110 EVENINGS AT TOE MICROSCOPE. 



labouring along, and lugging their feet on windows as if 

 they stuck fast to the glass : and it is with the utmost 

 difficulty they can draw one foot after another, and dis- 

 engage their hollow cups from the slippery surface." * 



But long ago another solution was proposed : for Hooke, 

 one of the earliest of microscopic observers, described the 

 two palms, pattens, or soles (as he calls the indvUl%), as 

 "beset underneath with small bristles or tenters, like the 

 Avire teeth of a card for working wool, which, having a 

 contrary direction to the claws, and both pulling different 

 Avays, if there be any irregularity or yielding in the surface 

 of a body, enable the fly to suspend itself very firmly." 

 He supposed that the most perfectly polished glass pre- 

 sented such irregularities, and that it was moreover 

 always covered with a " smoky tarnish," into which the 

 hairs of the foot penetrated. 



The " smoky tarnish," is altogether gratuitous ; and 

 Mr. BlackAvall has exploded the idea of atmospheric pres- 

 sure, for he found that flies could Avalk up the interior of 

 the exhausted receiver of an air-pump. He had explained 

 their ability to climb up A'ertical polished bodies by the 

 mechanical action of the minute hairs of the inferior sur- 

 face of the palms; but further experiments haA T ing shown 

 him that flies cannot Avalk up glass Avhich is made moist 

 by breathing on it, or which is thinly coated with oil or 

 flour, he was led to the conclusion that these hairs are in 

 fact tubular, and excrete a viscid fluid, by means of which 

 they adhere to dry polished surfaces ; and on close insjDec- 

 tion with an adequate magnifying power, he was always 

 able to discover traces of this adhesive material on the 

 track on glass both of flies and various other insects 

 furnished with pul-cUli, and of those spiders which 

 possess a similar faculty, f 



In the earlier editions of Kirby and Spence's " Intro- 

 duction to Entomology," Mr. Kirby had adopted the 



* " Anim. Biogr." t "L : nn. Trans.," xvi. 490, 768. 



