CH. XV.] SPECIES OF DIPTEROUS INSECTS. 217 



Another interesting peculiarity observable in the 

 domestic 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 ceil- 

 ings of rooms, back downwards, without its posi- 

 tion being disturbed in consequence of being con- 

 trary to gravity. Much diversity of opinion has 

 taken place among naturalists upon this curious 

 subject, and even in the latest works we find the 

 matter still forming a " questio vexata." Dr. Der- 

 ham, in his u Physico-Theology," speaking of the 



Leg of the fly highly magnified, with the terminal joint still more 

 increased, seen in different positions, to show the suckers. 



means 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 

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