The Life of the Fly 



the Dytiscus and other pirates. Necessity is 

 the mother of industry. 



Certain Caddis-worms, of the Sericostoma 

 and Leptocerus species, clothe themselves in 

 grains of sand and do not leave the bed of the 

 stream. On a clear bottom, swept by the cur- 

 rent, they walk about from one bank of ver- 

 dure to the other and do not think of coming 

 to the surface to float and sail in the sun- 

 light. The collectors of sticks and shells are 

 more highly privileged. They can remain on 

 the level of the water indefinitely, with no 

 other support than their skiff, can rest in in- 

 submersible flotillas and can even shift their 

 place by working the rudder. 



To what do they owe this privilege? Are 

 we to look upon the bundle of sticks as a sort 

 of raft whose density is less than that of the 

 water? Can the shells, which are always 

 empty and able to contain a few bubbles of air 

 in their spiral, be floats? Can the big joists, 

 which break in so ugly a fashion the none too 

 great regularity of the work, serve to buoy up 

 the over-heavy raft? In short, is the Caddis- 

 worm versed in the laws of equilibrium and 

 does it choose its pieces, now lighter and now 

 heavier as the case may be, so as to constitute 

 a whole that is capable of floating? The fol- 

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