INSECTS: THEIR FEET 131 



that they are able easily to overcome the pressure of the 

 air "in warm weather, when they are brisk and alert; but 

 toward the end of the year this resistance becomes too 

 mighty for their diminished strength; and we see flies 

 laboring along, and lugging their feet on windows as if 

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

 culty 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, 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 sur- 

 face of a body, enable the fly to suspend itself very 

 firmly." He supposed that the most perfectly polished 

 glass presented such irregularities, and that it was more- 

 over 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 pressure, 

 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 showed 

 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 



1 \\uim. Bior, " 



