391 MR. TUFFEN WEST ON THE FOOT OF THE FLY 
in an interesting little work by George Power (1661) *. He held that the claws enabled 
the Fly to take hold of any roughness of the surface on which it might be moving ; and 
when the latter was too smooth to afford any advantage of this kind, that there was "a 
fuzzy kind of substance like little sponges, with which she" (Nature) "hath lined the 
soles of her" (the Ply's) "feet, which substance is always repleaded with a whitish viscous 
liquor, which she can at pleasure squeeze out, and so sodder and be-glew herself to the 
plain she walks on, which other ways her gravity would hinder (were it not for this con- 
trivance) especially when she walks in those inverted positions." 
The near approach gained by Power, at one stride, towards the true structure of the 
minute parts in question, as well as towards what I believe to be the true explanation of 
their action, is indeed remarkable, when the early date of his observations is considered. 
Had he only seen the hairs on the Ply's cushions, nothing would have been left for suc- 
ceeding observers to discover, except such minute details of structure as are revealed by 
the higher powers of our modern microscopes. 
The next observer was the well-known Hooke, who wrote only three years later (1667) f. 
He saw, in addition to what Power had made known, the minute hairs on the under sur- 
face of the " soles," and recognized (as later observers hardly appear to me to have done) 
the importance of the grasp gained by the claws, when drawing against the strong, for- 
ward-pointing and sharp spinous hairs (one at either side of the basal line of each tarsal 
joint, underneath), wherever any projections, or a yielding surface, are presented by the 
plane on which the insect may be moving. The effect gained in such an action of the 
claws, when drawn backwards and inwards against the " soles with their small bristles or 
tenters, which have their points looking the contrary way, that is, forwards and out- 
wards," is distinctly mentioned and insisted upon. " If there be any irregularity," he 
continues, " or yielding in the surface of the body, the Ply suspends itself very firmly 
and easily, without the access or need of any sponges filled with an imaginary gluten." 
Further on, he proceeds to show how these " tenters " (a better name could not he 
devised for them)— how these " tenters or points, whereof a Ply has about ten in each foot, 
to wit two in every joint," assist in the action. This was the "mechanical theory," hi 
its purest expression, as the preceding had been the « viscid " one. It was a decided 
advance towards a complete explanation, although manifestly insufficient for all the facts 
of the case. He believed in the existence of a « smoky substance on glass," into which 
he thought that the minute hairs of the « soles " penetrated, and so assisted the Ply » 
holding on to that smooth and slippery material. This belief in a " smoky substance on 
glass has been considered a mistake by every successive writer on the subject ; and yet 
tarn that glass very frequently undergoes a slow decomposition on its 
moist atmosphere, from the excess of alkali in its composition. Such a change is speedily 
produced in glass exposed to the action of the weather, as in our window-panes, and con- 
veys the appearance as if a « smoky substance " were condensed upon it. It has been 
proved by most careful observations, which may be readily verified by any one desirous oi 
getting at the truth, that this tarnish does very materially assist a Ply when in a weak 
state m maintaining its hold, and in freely moving upon the glass. To keep our windows 
* Experimental Philosophy, p. 5 . f Micrograpllia , p . , 7 0. 
