IN THE PUTSIOLOGY Or SPIDKKS AND INSECTS. 159 



Feet of Insects subservient to Holding or Climbing"* advocates 

 the hypotliesis that the papillae distributed over the inferior sur- 

 face of the pul villi of flies and other species of the class Insecta 

 act separately as independent suckers, adhesion being assisted 

 by the emission from each of a small quantity of fluid. This view 

 of the subject, being absolutely irreconcilable with the results of 

 observations and experiments regarded as having established a 

 widely different conclusion, I am not prepared to adopt. That 

 fluid is emitted from the papilla3 connected with the pulvilli of the 

 House-fly and Flesh-fly when in motion is unquestionable, as 

 finely pulverized nitrate of silver brought into contact with those 

 parts is immediately acted upon by it ; but that its agency is 

 merely to effect a more complete vacuum between the climbing 

 apparatus and the plane of position is evidently inadmissible. 

 This fluid, which appears to possess a moderate degree of viscidity, 

 assumes a gelatinous consistency when coagulated by exposure to 

 the atmosphere, and by its adhesive property enables animals pro- 

 vided with the requisite organs to move with facility and security 

 on the vertical surfaces of dry, highly polished bodies. In confir- 

 mation of the explanation of the phenomenon here insisted on, 

 many facts might be advanced ; but it will suffice, in the present 

 instance, to direct attention to one, the decisive character of which 

 cannot be mistaken. 



That flies are not supported on the vertical sides of highly- 

 polished bodies by the pressure of the atmosphere, experiments 

 with the air-pump plainly demonstrate ; for they can not only tra- 

 verse the upright sides and the interior of the dome of an exhausted 

 receiver, while their physical energy is unimpaired, but individuals 

 occasionally remain fixed to the sides of the glass after they have 

 entirely lost the power of locomotion — a circumstance which can 

 only be explained by admitting the adhesive property of the fluid 

 emitted from the extremity of the papillae on the inferior surface 

 of their pulvilli. 



To the same cause must also be attributed the power of Spiders 

 that are provided with scopulse or tarsal brushes to run with 

 celerity on the vertical surfaces of highly polished bodies, as 

 those instruments consist of numerous appendages slightly curved 

 downwards and somewhat enlarged towards their extremity, 

 which is densely covered on its inferior surface with minute hair- 

 like papillae for the emission of a viscid fluid, but which, from 



* Journal of the Proceedings of the Linnean Society, vol. ri. p. 26. 



13* 



