MOTIONS OF INSECTS. _ 232e 
those that are furnished with an apparatus by which they 
can form a vacuum, so as to adhere to the plane on which 
they are moving by atmospheric pressure. That flies 
can walk upon glass placed vertically, and in general 
against gravity, has long beena source of wonder and in- 
quiry; and various have been the opinions of scientific 
men upon the subject. Some imagined that the suckers 
on the feet of these animals were spunges filled with a 
kind of gluten, by which they were enabled to adhere to 
such surfaces. ‘This idea, though incorrect, was not so 
absurd as at first it may seem; since we have seen above 
in many instances, and very lately in that of the Smznthu- 
rus fuscus, that insects are often aided in their motions by 
. a secretion of this kind.. Hooke appears to have been 
one of the first who remarked that the suspension of 
these animals was produced by some mechanical con- 
trivance in their feet. Observing that the claws alone 
could not effect this purpose, he justly concluded that it 
must be principally owing to the mechanism of the two 
palms, pattens, or soles as he calls the suckers; these he de- 
scribes as beset underneath with small bristles or tenters, 
like the wire teeth of a card for working wool, which hav- 
ing a contrary direction to the claws, and both pulling 
different ways, if there be any irregularity or yielding in 
the surface of a body, enable the fly to suspend itself very 
firmly. That they walk upon glass, he ascribes to some 
ruggedness in the surface; and principally to a smoky 
tarnish which adheres to it, by means of which the fly 
gets footing upon it?. But these tenter-hooks in the 
suckers of flies, and this smoky tarnish upon giass, are 
a Microgr. 170. 
Nees 
