FECBI.IAR MOTIONS OF INSECTS. 389 



smoky substance, adhering to the surface; so that, 

 though the sharp points on the sponges cannot pene- 

 trate the surface of the glass, it may easily catch hold 

 of the tarnish.'* This is evidently brorrovved from 

 Hook.j But it is singular that none of these fanciers 

 ever took the trouble to ascertain the existence of either 

 a gluten squeezed out by the fly, or of the smoky tar- 

 nish on glass. Even the shrewd Reaumur could not 

 give a satisfactory explanation of the circumstance. 



The earliest correct notion on this curious subject 

 was entertained by Derham, who, upon mentioning 

 the provision made for insects that hang on smooth 

 surfaces, says, 'I might here name divers flies and 

 other insects, who, besides their sharp-hooked nails, 

 have also skinny palms to their feet to enable them 

 to stick to glass and other smooth bodies, by means 

 of the pressure of the atmosphere — after the manner 

 as I have seen boys carry heavy stones with only a 

 wet piece of leather clapped on the top of the stone. 'J 

 The justly celebrated Mr White, of Selborne, ap- 

 parently without the aid of microscopical investiga- 

 tion, adopted Derham's opinion, adding the interest 

 ing illustration, that in the decline of the year, when 

 the flies crowd to windows and become sluggish and 

 torpid, they are scarcely able to lift their legs, which 

 seem glued to the glass, where many actually stick till 

 they die; whereas they are, during warm weather, so 

 brisk and alert that they easily overcome the pressure 

 of the atmosphere. § 



This singular mechanism, liowever, is not peculiar 

 to flies, for some animals, a hundred times as large, 

 can walk upon glass by the same means. St Pierre 



* Nature Displ., vol. iii, p. 98. Loud. 1823. 



t Micrographia, p. 170. 



t Physico-Theology, vol. ii, p. 194, note (6), 11th ed. 



§ Nat. Hist, of Selborne, vol. ii, p. 274. 



VOL. VI. 33^ 



