70 MECHANICAL FUNCTIONS. 



191. Fig. 59 represents another of Fig, 59- 

 these genuises enveloped in a riband, 



made of pieces of leaves joined to- 

 gether, and rolled on like a bandage, 

 the folds being laid with as much re- 

 gard to symmetry and skill, as the neatest surgeon displays 

 in dressing a limb. 



192. Sometimes these mantles are constructed with small 

 shells cemented together, as seen by Fig. 60. These shells 

 are commonly empty, but it seems the Fig . g . 

 builder does not always wait for this, 



sometimes employing living snails, 

 when their sizes happen to suit his 

 wants. 



193. It appears that this insect, when out of its case, 

 can do little more than crawl along the bottom of the 

 brook, where it lives. But when clothed in the manner 

 represented, it floats along near the surface, or sinks toward 

 the bottom at pleasure, generally retaining the perpendicu- 

 lar position, but changing it to the horizontal, or turning 

 t je head downward, at will. These different positions, as 

 well as some motion in any direction, the insect gains by 

 using its feet as paddles, these parts being always out of 

 the case and free. 



194. But the most wonderful point in this history is the 

 judgment involved in the selection of materials, which, 

 when united to the body of the insect, will exactly coun- 

 terpoise the whole, so that it neither rises to the surface, 

 nor sinks to the bottom, but may be made to do either by 

 the small degree of force exerted by the feet. 



195. A vast number of instances might be selected of the 

 ingenuity, craft, and seeming discretion of the insect tribes, 

 especially of the bee, ant, spider, and wasp, but for these, 

 we must refer the reader to works on Entomology. 



What is said to be the most wonderful point in the history of the caddis- 

 worm? 



