400 MR. TUFFEN WEST ON THE FOOT OF THE FLY. 
aptly represent the condition of a fly struggling ineffectually to ascend a bottle strewed 
with these powders. So with watery vapour and oil in minute quantity ; whatever the 
true explanation, whether by vacuum, by viscid secretion, or by mechanical means, these 
substances, by preventing intimate contact between the acting parts of the foot and the 
fixed surface of support, i. e, the glass, would interfere equally with the Ply's ascent on 
all three suppositions. 
The belief that the presence of an adhesive secretion was requisite to explain all the 
observed facts led to the discovery that " spiders, and insects in the larva and imago state, 
when moving in a vertical direction on clean glass, leave " a "visible track behind them." 
This was found invariably to be the case, though sometimes very difficult of detection, on 
account of the minute quantity present. The truth of this assertion can be readily veri- 
fied, if due precautions be observed to ensure a satisfactory result. " On submitting this 
secretion to the direct rays of the sun in the month of July, and to brisk currents of air 
whose drying power was great, I ascertained,' ' says Mr. Blackwall, " that it did not 
suffer any perceptible diminution by evaporation under those circumstances." The pro- 
bable reason why it did not, 1 shall mention in due course. 
To the experiment with a noxious powder (nitrate of silver), mentioned subsequently by 
Mr. Blackwall in another place*, the objection, similar in kind, though still stronger in 
effect, must be made as to the other experiments with powders : the moisture (which is ad- 
mitted to be present, although varying in its quantity at different times and under different 
circumstances) must cauterize the parts after a little while, and so destroy their action, 
of whatever kind it consist. 
" Now it is reasonable to infer, from the foregoing researches, that the hair-like append- 
ages constituting the brushes of spiders, and occurring in such profusion on the inferior 
surface of the pulvilli of insects, are tubular," is the just inference drawn from the above. 
Mention is also made of the means by which larva?, some apodous, others with both true and 
false feet {prolegs), and others again with true legs furnished with " hair-like appendages,' 
are enabled to ascend perpendicular glass surfaces. In the case of the larva? destitute of 
legs, a viscid mucus is emitted in great abundance, respecting which, and its action per se 
and as an aid in producing a vacuum, I shall have some remarks to make from independent 
observations. 
The general possession of " hair-like appendages " by climbing insects was recognized, 
as was also the important fact that, " in structure and function, the apparatus by whose in- 
strumentality " spiders " accomplish their movements " " on highly polished perpendicular 
surfaces " « bears the closest analogy to the pulvilli of insects, which, if named in reference 
to the most important office they perform, should be termed holders or supporters " ^ 
note-worthy observation this, which I can but further exemplify by going more into 
detail respecting the minute structure of the parts. 
" A 
These remarks by Mr. Blackwall possess a very high value ; his experiments and 
observations were made with the express desire to ascertain simply the hard, dry facts ot 
the case, « eschewing all mere conjecture, however plausible," as « the bane of Natnra 
History :" they were frequently repeated, and the greatest care was taken to avoid every 
* Annals and Magazine of Natural History, vol. xv. p. 119. 
