LETTERS ON ENTO3I0L0GY. Ill 
suckers which support them by the pressure of 
the atmosphere. These suckers consist of a 
membrane capable of extension and contraction ; 
they are concavo-convex, with scollopped 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 glass, the air is sufficiently expelled 
to produce the pressure necessary to keep the 
animal from falling. When the suckers are dis- 
engaged, they are brought together again so as 
to be confined within the space between the two 
claws : this may be seen by watching the move- 
ments of a fly, inside a glass tumbler, with a 
common microscope. If you wet a piece of 
leather, and apply it closely to any thing, you 
will see an example of this suction in the dif- 
ficulty you will find in detaching it. You must 
have often observed that in the autumn the flies 
begin to move more slowly, and appear some- 
times to stick to the glass ; Mr. White remarks 
on this, that when their strength diminishes, 
the atmospheric pressure proves too strong for 
them to overcome, and they appear to labour 
along. They are besides furnished with a cushion 
of bristles or hooks, and claws. 
