134 THE BROOK BOOK 



if it were wound closely in a spiral. The exter- 

 nal openings of the respiratory system are called 

 spiracles. These vary in number, size and posi- 

 tion. They may easily be seen on the body of a 

 grasshopper, one on each side of every segment 

 of the abdomen. Since the blood bathes every 

 organ constantly and these organs are ramified by 

 delicate traches like silvery threads, it is not diffi- 

 cult to conceive how the purification of the vital 

 fluid takes place. The thin walls of the air-tubes 

 allow the exchange of gases in practically the 

 same manner as in our own lungs, though the air 

 and not the blood is inclosed in tubes in the in- 

 sect's body. 



It is evident that animals which live in the 

 water must be provided with special organs for 

 obtaining air. For example, the gills of fishes 

 are complicated networks of blood-vessels, with 

 very thin walls. The water of the brook, lake 

 or river contains much air — more than we think. 

 Every breeze that ripples the surface, every water- 

 fall that churns the stream into foam, every drop 

 of rain that strikes the water, adds to its supply 

 of air. The gills of the fish are so constructed as 

 to expose the greatest surface to the air -and -water 

 mixture. 



With reference to their method of obtaining 

 air, aquatic insects may be placed in two groups : 

 those which obtain supply from above the surface, 

 and those which breathe the air that is mechani- 

 cally mixed with the water. To the latter class 

 belong the damsel-fly nymphs with their plate-like 



