CADDIS-WORMS. 163 



stones full of angles and irregularities, the difficulty of 

 performing 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, 



stone Nest of Caddis- Worm. 



is only part of the problem, 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, fi-om 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 choice 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, when nearly finished, a large stone is 



Sand Nest balanced with a Stone. 



added by way of ballast. 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. 



Nest ot Caddis Worm balanced with Straws. 



It is worthy of remark, that the cement, used in all 



these cases, is superior to pozzolana * in standing water, 



in which it is indissoluble. The grubs themselves are 



also admirably adapted for their mode of life, the portion 



* A cement prepared of volcanic earth, or lava. 



