v CADDIS-WORMS 245 



with living musk-rats and moles ? This is what these 

 Caddis-worms do. The snails and small mussels are 

 still alive within their shells, though they are fastened 

 down to the sheath of the Caddis-worm so completely 

 that they cannot move on their own account. 



" It is common enough for the Caddis-worms which 

 make their sheaths of fragments of shell to fasten 

 along each side a stick which passes beyond the 

 sheath at each end. [Limnophilus lunatus, Anabolia, 

 Halesus radiatus, &c.] These sticks may be as long 

 again as the rest of the sheath, and almost equal to 

 it in diameter. Such a sheath is like a house fixed 

 between two beams higher than itself. In other 

 instances there is only one such stick, and sometimes 

 the sticks thus added are short and stout. 



" Most of the sheaths of Caddis-worms are so 

 heavy that they would make a trying load for the 

 Insect if it walked upon dry land. Living in the 

 water, however, the Caddis-worms can creep along 

 the bottom or go up and down the weeds with an 

 ease which shows they are not over-burdened. [There 

 are species which make heavy cases of gravel, and 

 never leave the bottom.] It is plain, when we come 

 to consider it, that a sheath which would be too 

 much for the strength of the Insect in air may be 

 easily carried about in water, if the mean specific 

 gravity of its different components is pretty nearly 

 equal to that of water. This is the reason for the 

 irregular and awkward pieces of wood which so often 

 spoil the symmetry of the sheath. The Caddis-worm, 

 though it cares little about the shape of the vegetable 

 fragments which it fastens to its sheath, is generally 



