NATURAL FOOD. 



35 



wich and Gravesend, they are very abundant. One of 

 the most surprising instances of their skill occurs 

 in the structures of which small stones are the princi- 

 pal materials. The problem is to make a tube about 

 the width of the hollow of a wheat straw, or a crow 

 quill, and equally smooth and uniform. Now the ma- 

 terials being small stones full of angles and irregulari- 

 ties, the difficulty of performing this problem will ap- 

 pear to be considerable, if not insurmountable: yet 

 the little architects, by patiently examining their stones, 

 and turning them round on every side, never fail to 

 accomplish their plans. In other instances, when the 

 materials are found to possess too great specific gravity, 

 a bit of light wood, or a hollow straw, is added to buoy 

 up the case. The grubs themselves are admirably 

 adapted for their mode of life, the portion of their 

 bodies which is always enclosed in the case, being soft 

 like a mealworm, or garden caterpillar, while the head 

 and shoulders, which are for the most part projected 

 beyond the door- way in search of food, are firm, hard, 

 and consequently less liable to injury than the protected 

 portion, should it chance to be exposed. These grubs 



Figures of four caddis worms in variously formed cases. 



