walk on the Vertical Surfaces of highly polished Bodies. 117 



to gravitation, are enabled to adhere to such surfaces, not by the 

 pressure of the atmosphere on their cUmbing apparatus, as is 

 commonly supposed, but by a viscous secretion emitted from the 

 papillae with which it is provided. 



A large, clean, open phial of transparent glass, containing the 

 larvae of several species of insects, capable of moving upon po- 

 lished perpendicular surfaces without the help of lines produced 

 by a spinning organ, was placed in the receiver of an air-pump, 

 from which the air was then exhausted by the usual process; 

 nevertheless, the larvae continued to traverse the inner surface of 

 the phial in every direction. This experiment was made in the 

 summer of 1827 ; in conducting it I was assisted by that distin- 

 guished philosopher and excellent man the late Dr. Dalton of 

 Manchester, who kindly allowed me to use his air-pump, and re- 

 marked, on witnessing the result, that it was physically impos- 

 sible that the larvse could be supported on the sides of the phial 

 by atmospheric pressure. 



In the next place, I put specimens of the common house-fly 

 into the receiver of an air-pump, and, after having exhausted the 

 air, observed that they walked readily upon its inner surface as 

 long as their vital powers were unimpaired, and that some indi- 

 viduals ultimately died adhering to its sides, from which it re- 

 quired a slight degree of force to detach them. Here, as in the 

 case of the larvse, it is evident that the insects could not be held 

 to the glass by the pressure of the atmosphere, so that this stri- 

 king fact supplies an experimentum crucis by which the insuffi- 

 ciency of the popular hypothesis to account for the phaenomenon 

 it is intended to explain is rendered manifest. 



Having cleared the way for a more exact investigation of the 

 subject by the detection of this prevailing error, it occurred to 

 me, that as the adhesion of insects to the upright sides of an ex- 

 hausted receiver cannot be occasioned by atmospheric pressure, 

 or by any exertion of muscular force, some individuals remaining 

 fixed even after life is extinct, it must be caused by the emission 

 of a viscous fluid from the papillae on the inferior surface of their 

 climbing apparatus. In order to ascertain whether this is the 

 case or not, I placed in clean phials of transparent glass, spiders 

 and various insects in the larva and imago states capable of 

 walking on their upright sides ; then breathing into the phials till 

 the aqueous vapour expelled from the lungs was copiously con- 

 densed on their inner surface, I found that the moisture totally 

 prevented the animals from obtaining any effectual hold on the 

 glass ; and a similar consequence ensued when the flour of wheat 

 or finely pulverized chalk was thinly distributed over their inte- 

 rior surface, the minute particles of those substances adhering to 

 the tarsal brushes of the spiders, the pul villi of the perfect in- 



