THE FIRST DIVERS 21 



by artificial means, by taking in a deep breath of air and 

 staying down until the supply of oxygen gives out, when 

 all of us must return to the surface. But the simile is not 

 exact, for frogs dive and swim chiefly with their hind 

 feet, penguins with their fore limbs, and dolphins by means 

 of a very special tail. 



Diving with definite apparatus seems to be confined to 

 insects, spiders, and human beings. The simplest method is 

 that adopted by the whirligig beetles of our ponds. They 

 skate about in dizzy circles on the top of the water, the 

 surface film bending like thin ice under their weight. 

 Then, suddenly, they take a header straight down, carry- 

 ing with them in a little cavity, between body and wing 

 covers, a pearly bubble of air. This is soon used up by their 

 breathing spiracles and when it is all gone they shoot up, 

 and again begin their eternal spinning. We can imitate 

 the beetle by putting on a helmet with an apparatus for 

 purifying the breath or providing a constant stream of 

 compressed air, and with this we can walk for a time 

 about the bottom in shallow waters. 



An advance over the beetles is found in the rat-tailed 

 larvae of the drone fly. These are unpleasant-looking mag- 

 gots which live on the mud of stagnant ponds. They swim 

 and creep about the bottom, but are air-breathers and 

 must maintain constant communication with the atmos- 

 phere. If one of these oval maggots is dropped into a tum- 

 blerful of water it creeps blindly about for a time. It is 

 almost as clear as the water so that all the body organs 



