326 MOTIONS OF INSECTS. 
polished perpendicular, like the glass im our windows 
and the chunam walls in India, or with their, backs 
downward 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 concayo- 
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 
confined 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 microscope?. 
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 extremity, by means of 
these organs it marches up and down at its pleasure, or 
as the state of the atmosphere inclines 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 orders 
have them, and some in greater numbers. As I lately 
4 Philos. Trans. 1816. 325. ¢. xviii. f. 1-7. > Ibid, f. 8-11. 
