TO POLISHED VERTICAL SURFACES. 219 
the production of a vacuum between each membrane 
and the plane of position is, therefore, clearly im- 
practicable, unless the numerous papillz on the under- 
side of these organs separately perform the office of 
suckers ; and there does not appear to be any thing in 
their mechanism which in the slightest degree coun- 
tenances such an hypothesis. When highly magnified, 
their extremities, it is true, are seen to be somewhat 
enlarged; but whether they be viewed in action or in 
repose, they never assume a figure at all adapted to 
the formation of a vacuum. 
Satisfied that this difficult problem must admit of a 
solution more consistent with the various phenomena 
it comprehends than the popular one here contro- 
verted, I determined to institute an experimental 
investigation of it. Accordingly, having procured 
living specimens of the House-fly (Musca domestica) 
and of the large Flesh-fly (Musca vomitoria), 1 en- 
closed them in clean jars and phials of transparent 
glass, the interior surface of which they traversed in 
every direction with the greatest facility, walking 
upon it, even with their backs downward, while they 
remained in full vigour; but when enfeebled by 
exposure to cold, or by over-exertion, the identical 
individuals ascended the sides of the same jars and 
phials with considerable difficulty, falling from them 
in numerous instances, and they were entirely incapa- 
citated for adhering to them in an inverted position ; 
yet when their physical energy was restored by re- 
