THE EPHEMERA. 



489 



glass not being found free from flaws and imperfec- 

 tions, 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 glutinous secretion, capable 

 of adhering to well-cleaned glass ; thus the Abbe de la 

 Pluche states, that when the fly marches over any pol- 

 ished 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 probable," 

 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 preservation of its claws, whose 

 pointed extremities would soon be impaired without this 

 prevention." Notwithstanding 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 ap- 

 plication of extremely small points to invisible irregular- 

 ities on the surface of glass, that the pulvilli or suckers 

 are attached, but by simple adhesion of the enlarged ends 

 of the hairs assisted by a fluid that is probably secreted 

 there, and the author is therefore reduced to refer the 



effect to molecular attraction only. It is also stated, 

 that when the foot of the fly is detached, a distinct fluid 

 trace will often be left by each individual hair, the spotty 

 pattern thus left on the glass appearing to be of an oily 

 character, for if breathed on, it remains after the moisture 

 is evaporated. The contrary opinion, although contained 

 in a review of Mr Blackwall's Memoir above noticed, 

 was evidently written in ignorance of the subsequent 

 observations of that author contained in the appendix of 

 the volume in which it appeared, and in which several 

 facts are stated, which appear " quite inexplicable, ex- 

 cept on the supposition that an adhesive secretion is 

 emitted by the instruments employed in climbing ;" 

 and it is subsequently affirmed, that careful and re- 

 peated examinations made with lenses of moderately 

 high magnifying powers, in a strong light and at a favour- 

 able angle, speedily convinced Mr Blackwall that his 

 conjecture was well founded, as he never failed to dis- 

 cover " unequivocal evidence of its truth." 



We have had a two-fold object in thus setting before 

 the reader, at considerable length, the various opinions 

 promulgated upon the subject, the first being occasioned 

 by the interest attached to so peculiar a phenomenon ; 

 and the second resulting from a desire to show that, even 

 in the commonest insects, there are most ample ma- 

 terials of no ordinary or uninteresting kiud for the full 

 exercise of the mind of the ingenious observer of nature 

 It will seem extraordinary, but it is nevertheless true, 

 that there is scarcely any domestic insect of whose econ- 

 omy we are more ignorant than that of the Mttsca do~ 

 mestica. History of Insects, London 1835. 



