50 Facti in the Phydology of Spiders and Inseda. — Blackwall. 



Sec. 3. My explanation of the means Avliereby various animals are 

 supported in their movements on the vertical surfaces of highly pol- 

 ished bodies having recently been called in question, I am induced to 

 offer a few remarks in vindication of its accuracy. 



Mr. Tuffen West, in treating "On certain Appendages to the Feet 

 of Insects subservient to Holding or Climbing,'' advocates the hypoth- 

 esis, that the papilla; distributed over the inferior surface of the pulvilli 

 of flies and other species of the class Insecta act separately as indepen- 

 dent suckers, adhesion being assisted by the emission from each of a 

 small quantity of fluid. This view of the subject, being absolutely 

 irreconcilable Avith the results of observations and experiments re- 

 garded as having established a wholly different conclusion, I am not 

 prepared to adopt. That fluid is emitted from the papilla connected 

 with the pulvilli of the house-fly and flesh-fly when in motion is un- 

 questionable, 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 ap- 

 paratus 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 provided with the requi- 

 site organs to move with facility and security on the vertical surfaces 

 of dry, highly polished bodies. In confirmation of the exijlanation of 

 the phenomenon here insisted on, many fiicts might be advanced ; but 

 it will suffice, in the present instance, to direct attention to one, the 

 decisive character of which can not be mistaken. 



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

 bodies by the pressure of the atmosphere, experiments Avith the air- 

 pump plainly demonstrate ; for they can not only traverse the upriglit 

 sides and the interior of the dome of an exhausted receiver, Avhile their 

 . j)hysical energy is unimpaired, but individuals occasionally remain fixed 

 to the sides of the glass after they have entirely lost the poAver of lo- 

 comotion — a circumstance Avhich can only be explained by admitting 

 the adhesive property of the fluid emitted from the extremity of the 

 papillse on the inferior surface of their pulvilli. 



To the same cause must also be attributed the poA\-er of spiders that 

 are provided Avith scopulae or tarsal brushes, to run Avith celerity on the 

 A'ertical surfaces of highly polished bodies, as those instruments consist 

 of numerous appendages, slightly curved doAA-nward, and someAvhaten. 

 larged toward their extremity, Avhich is densely covered on its inferior 

 surface with minute, hair-like papilla^ for the emission of a viscid fluid, 

 but which, from their organization, can not possibly contribute to the 

 formation of a vacuum. 



