488 



HISTORY OF INSECTS. 



In figure D. we have exhibited the mouth in another 

 position, showing the sharpened upper lip raised from 



E D 



the fleshy proboscis, and in figure E the mouth of the 

 hlue-bottle fly is represented, having the tongue disen- 

 gaged from the superior stronger labrum. 



Another interesting peculiarity observable in the do- 

 mestic fly, arises from the structure of its feet, enabling it 

 to walk with the greatest facility, not only upon upright 

 surfaces, but also upon the ceilings of rooms, back down- 

 wards, without its position being disturbed inconsequence 

 of being contrary to gravity. Much diversity of opinion 

 has taken place amongst naturalists upon this curi- 

 ous subject, and even in the latest works we find the 

 matter still forming a " questio vexata." Dr Derham, 

 in his " Physico-Theology," speaking of the means 



Leg of the fly highly m,,fnified,u'ith the terminal joint MUmore 

 tncreated, teen tn different position*, to show theaucken. 



whereby insects maintain their position upon smooth 

 substances, states, that "divers flies and other insects, 

 besides their sharp-hooked nails, have also skinny palms 

 to their feet, to enable them to stick on glass and other 

 smooth bodies by means of the pressure of the atmosphere, 

 after the manner as I have seen boys carry heavy stoni'S 

 with only a wet piece of leather clapped on the top of the 



stone." Gilbert White, of Selborne, adopted Derham's 

 opinion, adding, that although the flies are easily enabled, 

 from their lightness and alertness, to overcome the 

 weight of air in warm weather, yet that in the decline 

 of the year this resistance becomes too mighty for their 

 diminished strength, and we see flies labouring along 

 and lugging their feet in windows as if they stuck fast to 

 the glass, and it is with the utmost difficulty that they 

 can draw one foot from another, and disengage their 

 hollow caps from the slippery surface. 



This opinion, which has been entertained by the 

 majority of Entomologists of the present day, has acquired 

 additional weight by the elaborate investigations of Sir 

 Everard Home, undertaken at the suggestion of Sir Joseph 

 Banks, with the assistance of that unrivalled microscopic 

 artist, M. Bauer, and published in the Philosophical 

 Transactions for 1816. The suckers, of which several 

 kinds of flies possess three to each foot, are attached, as 

 will be seen from our figures, beneath the base of the 

 claws, and are of an oval shape and membranous texture, 

 being convex above, having the sides minutely serrated, 

 and the under concave surface covered with down or 

 hairs. In order to cause the alleged vacuum, these 

 suckers are extended, but, when the fly wishes to raise 

 its legs, they are brought together and folded up as it 

 were between the hooks. Messrs. Kirby and Spence 

 have likewise adopted this opinion, considering it as 

 " proved most satisfactorily." Other authorsof no mean 

 repute have, however, entertained a different opinion, 

 and have entirely rejected the idea of a vacuum being 

 produced; thus Dr Hooke describes the suckers as palms 

 or soles, beset underneath with small bristles or tenters 

 like the cone teeth of a card for working wool, which he 

 conceived gives them a strong hold upon objects having 

 irregular or yielding surfaces ; and he imagined that 

 there is upon glass a kind of smoky substance, penetrable 

 by the points of their bristles. The same opinion is also 

 given by Shaw in his " Nature Displayed," and, more 

 recently, Mr Blackwall has considered that the motions 

 of the fly are to be accounted for upon mechanical prin- 

 ciples alone ; thus upon inspecting the structure of the 

 parts of the suckers (regarding which great want of ac- 

 cordance exists in the descriptions of authors), " it was 

 immediately perceived that the function ascribed to them 

 by Dr Derham and Sir E. Home is quite incompatible 

 with their organisation. Minute hairs veiy closely set 

 and directed downwards so completely cover the inferior 

 surface of the expanded membranes, improperly denom- 

 inated suckers, with which the terminal joint of the foot 

 of flies is provided, that it cannot possibly be brought 

 into contact with the object on which those insects move, 

 by any muscular force they are capable of exerting ; the 

 production of a vacuum between each membrane and the 

 plane of position is therefore clearly impracticable, unless 

 the numerous hairs on the underside of these organs 

 individually perform the office of suckers ; and there 

 does not appear to be any thing in their mechanism, 

 which in the slightest degree countenances such a hypo- 

 thesis. When highly magnified, their extremities, it is 

 true, are seen to be somewhat enlarged, but when they 

 are viewed in action or in repose, they never assume a 

 figure at all adapted to the formation of a vacuum." 

 Moreover, on enclosing a house-fly in the receiver of an 

 air-pump, " it was demonstrated to the entire satisfac- 

 tion of several intelligent gentlemen present, that the 

 house-fly, while it retains its vital powers unimpaired, 

 cannot only traverse the upright sides, but even the in- 

 terior of the dome of an exhausted receiver, and that the 

 cause of its relaxing its hold, and ultimately falling from 

 the station it occupied, was a diminution of muscular 

 force, attributable to impeded respiration." Hence Mr 

 Blackwall is induced to believe, in the memoir above 

 referred to, that insects are enabled to take hold of any 

 roughness, or irregularity of surface, by means of the fine 

 hairs composing the brushes, the most carefully polished 



