MOTIONS OF INSECTS. 



259 



firmly. That they walk upon glass he ascribes to some rug- 

 gedness in the surface ; and principally to a smoky tarnish 

 which adheres to it, by means of which the fly gets footing 

 upon it. 1 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. 2 That learned and 

 pious naturalist, Dr. Derham, seems to have been one of the 

 first who gave the true solution of this enigma. " Flies," says 

 he, (( besides their sharped hooked nails, have also skinny 

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

 smooth bodies, by the pressure of the atmosphere." 3 He com- 

 pares these palms to the curious suckers of male Dytisci, 

 before alluded to, and illustrates their action by a common 

 practice of boys, who carry stones by a wet piece of leather 

 applied to their top. Another eminent and excellent natural- 

 ist, the late Mr. White, adopted this solution. He observes 

 that in the decline of the year, when the mornings and even- 

 ings become chilly, many species of flies retire into houses and 

 swarm in the windows ; that at first they are very brisk and 

 alert ; but, as they grow more torpid, that they move with 

 difficulty, and are scarcely able to lift their legs, which seem 

 as if glued to the glass ; and that by degrees many do actually 

 stick till they die in the place. Then, noticing Dr. Derham's 

 opinion as just stated, he further remarks, that they easily 

 overcome the atmospheric pressure when they are brisk and 

 alert. But, he proceeds, in the decline of the year this resist- 

 ance 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. 4 



Sir Joseph Banks, to whom every branch of Natural 

 History has been so much indebted, excited an inquiry, the 

 results of which confirmed Derham's system concerning this 

 motion of animals against gravity. When abroad, he had 

 noticed that a lizard, on account of the sound that it emits 



i Microgr. 170. % iv.259. 



3 Physico-Theol. ed. 13. 363. note b. 4 Nat. Hist. ii. 274. 



S 2 



