492 MOTIONS OF INSECTS. 



that at first they are very brisk and alert ; but, as they grow more torpid, 

 that they move with difficuhy, 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. Derhani's opinion as 

 just slated, 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 resistance becomes too mighty for their diminished strength ; 

 and we see flies laboring along, and luororin'j their feet in windows as if 

 they stuck fast to the glass. ^ 



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^ {Laccrta Gecko), could walk against 

 gravity 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 mo^t satisfactorily that it is by producing a 

 vacuum between certain organs destined for that purpose and the plane 

 of position, sufficient 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 ceHing, without being brought 

 to the ground by the weight of their bodies. 



The instruments by which a fly efl^ects 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 

 concave 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."* Thus the 



» Not. Hist. ii. 274. 



' Aman. Acad. i. 519. The Gecko, probably, is not the only lizard that walks against 

 gravity. Si. Pierre mentions one not lonjjer than a finger, that, in the Isle of France, clinabs 

 along the walls, and even up the gla^s, alter the flies and other insects, for which it watches 

 with great patience. These lizards are sometimes so tame that ihey will feed out of the 

 hand. ( Fwy^pe, itc. 73.) Major Moor and Captain Green observed similar lizards in India, 

 that ran up the walls and over the ceilings after the mosquiios. Hasselquisi 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 loes. 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 endeavored to catch it ; and immediately little pustules, 

 resembling those occasioned by the stinging-nettle, ro.se all over the parts the creature had 

 touched. {Voijasfy 220 ) J\l. Savigny. however, who examined this animal in Egypt, 

 assures me that this account of Hasselquisi's, as far as ii relates to the venom of the Gecko, 

 is not correct. 



3 PhUos. Trans. 1816, 325. t. xviii. f. 1—7. 



