490 Mr. Blackwall on the Pulvilli of Insects. 



highly polished bodies will frequently adhere firmly, with their 

 backs downward, to glass rather defective in polish, or slightly 

 soiled ; indeed, I may remark generally, that the results of ex- 

 periments, similar to those detailed above, will always be modified 

 by the vigour of the insects and the state of the glass vessels 

 with regard to cleanness and polish. 



. These facts plainly indicate that flies are not supported on the 

 vertical sides of smooth bodies by the pressure of the atmo- 

 sphere, nor by the aid of a glutinous secretion, but by means 

 strictly mechanical, as Dr. Hooke has suggested: he erred, how- 

 ever, in supposing that the hairs on the underside of the tarsal 

 membranes are pointed, and that there is a smoky substance on 

 glass which they penetrate. One other link in the chain of evi- 

 dence was wanting to place the matter beyond all dispute, and 

 that, the kindness of Mr.W. Hadfield of Cornbrook has enabled 

 me to supply. With his assistance, and the help of his air-pump, 

 it was demonstrated to the entire satisfaction of several intelli- 

 gent gentlemen present, that the House-fly, while it retains its 

 vital powers unimpaired, can not only traverse the upright sides, 

 but even the interior of the dome of an exhausted receiver; and 

 that the cause of its relaxing its hold and ultimately falling 

 from the station it occupies, is a diminution of muscular force 

 attributable to impeded respiration. 



Having thus established the mechanical theory of the move- 

 ments of flies on polished perpendicular surfaces, I shall offer a 

 few remarks on the apparatus by whose instrumentality they 

 accomplish their purpose. In structure and function it bears 

 the closest analogy to the pulvilli of insects, which, if named 

 with reference to the most important office they perform, should 

 be termed holders or supporters. It consists of expanded mem- 

 branes, varying in size, figure, and number in difterent species, 

 the edges of which are plain, not serrated, as Sir E. Home 



asserts. 



