POND LIFE. 143 



in the same way that the air in the tracheae of an 

 ordinary tracheal gill is purified. By watching a 

 living specimen it can be seen to alternately draw in 

 the water and force it out again. 



This arrangement serves as an organ of locomo- 

 tion as well as an organ of respiration. For the 

 insect, by suddenly forcing out the water from the 

 rectum, can cause itself to shoot forward. The jet 

 of water forced out from the rectum when the insect 

 jumps forward is most easily seen when the insect is 

 on the bottom of the aquarium ; in such a case the 

 fine dirt will be disturbed by it for a considerable 

 distance back of the insect. 



If you can find a large number of nymphs of 

 dragon-flies, do so, and, keeping them in aquaria, try 

 to observe the emergence of the adult. Of this 

 Tennyson wrote : 



To-day I saw the dragon-fly 

 Come from the wells where he did lie, 

 An inner impulse rent the veil 

 Of his old husk ; from head to tail 

 Came out clear plates of sapphire mail. 

 He dried his wings : like gauze they grew, 

 Through crofts and pastures wet with dew 

 A living flash of light he flew. 



