 ber of very fine tracheal tubes with very thin walls. Through 

 them the insect absorbs the oxygen contained in the water itself, 

 and we have exactly the same process that we find in the fishes, 

 except that here air instead of blood is brought into contact with 

 the oxygen in the water. These gill-like appendages vary greatly 

 in number and shape, but do not differ in function, and will be 

 further described when necessary in connection with the insects 

 themselves. 



Yet another type of structure is found where the absolute con- 

 nection with the air really never ceases, — that is to say, the in- 

 sects have a rigid or an extensile tube attached to the anal end 

 of the body, and this they keep pointing above the surface while 

 they walk about on the bottom of the ponds. Insects so pro- 

 vided are inhabitants of shallow water, and the tubes rarely ex- 

 ceed an inch or an inch and a half in length. The common 

 "rat-tailed larvae, " often found in the filthiest of all excrementi- 

 tious material, are good examples of this method of breathing. 

 They may be entirely imbedded in the pasty mass ; but the 

 tip of the ' ' tail' ' is kept above the surface to enable them to 

 secure fresh air. Finally, we have those cases where insects live 



