228 ON THE MOVEMENTS OF INSECTS 
vertical surface iz vacuo, thus conceded, must be 
ample to enable them to move on the glass of our 
windows in perfect security, under ordinary circum- 
stances, without the adventitious aid of atmospheric 
pressure ; the question of specific gravity, therefore, 
may be safely eliminated as being of no moment in 
any attempt to solve this interesting physiological 
problem. 
The argument so much relied upon by opponents 
is, that if flies retained their position on polished 
vertical surfaces by means of an adhesive fluid emitted 
from the hair-like papillae on the inferior surface of 
their pulvilli, they would, after remaining long in one 
situation, be unable to quit it by any muscular effort 
they could employ without seriously injuring those 
delicate parts, in consequence of the tenacity that the 
fluid would acquire by desiccation ; whereas it is well 
known that their movements are not in the least 
impeded by this circumstance. Plausible as this 
reasoning is, it appears to be based on the erroneous 
supposition that the properties of the fluid resemble 
those of animal-glue or vegetable-gum, an assump- 
tion which is at variance with all the particulars 
that have been ascertained in connexion with the 
phenomenon; in fact the fluid merely assumes a 
gelatinous consistency on exposure to the atmosphere, 
and is readily removed from the pulvilli, when re- 
dundant, by the customary mode of cleansing those 
organs employed by insects, which it could not 
