ON DEr, POLISHED, VERTICAL STJHFACES. 139 



therefore, either to feebleness resulting from some other cause, or 

 to an increase in the adhesiveness of the fluid secretion emitted 

 from the papillae in the act of climbing. If it should still be in- 

 sisted upon that the, phenomenon is the result of atmospheric 

 pressure, it behoves the advocates of that hypothesis to explain in 

 what manner a little condensed vapour causes the liberation of 

 insects that are unable to accomplish the act by their own unaided 

 efforts. That an organ deemed to be capable of so entirely ex- 

 pelling the air from the space between its extremity and smooth 

 surfaces with which it is brought in contact as to produce a va- 

 cuum, should yet be incompetent to effect the exclusion of so 

 dense a fluid as water, does certainly appear to be in the highest 

 degree improbable*. 



The promptness and celerity of the movements of flies in an 

 inverted position, or with their backs downwards, on highly po- 

 lished surfaces, and the certainty with which their hold is imme- 

 diately secured when they alight upon them, would seem to pre- 

 clude the possibility of the employment of muscular force on such 

 occasions adequate to the instantaneous expulsion of the air be- 

 tween their delicate climbing apparatus and the plain on wbich 

 they move, to the extent required for the formation of an efficient 

 vacuum ; but every difficulty is at once obviated by admitting 

 that a minute qiiantity of moderately adhesive fluid, which ac- 

 quires a gelatinous consistency on exposure to the atmosphere, is 

 emitted from the organs of sustentation. Unexceptionable evi- 

 dence that such actually is the case has been obtained by obser- 

 ving that the extremity of each papilla becomes cauterized when 

 subjected to the action of finely pulverized nitrate of silver ; and 

 that insects, when traversing a vertical surface of glass, leave upon 

 it a visible and enduring trace of their path, for the better per- 

 ception of which a lens having a high degree of magnifying power 

 should be employed. 



Though perfectly satisfied that the conclusion deduced by me 

 from the experiment with the air-pump rests on too secure a basis 

 to be subverted, yet a desire to remove aU apparent difficulties 

 which may be thought to militate against the view that I have 

 promulgated of the means by which numerous species of insects 

 and spiders, and even some reptiles, are enabled to move on dry, 



* The adhesion of flies to the glass of windows and to other surfaces, towards 

 the end of summer and in autumn, is usually caused by the growth from the 

 interior of the body of a parasitic fungus {Sporendonema mttscee, Fries ; JSm- 

 pusa musccE, Colin). — G. B. 



