20 OK THE ANATOMY OF THE FLY. 



this was first ascribed by Dr. Derham* 4 to the exhaustion of air 

 from the foot pads; recently, it has been supposed to be due to the 

 exhaustion of air from the extremities of the hairs with which 

 the pad is clothed ;f others have ascribed it to be the holdj which 

 these minute hairs take of trifling irregularities of surface, but none 

 of these explanations are correct, and one of the earliest notions upon 

 the subject is the nearest to the truth; that is, that the feet se- 

 crete a glutinous fluid which glues them to the surface on which the 

 insect walks. When the pads are carefully examinod it will be seen 

 that they have no cup-shaped cavity beneath them, but that they 

 are hollow with a nipple-like protuberance projecting into each. 

 This will be seen more plainly by pressing upon the tarsus which 

 forces it into the pad ; by cutting off the end of the pad first, it may 

 be exposed in this manner, and will be found to consist of a 

 closed sac. 



This sac fills the whole of the last four tarsal joints and 

 is lined with pavement epithelium ; it secretes a perfectly clear 

 viscid fluid which exudes from it into the pad, and fills its cavity 

 as well as the hollow hairs with which its under surface is covered. 

 These hairs open by trumpet-shaped mouths, and the disc of each 

 mouth is kept full of the fluid. Sometimes, when the insect is cap- 

 tured and held between the finger and thumb, it exudes so rapidly 

 that the pads are soon covered with a little glistening drop of it, 

 which may be collected upon a glass slide where it rapidly solidi- 

 fies ; it is insoluble in water and solidifies under that fluid. The 

 whole contents of the tarsus becomes solid very rapidly as soon 

 as the insect is dead, or the part is removed. 



* Kirby and Spence, Introduction to Entomology. The whole history 

 of the various opinions of different Naturalists upon this subject are given 

 at length. 



t J. Hepworth, Koy. Micros. Soc. Journal, vol. iii. 314. T. West. Linn 

 Soc. Trans, vol. xxiii. 393. 



t Blackwall, Linn. Soc. Trans, vol. xvi , on the pulvilli of insects. 



