220 NATURAL HISTORY. [CH. XV. 



and ultimately falling from the station it occupied, 

 was a diminution of muscular force, attributable to 

 impeded respiration." Hence Mr. Blackwall is in- 

 duced to believe, in the memoir above referred to, 

 that insects are enabled to take hold of any rough- 

 ness or irregularity of surface, by means of the fine 

 hairs composing the brushes, the most carefully 

 polished glass not being found free from flaws and 

 imperfections, when viewed in a favourable light 

 with a powerful lens. 



A still different opinion has been maintained by 

 other authors upon this subject; who, setting aside 

 all idea of a vacuum, have conjectured that the 

 suckers, as they have been termed, contain a gluti- 

 nous secretion, capable of adhering to well-cleaned 

 glass ; thus the Abbe de la Pluche states, that when 

 the fly marches over any polished body, on which 

 neither her claws nor her points can fasten, she 

 sometimes compresses her sponge, and causes it to 

 evacuate a fluid, which fixes her in such a manner 

 as prevents her falling, without diminishing the 

 facility of her progress ; " but it is much more prob- 

 able," he adds, " that the sponges correspond with 

 the fleshy balls which accompany the claws of dogs 

 and cats, and that they enable the fly to proceed 

 with a softer pace, and contribute to the preserva- 

 tion of its claws, whose pointed extremities would 

 soon be impaired without this prevention." Not- 

 withstanding the ridicule which has been thrown 

 upon this opinion in a recent entomological work, it 

 appears, from still more recent investigations, to be 

 the best founded of any hitherto advanced. Thus, 

 an anonymous writer has published an account of 

 various experiments and examinations upon this 

 subject, which appear satisfactorily to prove, that it 

 is not by the application of extremely small points 

 to invisible irregularities on the surface of glass, 

 that the pulvilli or suckers are attached, but by sim- 

 ple adhesion of the enlarged ends of the hairs, as- 



