138 INSECT LIFE. 



hand, they frequently rub their hind legs down their 

 backs, thus forcing the air from under the wings to 

 the ventral surface of the body. 



5. Note that while a water-boatman is resting 

 near the bottom of the aquarium, it frequently moves 

 its oarlike hind legs backward with a quick sweeping 

 motion, causing a current of water to flow over the 

 layer of air on the ventral side of the body. 



Although the water-boatmen breathe air, which 

 they carry with them in a way very similar to that 

 of the back-swimmers, they are able to remain under 

 water without going to the surface to renew the sup- 

 ply of air for a very long period. This, I think, is 

 explained by the fact that the air on the lower side 

 of the body and along the outer edge of the wing- 

 covers is in direct contact with the water, so that it 

 can be purified by the air that is mixed with the 

 water. And probably it is to insure this purification 

 of the layer of air that the insect causes a current of 

 water to flow over it by the sweeping motion of its 

 hind legs. This also explains the reason for forcing 

 the air out from under the wings and drawing it 

 back again. 



Sometimes, however, the insects dart to the sur- 

 face and return to the bottom as if going after a fresh 

 supply of air ; but the movement is so rapid and the 

 stay at the surface is so short that I have been unable 

 to determine the manner of taking the air. 



6. Observe the mode of life of these insects as 

 completely as possible, and write an account of them. 



The Structure of Water-boatmen {School 

 Work). — Kill and pin some specimens, and study 

 their structure. 



