MOTIONS OF INSECTS. 323 



That flies can walk upon glass placed vertically, and 

 in general against gravity, has long feeen a source of 

 wonder and inquiry ; and various have been the opi- 

 nions of scientific men upon the subject. Some ima- 

 gined that the suckers on the feet of these animals 

 were spunges filled with a kind of gluten, by which 

 they were enabled to adhere to such surfaces. This 

 idea, though incorrect, was not so absurd as at first it 

 may seem ; since we have seen above in many instances, 

 and very lately in that of the Sminthurus fuscus, that 

 insects are often aided in their motions by a secretion 

 of this kind. Hooke appears to have been one of the 

 first who remarked that the suspension of these animals 

 was produced by some mechanical contrivance in their 

 feet. Observing that the claws alone could not effect 

 this purpose, he justly concluded that it must be prin- 

 cipally owing to the mechanism of the two palms, pat- 

 tens, or soles as he calls the suckers ; these he de- 

 scribes as beset underneath with small bristles or ten- 

 ters, like the wire teeth of a card for working wool, 

 which, having a contrary direction to the claws, and 

 both pulling different ways, if there be any irregula- 

 rity or yielding in the surface of a body, enable the fly 

 to suspend itself very firmly. That they walk upon 

 glass, he ascribes to some ruggedness in the surface ; 

 and principally to a smoky tarnish which adheres to it, 

 by means of which the fly gets footing upon it^. But 

 these tenter-hooks in the suckers of flies, and this 

 smoky tarnish upon glass, are mere fancies, since they 

 can walk as well upon the cleanest glass as upon the 

 most tarnished. Reaumur also attributes this faculty 



* Microgf. 170. 

 Y 2 



