FOOD AND AIR TURNED INTO FLESH AND ENERGY 83 



and go out through these by an alternate contraction and 

 expansion of the body readily seen in a bee or dragon-fly 

 at rest. The walls of the tracheae are in part supported 

 by a fine spirally wound elastic thread which keeps the 

 tubes always open to the air. The spiracles are often 

 guarded by little tufts of hairs, which being oily prevent 



Fig. 47. 



Fig. 48. 



Fig. 47. — Diagram of trachese in head of a cockroach; note branches to all 

 mouth parts and feelers; i, tracheae or air-tubes. (After Miall and 

 Denny.) 



Fig. 48. — Piece of trachea (air-tube) from an insect. (Photomicrograph 

 by Geo. O. Mitchell.) 



water from entering easily, though oil will enter readily, 

 and a drop of oil running over the spiracles will quickly kill 

 an insect. Very fine dust will also choke up the spiracles 

 and smother it. Some ' ' insect powders ' ' act in this way. 

 The amphibians, which class includes frogs, toads, 

 and salamanders, are water-breathers during their young 

 life and in this period have gills. But early in the tad- 

 pole stage there develops by growth from a point in the 

 throat what is first a pouch, later a small pair of lungs, 

 which are soon put in use. By the time the animal 



