POND LIFE. 



127 



little furrow on each side ; these are air-passages 

 extending between the chambers on the belly side 

 of the abdomen to that beneath the wings. (In 

 addition to the spir.acles in the abdomen there are 

 spiracles in the thorax ; but as these are exceed- 

 ingly difficult to find, they will not be described 

 here.) 



13. Observe living specimens in water and note 

 that they carry air among the hairs on the lower side 

 of the thorax, and in the spaces between the head 

 and prothorax and between the prothorax and the 

 mesothorax. In fact, a large part of the body is en- 

 veloped with air. But the most capacious air-cham- 

 bers are those inclosed by the fringes of hairs on the 

 belly side of the abdomen. It is to the buoyant ac- 

 tion of the air in these that the insect owes its pecul- 

 iar position in the water. 



14. If you will watch with a lens living specimens 

 in a glass of water you will be able to see them force 

 the air out of the chambers beneath the fringes of hair, 

 using their hind legs for this purpose, and sometimes 

 an entire fringe will be lifted like a lid. 



15. Watch living specimens as they rise to the 

 surface from the depths of the aquarium, and see 

 how they bump against the surface film. 



16. Throw living flies into the water where the 

 back -swimmers are and see what the back -swim- 

 mers will do. If flies are abundant in the room, 

 observe the fate of those that come to drink from the 

 water. 



17. Kill and pin several specimens of each species 

 of back-swimmers that you have collected, and ar- 

 range them in your collection under a copy of the 



