MOTIONS OF INSECTS. 319 



can walk upon glass placed vertically, and in general 

 against gravity, has long been a source of wonder and 

 inquiry; and various have been the opinions of scientific 

 men upon the subject. Some imagined 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 Sminthu- 

 rus fiiscus, 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 con- 

 trivance in their feet. Observing that the claws alone 

 could not effect this purpose, he justly concluded that 

 it must be principally owing to the mechanism of the 

 two palms, pattens, or soles as he calls the suckers ; 

 these he describes as beset underneath with small bristles 

 or tenters, 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 irregularity or 

 yielding in the surface of a body, enable the fly to sus- 

 pend itself very firmly. That they walk upon glass, he 

 ascribes to some ruggedness in the surface ; and princi- 

 pally 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 of these animals to the hairs 

 upon their suckers ^ That learned and pious naturalist, 

 * Microgr. 1 70. " iv. 259. 



