458 MOTIONS OF INSECTS. 
Dipterous insects, which in general have these organs, and some three 
on each foot?, are not exclusively gifted with them ; for various others 
could walk up the sides of an exhausted receiver, denies that their suckers have any 
such power of forming a vacuum as is above ascribed to them, and explained their 
ability to climb up vertical polished bodies, such as glass, by the mechanical action 
of the minute hairs which clothe the inferior surfaces of the suckers, nearly ag 
Dr. Hooke had suggested ; but further experiments having shown him that flies 
cannot walk up glass which is made moist by breathing on it, oris thinly coated 
with oil or flour, he was led to the conclusion that these hairs are in fact tubular, 
and excrete a yiscid fluid, by means of which they adhere to dry polished surfaces ; 
and on close inspection with an adequate magnifying power, ke was always able to 
discover traces of this adhesive material on the track on glass both of flies and 
various insects with pulvilli, and of those spiders which have the same power of 
climbing polished surfaces, such as Salticus scenicus, &c, (Linn. Trans. xvi. 490, 
768. ; compare also Lntom. Mag. i. 557.) i 
On repeating Mr, Blackwall’s experiments, I found, just as he states, that when 
a pane of glass of a window was slightly moistened by breathing on it, or dusted 
with flour, blue-bottle flies, the common house-flies, and the common bee-fly (Lris- 
talis tenax) all slipped down again the instant they attempted to walk up theso 
portions of the glass ; and I moreover remarked that eacl time after thus slipping 
down, they immediately began to rub first the two fore tarsi, and then the two hind 
tarsi, together, as flies are so; often seen to do, and continued this operation for some 
moments before they attempted again to walk. This last fact. struck me very 
forcibly, as appearing to give an importance to these habitual procedures of flies 
that has not hitherto, as far as I am aware, been attached to them. These move- 
ments I had always regarded as meant to remove any particle of dust from the legs, 
but simply as an affair of instinctive cleanliness, like that of the cat when she licks 
herself, and not as serving any more important object ; and such entomological 
friends as I have had an opportunity of consulting tell me that their, view of the 
matter was precisely the same; nor does Mr, Blackwall appear to have seen it 
in a different light, since, though so strongly bearing on his explanation of the 
way in which flies mount smooth vertical surfaces, he never at all refers to it. 
Yet, from the absolute necessity which the flies on which I experimented appeared 
to feel of cleaning their pulvilli immediately after being wetted or clogged with 
flour, however frequently this occurred, there certainly seems ground for suppos- 
ing that their usual and frequent operation for effecting this by rubbing their 
tarsi together is by no means one of mere cleanliness or amusement, but a very 
ene point of their economy, essentially necessary for keeping their pul- 
villi in a fit state for climbing up smooth vertical substances by constantly removing 
from them all moisture, and still more all dust, which they are perpetually liable to 
collect. In this operation the two fore and two hind tarsi are respectively rubbed 
together for their whole length, whence it might be inferred that the intention is to 
remove impurities from the entire tarsi ; but this I am persuaded is not usually 
the object, which is simply that of cleaning the under side of the pulvilli by rubbing 
them backward and forward along the whole surface of the hairs with which the 
tarsi are clothed, and which seem intended to serve as a brush for this particular: 
purpose. Sometimes, indeed, when the hairs of the tarsi are filled with dust 
throughout, the operation of rubbing them together is intended to cleanse these 
hairs; because without these brushes were themselves clean, they could not act 
upon the hairs of the under side of the pulvilli. Of this I witnessed an interesting 
instance in an Lristalis tenax, which by walking on a surface dusted with flour had 
the hairs of the whole length of the tarsi, as well as the pulvilli, thus clogged with 
it. After slipping down from the painted surface of the window-frame, which she 
in vain attempted to climb, she seemed sensible that before the pulvilli could be 
brushed it was requisite that the brushes themselves should be clean, and full two 
1 Philos, Trans, 1816, 826. t. xviii. f, 8—11, 
