Nov. 1, 1865.1 



SCIENCE-GOSSIP. 



253 



THE FOOT OE A ELY. 



"D ECENTLY an article appeared in these pages 

 -^*^ ou the Breeze-ily (page 194) in which some 

 interestiug particulars were detailed of the species 

 of Breeze-fly, with which the writer had been 

 brought into impleasaut contact. On the present 

 occasion we have selected a portion from our com- 

 mon species {Tahanus bovimis) to illustrate a few 

 observations on the feet of flies in general. How 

 can a fly walk upon the ceiling ? is a question which 

 has often been asked, and a great many successive 

 efforts have been made to answer it. The most 

 recent researches in this direction are those of Mr. 

 Tuffen West, which were published in the " Tran- 

 sactions of the Linuean Society, for 1861," profusely 

 and beautifully illustrated, and we shall freely avail 

 ourselves of the results of those reseai'ches. 



The foot of the fly differs a little in minor parti- 

 culars in the house-fly, the blow-fly, and the drone- 

 fly, but there is a still greater divergence from this 

 type in the foot of the breeze-fly. In the former 

 instances the fifth tarsal joint is terminated by a 

 doable pad, but in the present the pad is treble. On 

 the upper surface at the base of these pads is a pair 

 of strong claws. Somehow or other the fly manages 

 to walk, head downwards by the aid of these pads 

 or these claws, or both ; but how this is accomplished 

 we can scarcely understand without a more minute 

 examination of the parts already named. It must 

 be premised that in our illustration the upper surface 

 of the foot is shown. 



The pad, cushion, or pulvillus is deeply cleft into 

 what seems to be three pads, but which is in reality 

 a single pad with three lobes. The under surface is 

 clothed with minute hairs, which are somewhat 

 trumpet-shaped, expanding at the outer extremity 

 into a membranous elastic disk. It has been sup- 

 posed that each of these hairs is hollow, and when 

 the fly moves over a highly-polished surface a minute 

 drop of fluid passes down it, to assist in adhesion, 

 by the more effectually excluding the air. There 

 was at one time a notion extant that the whole sur- 

 face of the pad was a kind of " sucker," and that 

 by means thereof the fly sustained itself, asascliool- 

 boy lifts a stone by means of a moistened disk of 

 leather at the end of a string. 



The pair of claws, shown in the figure, are move- 

 able, and both to them and the pad, flexor and ex- 

 tensor muscles are attached. 



The mode in which these parts are employed, is 

 thus graphically described by Mr. West : — " When 

 a fly is not making use of its pads {piilvilli), as on a 

 surface sufficiently rough to afford it foothold with 

 its claws alone, these only are made use of. On a 

 smooth surface, perpendicular or horizontal, the 

 pulvilli are brought down, and the teneut (holding) 

 hairs applied to such surface ; a slight push forwards 



of these, succeeded by a gentle draw backwards, at 

 each application, removes the air between their soft 

 elastic expansions and then- plane of motion, and 

 tbus a firm hold is gained. Access of aii- is prevented 

 by the minute quantity of moisture which exudes 

 from the expanded tips of the tenent appendages ; 

 and thus a vacuum is formed. When the fly wishes 

 to move a leg from its place of attachment, tlie 

 claws are brought down and pressed against the 

 surface ; from their position they raise the hinder 

 part of the pulvillus, v^here the tenent hairs""' are 

 least developed, first, and so on forwards. I think 

 a fly when once stuck fast, if it had no claws might 

 remain so." 



" That the pressure of the atmosphere is the main 

 agent by which a fly is enabled to adhere to perfectly 





-> / 



^^.«f->\5 



Foot of the Breeze-fiy. 



smooth surfaces, cannot, I conceive, be doubtea. 

 Careful experiments on the weights of numerous 



* The term " tenent hairs " is applied to the trumpet- 

 shaped liairs wliich cover the lower surface of the pads of 

 pulviUi. 



