490 Mr. BrAckwarr on the Pulvilli of Insects. 
highly polished bodies will frequently adhere firmly, with their 
backs downward, to glass rather defective in polish, or slightly 
, soiled; indeed, I may remark generally, that the results of ex- 
periments, similar to those detailedabove, will always be modified 
by the vigour of the insects aud the state of the glass vessels 
with regard to cleanness and polish. 
These facts plainly indicate that flies are not supported on the 
vertical sides of smooth bodies by the pressure of the atmo- 
sphere, nor by the aid of a glutinous secretion, but by means 
strictly mechanical, as Dr. Hooke has suggested: he erred, how- 
ever, in supposing that the hairs on the underside of the tarsal 
membranes are pointed, and that there is a smoky substance on 
glass which they penetrate. One other link in the chain of evi- 
dence was wanting to place the matter beyond all dispute, and 
that, the kindness of Mr.W. Hadfield of Cornbrook has enabled 
me to supply. With his assistance, and the help of his air-pump, 
it was demonstrated to the entire satisfaction of several intelli- 
gent gentlemen present, that the House-fly, while it retaius its 
vital powers unimpaired, can not only traverse the upright sides, 
but even the interior of the dome of an exhausted receiver; and 
that the cause of its relaxing its hold and ultimately falling 
from the station it occupies, is a diminution of muscular force 
attributable to impeded respiration. 
< Having thus established the mechanical theory of the move- 
ments of flies on polished perpendicular surfaces, I shall offer a 
few remarks on the apparatus by whose instrumentality they 
accomplish their purpose. In structure and function it bears 
the closest analogy to the pulvilli of insects, which, if named 
with reference to the most important office they perform, should 
be termed holders or supporters. It consists of expanded mem- 
branes, varying in size, figure, and number in different species, 
the edges of which are plain, not serrated, as Sir E. Home 
asserts, 
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