INSECTS : THEIR FEET. 125 



ther clapped on tlie top of a stone." Bingley, citing 

 this opinion, adds that they are able easily to overcome 

 the pressure of the air " in warm weather, when they 

 are brisk and alert ; but towards the end of the year 

 this resistance becomes too mighty for their diminished 

 strength ; and we see flies 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 disengage their hollow cups from 

 the slippery surface." * 



But long ago another solution was proposed : ' for 

 Hooke, one of the earliest of microscopic observers, de- 

 scribed 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 presented such irregular- 

 ities, and that it was moreover always covered with a 

 " smoky tarnish," into which the hairs of the foot pene- 

 trated. 



The " smoky tarnish" is altogether gratuitous ; and 

 Mr. Blackwall has exploded the idea of atmospheric 

 pressure, for he found that flies could walk up the in- 

 terior 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 surface of the palms ; but further experi- 

 ments having showed him that flies cannot walk up 

 glass which is made moist by breathing on it, or which 

 * Anim. Biogr. 



