218 NATURAL HISTORY. [cH. XV. 



manner as I have seen boys carry heavy stones 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 investi- 

 gations 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 mi- 

 nutely serrated, and the under concave surface cov- 

 ered 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 like- 

 wise adopted this opinion, considering it as " proved 

 most satisfactorily." Other authors of no mean re- 

 pute 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 



