The Life of the Fly 



The admission of a bubble of air into the 

 back-chamber, which the animal ceases to oc- 

 cupy, allow it, without further to-do, to re- 

 main for an indefinite period on the surface. 

 To dive down again, the Caddis-worm has 

 only to retreat entirely into its sheath. The 

 air is driven out; and the canoe, resuming its 

 mean density, a greater specific density than 

 that of water, goes under at once and descends 

 of its own accord. 



There is, therefore, no choice of materials 

 on the builder's part, no nice calculation of 

 equilibrium, save for one condition, that no 

 stony matter be admitted. That apart, every- 

 thing serves, large and small, joist and shell, 

 seed and billet. Built up at haphazard, all 

 these things make an impregnable wall. One 

 point alone is essential : the weight of the 

 whole must slightly exceed that of the water 

 displaced; if not, there could be no steadiness 

 at the bottom of the pond, without a perpetual 

 anchorage struggling against the pull of the 

 water. In the same manner, quick submersion 

 would be impossible at times when the surface 

 became dangerous and the frightened creature 

 wanted to leave it. 



Nor does this important heavier-than-water 

 question call for lucid discernment, seeing that 



210 



