HOW FLIES HANG ON. 



69 



ulty of running over smooth bodies, but that the power should be 

 attributed to the molecular action between solid and liquid bodies ; 

 or, in other words, to capillary adhesion. 



If we examine the under part of the pulvilli (Fig. 1) with a micro- 

 scope, we shall see distinctly that it is furnished with numerous hairs, 

 regularly distributed. These hairs terminate, at their lower end, in a 



Fig. 1.— UfTDEB Part op a Fly's Foot.— 1. Palvilli, 200 times. 2. Hairs found on the eides, 



670 times. 3. Different forms of hairs. 



kind of bulb, the form of Avhich varies, whence flows an oily liquid that 

 dries slowly and does not harden for a long time. The minute drops 

 left on the glass by the hairs may be taken away, even after two or 

 three days have passed, without our having to moisten them, by sim- 

 ply rubbing a piece of fine paper over them. I have devised an ap- 

 paratus for collecting these drops by cutting a hole in a piece of board 

 over which I fix a glass slide. Turning the board over so that the 

 glass shall be at the bottom, I have a little cell with a glass floor. 

 With the aid of a piece of paper gummed to the wings, I introduce a 

 fly into this cavity in such a manner that the pulvilli shall rest upon 

 the floor. Then, putting the board under the microscope with the 

 glass slide uppermost, we have the fly's feet under our eyes. The in- 

 sect, struggling for liberty, places his pulvilli against the glass, and 

 leaves after each effort, traces that may be observed very distinctly, 

 for they are perfectly visible in a good light (Fig. 2). 



