HOW INSECTS BREATHE 135 



tracheal gills, and the dobsons and May-fly nymphs 

 with their tufts of tracheae. To the other class 

 belong the water -boatmen, the back- swimmers and 

 a great variety of insects, both adults and young. 



Many of the insects living in swift streams use 

 the air that is mixed with the water. It is easy 

 to appreciate the difficulties of living in a strong 

 current if one must come to the surface occa- 

 sionally. Imagine,, if you will, a water-tiger try- 

 ing to settle himself for a quiet draught of much- 

 needed air after an encounter with a poUiwog. 

 How disconcerting to be ordered by the current 

 to " move along ! " Is it any wonder that these 

 voracious water-tigers prefer the quieter reaches of 

 the stream, since they cannot breathe under water? 

 The dobson, however, gives itself no concern about 

 air supply. The swifter the current the more 

 abundant the food and the less difficulty in breath- 

 ing. The tufted tracheal gills spread out and the 

 current flows over them, keeping them constantly 

 wet with freshly aerated water. The caddice-worm 

 has tracheal gills of a different pattern and wisely 

 keeps both ends of its house open to insure the 

 free passage of water. The black -fly larva sticks 

 to its rock, waves its fans as if beckoning all to 

 enter, and breathes through its own special form 

 of tracheal gills without knowing it. 



To secure air enough from above the surface of 

 the water is quite a different proposition. If you 

 would see how it is managed you should visit a 

 sluggish stream or quiet bay that teems with insect 

 life. How such a place fascinates ! Can we not 



