MOTIONS OF INSECTS. 457 



But, he proceeds, 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. 1 



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 

 before rain, named the Gecko 2 (Lncerta Gecko), could walk againstgravity up 

 the walls of houses ; and comparing this with the parallel motions of flies, he 

 was desirous of having the subject more scientifically illustrated than it 

 had been. This inquiry was put into the hands of Sir Everard Home, 

 who was assisted in it by the incomparable pencil of Mr. Bauer ; and it 

 was proved most satisfactorily that it is by producing a vacuum between . 

 certain organs destined for that purpose and the plane of position, suffi- 

 cient to cause atmospheric pressure upon their exterior surface, that the 

 animals in question are enabled to walk up a polished perpendicular, like 

 the glass in our windows, and the chunam walls in India, or with their 

 backs downward on a ceiling, without being brought to the ground by the 

 weight of their bodies. 



The instruments by which a fly effects this purpose are two suckers 

 connected with the last joint of the tarsus by a narrow infundibular neck, 

 which has power of motion in all directions, immediately under the root of 

 each claw. These suckers consist of a membrane capable of extension 

 and contraction ; they are concavo-convex, with serrated edges, the con- 

 cave surface being downy, and the convex granulated. When in action 

 they are separated from each other, and the membrane expanded so as to 

 increase the surface ; by applying this closely to the plane of position, the 

 air is sufficiently expelled to produce the pressure necessary to keep the 

 animal from falling. When the suckers are disengaged, they are brought 

 together again so as to be confined within the space between the two claws. 

 This may be seen by looking at the movements of a fly in the inside of a glass 

 tumbler with a common microscope. 3 Thus the fly, you see, does no more 

 than the leech has been long known to do, when moving in a glass vessel. 

 Furnished with a sucker at each extremity, by means of these organs it 

 marches up and down at its pleasure, or as the state of the atmosphere 

 inclines it. 4 



1 Nat. Hist. ii. 274. 



3 Amcen. Acad. i. 549. The Gecko, probably, is not the only lizard that walks 

 against gravity. St. Pierre mentions one not longer than a finger, that, in the Isle 

 of France, climbs along the walls, and even up the glass, after the flies and other 

 insects, for which it watches with great patience. These lizards are sometimes so 

 tame that they will feed out of the hand. ( Voyage, &c. 73.) Major Moor and 

 Captain Green observed several lizards in India, that run up the walls and over the 

 ceilings after the mosquitos. Hasselquist says that the Gecko is very frequent at 

 Cairo, both in the houses and without them, and that it exhales a very deleterious 

 poison from the lobuli between the toes. He saw two women and a girl at the 

 point of death, merely from eating a cheese on which it had dropped its venom. 

 One ran over the hand of a man, who endeavoured to catch it ; and immediately 

 little pustules, resembling those occasioned by the stinging-nettle, rose all over the 

 parts the creature had touched. (Voyage, 220.) M. Savigny, however, who exa- 

 mined this animal in Egypt, assures me that this account of Hasselquist's, as far as 

 it relates to the venom of the Gecko, is not correct. 



3 Philos. Trans. 1816, 325. t. xviii. f. 17. 



4 Mr. Blackwall, in a paper " On the Pulvilli of Insects," having found that flies 



