Flies have four ways of detaching a foot from a surface, as Stanisiav Gorb and 

 his group observed. A fly can push a foot away from its body, causing the 

 footpads to fold up and lift free (a); twist its footpads free (b); peel off the back 

 of its footpads by planting its claws and rocking its foot forward (c): or simply 

 yank backward, scraping its claws across the top of its footpads (d). 



Strong bond between hair and surface 

 through capillary adhesion. This 

 form of adhesion is familiar to any- 

 one who has ever found an advertis- 

 ing flyer on the windshield: a dry 

 one flutters off", whereas soggy paper 

 clings stubbornly to the glass, even 

 while you drive down the highway. 



That same stubborn adhesion pre- 

 sents a problem for the fly. If its feet 

 stuck too well to whatever it landed 

 on, it would have to just stay put and 

 order room service. So how does it 

 get unstuck? Gorb watched hundreds 

 of videotaped detachment events at a 

 slow speed. What he found is that the 

 fly has not one strategy, but four, for 

 freeing up a stuck foot. Pushing the 

 foot away from the bociy tencis to 

 scrunch up the footpads, popping 

 them free. The other options are 

 twisting the pads loose, prying them 

 up with the help of two little claws on 

 the end of the foot, or simply yanking 

 them away from the surface with 

 brute force [sec illustratiivis at r(sj//r]. 



No fly in its right mind is going to 

 want to go to all that trouble unless 

 it's absolutely necessary. Usually, 

 when walking on a ceiling or a wall, 

 the fly has a relatively slow gait be- 

 cause four of its feet are attached to 

 the surface at any one time. On the 

 ground, though, flies can save energy 

 by walking like most other insects, 

 with only three feet on the ground. 

 The insect's six legs form alternating 

 tripods, with the body supported by a 

 fore and a hind limb on one side, in 

 concert with the middle limb on the 

 other. The ground gait is the six- 

 legged equivalent of a trotting horse 

 that has two feet planted at a time — 

 and, just as a trot is faster than a walk, 

 the trotting fly is a faster fly. 



The damp nature of fly-foot con- 

 tact has other consequences as well. 

 In high humidity and under strong 

 pressure, the fluid between the tenent 

 setae can act like grease, causing the 

 foot to slide. So flies have adopted a 

 cockeyed strategy for hanging onto 

 walls. If a fly stood vertically on a 

 wall, the forces on its footpads would 

 tend to detach the setae. Next time 



one lands on a fridge door near you, 

 notice that the fly stands at an angle. 

 In that position, the setae are pulled 

 in the direction of strongest attach- 

 ment — diagonally. For the same rea- 

 son, the hardest thing for flies to do is 

 walk headfirst down a wall; they can 

 do it, but they are barely hanging on. 



Gorb's group is now working on 

 patterning various materials to scale 

 fly feet up to human size. With pho- 

 tolithography and laser drills they 

 etch a mold of tenent-setae look- 



alikes, then pour m a liquid poh nier 

 that solidifies into a flat sheet studded 

 with hundreds of thousands of tiny, 

 flanged columns. These prototypes 

 have a long way to go before any of 

 them is ready for wall walking. But 

 I'm certain my mom w ill be fust in 

 line tor a full set ot fly feet. 



Aihiiii Siiniiiu'is (asummers@uci.edu) is <iii 

 iissisiaiit projcssor oj biociniiiiccriii{i and ccol- 

 oijy and ctvhitioiiary /'/c/i'ij)' at tlic I 'iiitrrsiiy 

 of Calitornid, Irrinc. 



February 2006 NATURAl HISTORY 



29 



