264 NATURAL HISTORY. [cH. XV. 



fixed in such a way as to prevent them from chang- 

 ing their place 



It is not, however, to be imagined, that the worm 

 loads its case with these singular materials without a 

 valid reason. This worm is an aquatic insect ; the 

 annexed figure will show that its form is but little 



adapted for swimming ; its long body, encumbered 

 with six legs, is specifically heavier than water, the 

 element in which it has to seek its food. As a com- 

 pensation, the Author of nature seems to have en- 

 dowed it with an instinctive faculty, which enables 

 it to know what substances are lighter than water : 

 these it attaches to its coat, in order to counterpoise 

 its own excess of gravity, and enable it to swim 

 on the surface, in the centre, or at the bottom of the 

 element in which it has to seek its food. This 

 seems to account for the variety and singularity 

 observable in the coats of those insects. When 

 they want to ascend, the quantity of hollow and 

 buoyant substances attached to them is increased : 

 in order to descend, the light and hollow substances 

 are either diminished or counterpoised by an addition 

 uf heavy materials, such as shells or gravel. 

 Another tribe of moths (the Galena %f Fabricius), 



