368 NATURAL HISTORY OF AQUATIC INSECTS CH. 



ticity would suffice if the Insect were of large size. 

 Take a stone of half a pound weight, and throw it 

 upon water ; it sinks at once. Crush the same stone 

 to fine powder, and scatter a small quantity upon 

 the water ; the lighter particles will float for a long 

 time. 



Can the aquatic Spring-tail descend beneath the 

 surface ? I was long persuaded that it could not. 

 In my aquarium this species abounds, and propagates 

 itself from year to year, feeding, as I suppose, upon 

 Duckweed and other floating vegetation. I have 

 never once found it beneath the surface. But out of 

 doors and in wintry weather I believe that it descends, 

 as so many other aquatic Insects do, for shelter from 

 the cold. It may also, I have some reason to sup- 

 pose, feed on submerged vegetation when the surface 

 is free of weeds. I once brought home a collecting 

 bottle full of decaying vegetable matter and water. 

 When I came to look at it closely, two or three 

 Podurse were floating on the surface. After an hour 

 or two there were eight or ten. This led me to think 

 that they must have come up from the depths where 

 they had previously been lurking. Once at the 

 surfao , they were unable to descend again. I placed 

 a number in a beaker half full of water and saw them 

 run and leap upon the surface. I chased them with 

 a small rod till they became excited and alarmed, but 

 they were wholly unable to escape by sinking. Even 

 when alcohol was added to the water, and the surface 

 tension lowered thereby, the dead bodies of the 

 Podurae floated as before. It was only when they were 

 placed upon strong alcohol that they became wetted, 



