WEST 
°>95 
'V 
clear for the admission of light, it requires to be constantly removed. To opticians, 
similar deposition of moisture on the glasses of their lenses is but too well known J th 
call it the " sweating of glass." 
Hooke clearly foresaw the difficulties presented by the " viscid secretion " theory, and 
put them in the following forcible manner : " I could not well comprehend how, if there 
were such a glutinous matter in those supposed sponges, as most (that have observed that 
object " [the Ely's cushions] " in a Microscope) have hitherto believ'd, how, I say, the 
Fly could so readily unglew and loosen its feet." 
Leeuwenhoek {circa 1690)* brought to this, as he did to every other subject he investi- 
gated, his usual clear-headed sa^aciousness. He saw flies, " almost as Urae as a bee " 
every year " in the month of August, sitting on a "lass, at the backside 
o ~~ " o 
of" ..." his house " (Uristalis tenax). " The extremities of their feet," lie found, " were 
w 
covered with an incredible number of hairy parts, by the help of which they are better able 
than other flies to climb up a glass though it be ever so free from impurities or irregularities 
of which they might take hold. I have therefore," he says, " often placed the feet of those 
flies before the microscope, in order to view the means by which they can fasten themselves 
to the glass, and run up it ; and I have for some years past thought that I could discover 
that these hairs were each of them provided with crooked parts like hooks, by the help of 
which they can take the firmer hold on glass ; but which parts have never, to my know- 
ledge, been described by any person, though the figures of those hairs may be seen in 
many authors." 
Here there is a distinct reference to the peculiar flexure downwards of these parts, 
which will presently have to be mentioned in the description that I shall give of them 
from my own observations. 
Leeuwenhoek, thinking that the action of these hairs was purely mechanical (*. e. that 
they acted only like so many minute hooks), illustrated his views by a description, which 
follows the above, of his observations on some remarkable hairs on "the hind-feet " of 
" large crabs "...." caught among the rocks in Norway •" which hairs are " many of them 
provided with a double row of parts like teeth, placed in very exact order beside each other. 
in like manner as if we were to imagine the back of a knife cut into a double row of teeth or 
notches." « This wonderful formation, I am persuaded, is intended for this pur- 
pose, that when the crab is climbing up the rocks, he may be enabled by this assistance 
to fix his feet firmly on the rocks or stones t." 
He also describes another form of insect-foot which bears distinctly on our present 
purpose, in the notice of a "Ply" infesting "the blossoms of fruit-trees, particularly 
apples " {Anthonomus pomorum ?). " Observing," he says, " that these insects " could run 
along or stand for a long time on any side of the glass, even with their feet upwards, I was 
desirous to examine accurately the formation of their feet, and, in this little creature, I saw 
such perfectly formed limbs, enabling it to adhere to the glass, and to run along upon its 
surface, as distinctly as I had ever seen in other larger flying animals." 
* Collected Works (translated by Samuel Hoole, 1800-1807), vol. ii. part 3. p. 7 
t Ibid. pp. 71, 72. 
71. 
vol. xxiii. 
3 ii 
