140 



prototype of a proper leg ; they have no joints, 

 however a total want of which, in all animals 

 of this class, constitutes one of their most cha- 

 racteristic distinctions. 



The motions of insects are much more perfect 

 than those of any of the preceding animals, and 

 perhaps also of any others. In the centipede, 

 indeed, the legs are not much more perfect than 

 the bristles of the sea-mouse ; but in the cray- 

 fish, scorpion, spider, &c., they are very highly 

 organized, and moved by regular muscles, aris- 

 ing from the calcareous or horny covering of the 

 animal, which is accordingly often represented 

 as its skeleton. This reputed skeleton, however, 

 is, as I have already shown, only a modification 

 of the corpus mucosum. All winged insects 

 have six legs of this description, two attached to 

 their corslet, and four to their thorax ; and many 

 of them have, either in the course of their legs, 

 or at their extremity, numerous suckers, by which 

 they form a vacuum every time the legs come in 

 contact with any surface ; and it is in this way 

 that flies are enabled to crawl upon a perpendi- 

 cular plane, however smooth a mirror, for ex- 

 ample or even to walk along the ceiling of a 

 room, being thus held up by the pressure of the 

 atmosphere. It is, of course, unnecessary to 

 say, that, under the exhausted receiver of an 

 air-pump, they remain always on the floor of the 

 instrument. The structure of these suckers is 

 strikingly beautiful, considering their excessive 

 minuteness thev are best seen in a kind of 



