456 MOTIONS OF INSECTS. 



(Dytisd\ as given for sexual purposes. 1 The three first joints of the anterior 

 tarsi of many of the larger rove-beetles (Staphylinus L.) are dilated so as 

 to form, as in the last-mentioned insects, an orbicular patella, but covered by 

 cushions. Since in them this is not peculiar to the males, it is probably 

 given that they may be able to support their long bodies when climbing. 



But the most remarkable class of climbers consists of those that are 

 furnished with an apparatus by which they can form a vacuum, so as to 

 adhere to the plane on which they are moving by atmospheric pressure. 

 That flies 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 sponges 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 Smin- 

 thurus 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 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 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. 2 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. 3 

 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 sharp 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." 4 He compares 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 naturalist, the late Mr. White, adopted 

 this solution. He observes that in the decline of the year, when the 

 mornings and evenings 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. 



1 See Kirby, in Fauna Soreali- Americana, on various modifications of these foot- 

 cushions amongst some tribes of beetles. 



2 Microgr. 170. 5 iv. 259. 

 < Physico-Theol ed. 13. 363. note b. 



