ON DRY, POLISHED, VERTICAL SURFACES. 231 
The promptness and celerity of the movements of 
flies in an inverted position, or with their backs down- 
wards, on highly polished surfaces, and the certainty 
with which their hold is immediately secured when 
they alight upon them, would seem to preclude the 
possibility of the employment of muscular force on 
such occasions adequate to the instantaneous expul- 
sion of the air between their delicate climbing-appa- 
ratus and the plain on which they move, to the extent 
required for the formation of an efficient vacuum ; 
but every difficulty is at once obviated by admitting 
that a minute quantity of moderately adhesive fluid, 
which acquires a gelatinous consistency on exposure 
to the atmosphere, is emitted from the organs of 
sustentation. Unexceptionable evidence that such 
actually is the case has been obtained by observing 
that the extremity of each papilla becomes cauterized 
when subjected to the action of finely pulverized 
nitrate of silver; and that imsects, when traversing a 
vertical surface of glass, leave upon it a visible and 
enduring trace of their path, for the better perception 
of which a lens having a high degree of magnifying- 
power should be employed. 
Though perfectly ‘satisfied that the conclusion de- 
duced by me from the experiment with the air-pump 
‘rests on too secure a basis to be subverted, yet a 
desire to remove all apparent difficulties which may 
be thought to militate against the view that I have 
promulgated of the means by which numerous species 
