394 MR. TUFFEN WEST ON THE FOOT OF THE FLY. 



in an interesting little work by George Power (IGGi)*. He lield 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 " (tlie Fly's) "feet, which substance is always repleated 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 otherways 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 liave been left for suc- 

 ceeding observers to discover, except such minute details of structure as are revealed by 

 the higher ])owers 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 spinoiis 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 draAvn 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 be 

 devised for them)— /^o^y these " tenters or points, whereof a Fly has about ten in each foot, 

 to wit two in every joint," assist in the action. This was the "mechanical theory," in 

 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 Fly in 

 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 

 it is certain that glass very frequently undergoes a slow decomposition on its surface in a 

 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 ])y most careful observations, which may be readily verified by any one desirous of 

 getting at the truth, that this tarnish does very materially assist a Fly when in a weak 

 state in maintaining its hold, and in freely moving upon the glass. To keep our windows 



♦ Experimental Philosophy, p. 5. t Micrographia, p. 1/0. 



