DIFFICULTY OF THE TASK. 247 



enclosed ; and a number of men must labour hard 

 to stop up the chinks, where the water comes 

 pouring in. Thus men, and machines of great 

 power, and in sufficient number, are requisite to 

 enable the most expert engineer to form a cavity 

 at the bottom of the river's bed. What if we were 

 to give him as a problem, to build a cell which 

 should be perfectly water-tight and filled with air, 

 without allowing him to employ any of these 

 means, and to insist that it should all be done with- 

 out his coming up to the surface of the water at all 

 for anything ? He would probably tell us it was 

 impossible. Not so the insect ; it, though on all 

 sides surrounded with water, actually spins a web 

 in the waves, and fastens it by cables of sufficient 

 strength to adjoining plants, and afterwards forms 

 a water-tight cell in which it shuts itself safely up 

 secure against the invasion of enemies, or the in- 

 trusion of a drop from the element in which its 

 cell floats and is moored. 



All pupge of aquatic habits do not, however, 

 possess this remarkable faculty of surrounding 

 themselves with air even in the midst of the water ; 

 and these would certainly perish unless means 

 existed for them also to inhale the vital air. These 



