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



aptly represent the condition of a fly strnggiing ineffectually to ascend a bottle strewed 

 witli these powders. So with watery yapoiir 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 Fly'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 ca^, though sometimes very difficult of detection, on 

 account of the minute qu.antity 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 Ije 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 hau'-like append- 

 ages constituting the brushes of spiders, and occiundng 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 larvae, some apodous, others with Ijoth true and 

 false feet {prologs), and others again with true legs fvirnished with " hair-like appendages," 

 are enabled to ascend perpendicular glass surfaces. In the case of the larvae destitiite of 

 legs, a viscid mucus is emitted in great abundance, respecting which, and its action per se 

 and as an aid in producing a vacwum, 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 

 sm'faces " " 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." A 

 note-worthy observation this, which I can but further exemplify by going more into 

 detail respecting the minute structure of the parts. 



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 of 

 the case, " eschewing all mere conjecture, however plausible," as " the bane of Natural 

 History :" they were frequently repeated, and the greatest care was taken to avoid every 



* Auuals and Magazine of Natural History, vol. sv. p. 119. 



