CADDIS-WORMS. 187 



them without mercy along with him. These grotto- 

 buildiiig grubs are by no means uncommon in ponds; 

 and in chalk districts, such as the country about 

 Woolwich 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 principal material. 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 materials being small stones full 

 of angles and irregularities, the difficulty of per- 

 forming this problem will appear 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. This, however, is only part of the pro- 



Slonc Nisi of Cuhlis-n'oriii. 



blem, which is complicated with another condition, 

 and which we have not found recorded by former 

 observers, namely, that the under surface shall be 

 flat and smooth, without any projecting angles which 

 might impede its progress when dragged along the 

 bottom of the rivulet where it resides. The selection 

 of the stones, indeed, may be accounted for, from this 

 species living in streams where, but for the weight of 

 its house, it would to a certainty be swept away. For 

 this purpose, it is probable that the grub makes 

 choise of larger stones than it might otherwise want ; 

 and therefore also it is that we frequently find a case 

 composed of very small stones and sand, to which, 



Sand rfesi bnlanced wilh a Stone- 



