MR. TUFFEN WEST ON THE FOOT OF THE FLY. 399 



appearance, it is extremely probable that they are analogous in structure, though from 

 the smallness of then- dimensions I have not yet been able to satisfy myself that this is 

 the case by direct observation, notwithstanding I have employed the highest magnifying 

 power at my command*." It appears to me that an error has crept in here, through the 

 inadequacy of the instrument employed in these investigations into structures so minute 

 and delicate as those in question. The under surface of the enlarged ends of the hairs is 

 always perfectly smooth in beetles, and all other insects, though the same surface is 

 minutely serrate in the climbing spiders. There are a few small scale-like processes on the 

 upper surface of the same part in species belonging to the Silphidse, Staphylinidas, and 

 Cerambycidte ; but, from the difference in their structure, I am compelled to think that 

 the hau-s on the under surface of the tarsi of these spiders and beetles must differ mate- 

 rially in their action. In Frionus coriaceus this serration on the upper surface of the hairs 

 forming the tarsal cushion presents an appearance somewhat like that described by Mr. 

 Blackwall, but there are certainly no serrations on the under surface. Mr. Blackwall admits 

 that " whoever examines the most carefully polished glass in a favourable light, with a 

 powerful lens, will speedily be convinced that it is not free from flaws and imperfections," 

 whence " the hold which insects are enabled to take of any roughness or irregularity 

 of surface must be very considerable." " The mechanical theory of the movements 

 of flies on polished perpendicular surfaces," considered to have been " established," 

 did not prove quite satisfactory on still further experiments being tried. Accordingly 

 Mr. Blackwall sent a third communication on the subject, which will be found amongst 

 the " Extracts from the Minute-Book " of the Society!. In it is announced the further 

 interesting discovery, that flies walking over well-cleaned glass- leave such indications 

 of their steps as would be produced by the emission of a fluid from the tips of the 

 hairs on the under surface of the tarsal cushions. On further trials with the air-pump, 



it was "observed" "in experimenting upon the House-fly" "that 



individuals frequently remained fixed to the sides of an exhausted glass receiver after 

 they had entirely lost the power of locomotion, and an evident distension of the abdomen 

 had been occasioned by the expansion of the aeriform fluids it contained. To detach 

 them from these stations, the employment of a small degree of force was found requisite." 



With respect to the circumstance of flies remaining attached to the bell-glass of the 

 air-pump, it may sufiice to advert to their very low specific gravity (a large Blue-bottle 

 Fly not weighing more than a grain), which renders it evident that a very slight degree 

 of viscidity in the fluid emitted from the tips of the hairs— a quality which it undoubtedly 

 possesses — would effect such an adhesion as occurred in the cases mentioned. 



Experiments were then made to determine the effects of moisture, oil, and powders of 

 an hinoxious nature ("flour of wheat," "finely pulverized chalk or gypsum"), in pre- 

 venting the flies from adhering to the surface of the glass, and their results are mentioned. 

 It appears to have been forgotten in these experiments that the particles even of what we 

 call 'impalpable' powders are mostly quite as large as, and often vastly larger than tlie 

 applied extremities of the hairs ; so that the attempt by a human being to wrxW up ver- 

 tical precipices covered with rolling shingle, were such precipices possible, would not in- 



* Loc. cit. p. 487. t IljiJ- P- '67. 



