396 



EXPERIMENTS ON FLIES. 



in clean jars, and phials of transparent glass; the interior surface of which they 

 traversed in every direction with the greatest facility, walking upon it even with 

 their backs downwards, while they remained in full vigour ; but when enfeebled 

 by exposure to cold, or when fatigued by over exertion, the identical individuals 

 ascended the sides of the same jars and phials with considerable difficulty, falling 

 from them in numerous instances, and they were entirely incapacitated for adhering 

 to them in an inverted position ; yet, when their physical energy was restored by 

 repose, or an increase of temperature, they again repeated their most extraordinary 

 feats with all their original promptness and dexterity. 



" Flies which are unable to maintain an inverted position on highly polished 

 bodies, will frequently adhere firmly, with their backs downward, to glass rather 

 defective in polish, or slightly soiled ; indeed, I remark generally, that the results 

 of experiments, 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 rot supported on the vertical sides 

 of smooth bodies by the pressure of the atmosphere, nor by the aid of a glutinous 

 secretion, but by means strictly mechanical, as Dr. Hooke has suggested : he 

 erred, however, in supposing that the hairs on the underside of the tarsal mem- 

 branes are pointed, and that there is a smoky substance on glass which they 

 penetrate. One other link in the chain of evidence was wanting to place the 

 matter beyond all dispute, and that, the kindness of Mr. W. Hadfield, of Coin- 

 brook, has enabled me to supply. With his assistance, and the help of his air- 

 pump, it was demonstrated to the entire satisfaction of several intelligent 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 movements of flies on 

 polished perpendicular surfaces, I shall offer a few remarks on the apparatus, by the 

 instrumentality of which 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 membranes, varying in size, figure, and number, in differ- 

 ent species; the. edges of which are plain, not serrated, as Sir E. Home asserts, 

 though when placed in such a situation relative to the eye of the observer, that 

 the hairs connected with them are fore-shortened ; they certainly present an appear- 

 ance which, on a superficial view, might lead to the latter conclusion. 



" The hold which insects are enabled to take of any roughness, or irregularity of 

 surface, by means of the fine hairs composing the brushes, must be very consider- 

 able ; and 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." p. 421. 



To us these details are highly interesting, and we cannot too 

 strongly recommend our readers, who may have an opportunity to 



