MR. TUFFEN WEST ON THE FOOT OF THE FLY. 399 
appearance, it is extremely probable that they are analogous in structure, though i'rom 
the smallness of their dimensions I have not yet been able to satisfy myself that this is 
the case by direct observation, notwithstanding I have employed the highest magnifyin 
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 Silphidic, Staphylinidaj, and 
Cerambycidse ; but, from the difference in their structure, I am compelled to think that 
the hairs on the under surface of the tarsi of these spiders and beetles must differ mate- 
rially in their action. In JPrionus coriaccus 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" ' 
individuals frequently remained fixed to the sides of an exhausted glass receiver 
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 suffice to advert to their very low specific gravity ^^^^ 
Fly not weighing more than a graii 
fter 
o A ""o 
l), which renders it evident that a very slight degree 
the tips of the hairs— a quality which it undoubtedly 
of viscidity in the fluid emitted from the tip 
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 innoxious nature ("flour of wheat," "finely pulverized chalk or gypsum ), m 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 v hat we 
call -impalpable' powders are mostly quite as large as, and often vastly arger than the 
applied extremities of the hairs; so that the attempt by a human being to walk up ver- 
tical precipices covered with rolling shingle, were such precipices possible, would not m- 
■f Ibid. p. / 6/. 
* Loc. cit. p, 487. 
