TRANSACTIONS OF THE LINN^AN SOCIETY. 449 



small portion only. The parts which touch commonly slide 

 very gradually over the surface, in the direction in which the 

 fly clings by them ; and when the foot is detached, a distinct 

 fluid trace will often be left by each individual hair. The 

 spotted pattern, thus left on the glass, seems of an oily character; 

 for, if breathed on, it remains after the condensed moisture is 

 evaporated. 



A slight dewiness from the breath, or an oily film on the 

 glass, disables the fly fi'om climbing it, if vertical, but not from 

 traversing it when flat ; and the insect ascends dry glass with 

 ease immediately after having done so ; but on watching one 

 which seemed stationary on an upright piece, having just 

 walked over a greased surface, it could be seen to be very 

 slowly gliding down without any change in the relative position 

 of its limbs. We have not found that the cleanness, or high 

 polish of glass, causes any difficulty to flies in climbing. 



The circumstances related imply, that it is not by the appli- 

 cation of extremely small points to invisible irregularities on 

 the surface of glass, that the pulvilli are attached, but by 

 simple adhesion of the enlarged ends of the hairs, assisted by 

 a fluid that is probably secreted there ; and we are therefore 

 induced to refer the effect to molecular attraction only — a 

 power difficult to estimate, but evidently great in some cases, 

 when an extremely thin flat surface is placed in contact with a 

 smooth body ; — and, in the present instance, the manner in 

 which the bent expanded ends of the congeries of elastic hairs 

 are applied, is particularly favourable for its exertion. 



When a fly would increase the firmness of its hold, it con- 

 tracts its leg so as to draw the tarsi to a direction more nearly 

 parallel to the glass, thus bringing into contact the hairs of the 

 whole indvillus, and a larger portion of each hair. On the 

 other hand, the foot is detached by an outward and upward 

 impulse given to the tarsus; the tendency of which must be to 

 draw off" the ends of the hairs in rapid succession, beginning 

 with those next it. This act is, as we suspect, assisted by a 

 strong blunt bristle that projects from a tuft of hair between the 

 pulvilli, and has a motion independent of them ; and by the two 

 claws that spring from one base, and branch over on each side, 

 their ends touching the glass, and preserving the position of 

 the foot as the insect clings. We have seen a blue-bottle fly, 

 {Musca vomiioria,) when so confined as not to be able to apply 



NO. V. VOL. I. o M 



