110 EVENINGS AT THE 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 pulvilli), as 

 " beset underneath with small bristles or tenters, like the 

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

 contrary direction to the claws, and both pulling different 

 ways, 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. Blackwall has exploded the idea of atmospheric pres- 

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

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

 their ability to climb up vertical polished bodies by the 

 mechanical action of the minute hairs of the inferior sur- 

 face of the palms ; but further experiments having shown 

 him that flies cannot walk up glass which 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 inspec- 

 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 pulvilli, and of those spiders which possess 

 a similar faculty, t 



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

 duction to Entomology," Mr. Kirby had adopted the suc- 



* " Anim. Biogr." t " Lum. Trans.," xvi. 490, 768. 



