64 MECHANICAL FUNCTIONS. 



from the mechanism of these parts, the most easy position 

 they can take. The beetle tribe, and the grasshoppers, 

 are furnished with this apparatus. They cannot climb 

 up smooth surfaces, as a polished door, or a pane of glass, 

 their hooks being useless, without some degree of rough- 

 ness. 



174. Some insects walk by atmospheric pressure. 

 Other insects are furnished with a curious and somewhat 

 complicated apparatus, by which they are enabled to walk 

 not only upon rough, but also upon the smoothest surfaces, 

 even with their backs downward. It is well known, that 

 the common house-fly (inusca domestica), prefers this posi- 

 tion to all others, for the purpose of repose. Hence we 

 may infer, that this is the easiest position the insect can 

 take, and therefore the one which requires the least mus* 

 cular exertion. 



175. There has been much diversity of opinion among 

 naturalists, by what means these insects are able thus to 

 suspend themselves on surfaces entirely smooth, with so 

 much ease as to prefer this position for sleeping. Dr. 

 Derham, in his Physico-Theology, speaking on this sub- 

 ject, says, that " divers flies and other insects, beside 

 their sharp-hooked nails, have also skinny palms to 

 their feet, to enable them to stick on glass and other 

 smooth bodies by means of the pressure of the atmo- 

 sphere, after the manner I have seen boys carry heavy 

 stones with only a wet piece of leather clapped on the 

 top of the stone." This theory acquired additional 

 weight, or rather was confirmed in the opinions of most 

 entomologists by the elaborate and celebrated experi- 

 ments of Sir Everard Home, in which he was assisted 

 by the microscopic observations, and drawings of M. 

 Bauer. 



176. Dr. Roget, in his Animal and Vegetable Physiol- 

 ogy, one of the most recently published " Bridgewater 

 Treatises," has given the following description of this 

 curious mechanism : 



177. Mechanism of the foot of the House-Fly. In the 

 house-fly, that part of the last joint of the tarsus, which 



Why cannot they walk on smooth surfaces 



