326 MOTIONS OF INSECTS. 



on a ceiling, without being brought to the ground by 

 the weight of their bodies. 



The instruments by which a fly effects this purpose 

 are two suckers connected with the last joint of the tar- 

 sus by a narrow infundibular neck, which has power of 

 motion in all directions, immediately under the root of 

 each claw. These suckers consist of a membrane ca- 

 pable of extension and contraction ; they are concavo- 

 convex with serrated edges, the concave surface being 

 downy, and the convex granulated. When in action 

 they are separated from each other, and the membrane 

 expanded so as to increase the surface : by applying 

 this closely to the plane of position, the air is suffi- 

 ciently expelled to produce the pressure necessary to 

 keep the animal from falling. When the suckers are 

 disengaged, they are brought together again so as to 

 be conflned within the space between the two claws. 

 This may be seen by looking at the movements of a fly 

 in the inside of a glass tumbler with a common micro- 

 scope*. Thus the fly you see does no more than the 

 leach has been long known to do, when moving in a 

 glass vessel. Furnished with a sucker at each extre- 

 mity, by means of these organs it marches up and down 

 at its pleasure, or as the state of the atmosphere in- 

 clines it. 



Dipterous insects, which in general have these or- 

 gans, and some three on each foot*", are not exclusively 

 gifted with them ; for various others in different or- 

 ders have them, and some in greater numbers. As I 

 lately observed, the cushions of the Buprestes are some- 



* jPMlos. Trans. 1816. 325. t.xvm.f, 1-1. " ibid. /. S-U, 



