POND INSECTS 73 



while the back-swimmers (fig. 44) have the back blu- 

 ish black with large creamy white patches. Both kinds come to 

 the surface to get air, the water-boatmen very much more rarely 

 than the back-swimmers. The water-bugs,like the water-beetles, 

 carry down with them into the water a supply of air held on the 

 outside of the body. The air clings to a large part of the body 

 lying both under the folded wings and ou the exposed surface of 

 the body. Note the difference in the disposition of the air in the 

 two kinds of bugs. Is there a difference in their weight as com- 

 pared with the weight of water ? What part of the body projects 

 above the surface when the bug comes up for air ? Note the cus- 

 tomary position of the body and legs as one of the back -swimmers 

 lies at rest just underneath the surface with the tip of the abdomen 

 projecting very slightly above it. Which pair of legs is the row- 

 ing pair? Throw some non- swimming insects, as house-flies, in- 

 to the water and observe the actions of the back-swimmers with 

 regard to them. Kill a few back-swimmers in the cyanide bottle 

 and examine them. Note, that altho they are aquatic insects, they 



have wings; they are thus able to 

 fly from pond to pond. Note the 

 oar- like character of the swim- 

 ming legs; note their broad fringes 

 of hair. Note the sharp-pointed 

 strong beak which the back- 

 swimmer thrusts into the bodies 

 of its prey, and thru which the, 

 blood of the captured insect is 



Fig. 45. A Water-boatman (Corisa). 1 j /~\ i_ 



sucked. Care must be exer- 

 cised in handling live back-swimmers, as they can inflict a pain- 

 ful sting with the sharp beak. 



Note that the water- boatmen keep usually to the bottom of 

 the aquarium, coming only rarely to the surface ior air. Note 

 their favorite resting position. In what way does this differ from 



