BIOMECHANICS 



Shoe Fly 



To walk on walls and ceilings, your feet 



have to stick, hut they have to get unstuck, too. 



By Adam Summers ~ Illustrations by Tom Moore 



My mother is a rock 

 climber, the familial hu- 

 man fly. She practices 

 endlessly on walls anci cliffs, refining 

 her ability to stick to vertical surfaces 

 and overhangs. Watching her, I've 

 haci plenty of time to contemplate the 

 biomechanics of her gravity defiance. 

 As she glides up a wall and then spi- 

 ders along upsicie down across a 



"roof" section at a local gym, her 

 aerial ballet testifies to the powers of 

 friction anci adhesion, hi fact, the 

 same interplay of forces enables a real 

 fly to stick to walls. One day human 

 climbers may borrow some of the 

 fly's tricks for holding fast. 



Fortunately for the fly, nothing in 

 nature is perfectly flat and smooth; 

 any interaction between surfaces is 



Fly foot gets a grip with sticky hairs and with daws shaped //ke a set of bull's 

 horns. In the false-color scanning electron micrograph, the footpads of a 

 syrphid fly (Eristalis pertinax), depicted in beige, are covered with minute 

 hairs, or tenent setae. The hairs both increase the contact area of the fly's foot 

 and secrete a sticky fluid, enabling the fly to walk upside down. The set of 

 claws can help the fly pry its footpads loose. The image is magnified 120X. 



really a story of bumps hitting lumps. 

 Friction is the force that results when 

 the bumps on one surface smack into 

 and snag on the lumps of another. 

 Adhesion, a close cousin to friction, 

 is the result of the molecular attrac- 

 tion between two materials as they 

 are being pulled apart. Usually, 

 though not always, increasing the 

 adhesion between two surfaces in- 

 creases the friction, too. Together 

 these forces enable flies to walk just 

 about anywhere. 



Stanislav Gorb and his research 

 group at the Max Planck Institute 

 for Metals Research in Stuttgart, 

 Germany, have examined the footfalls 

 of dozens of insect species. With elec- 

 tron microscopy, high-speed video, 

 and clever devices for measuring 

 forces, Gorb's group has looked at 

 how these creepy crawlers attach and 

 detach themselves from surfaces. Flies, 

 beetles, and thousands of other insects 

 depend on a system of hairs to hang 

 on. For all their clinging power, 

 though, sticky feet do exact a cost: the 

 better the fly sticks to a surface, the 

 harder it is to get unstuck. 



Under the electron microscope 

 you can see that a fly's foot ends 

 in a soft pad covered with tiny hairs, 

 called tenent setae. Each hair termi- 

 nates in a delicate spatula that maxi- 

 mizes the contact area of the foot by 

 flattening against the surface on which 

 It stands [see micrograph at left]. Increas- 

 ing the contact area increases the fric- 

 tional forces that keep the foot rooted 

 down. Rock climbers, too, try to 

 increase surface area for a better hold 

 by "smearing," or spreading the balls 

 of their feet over rocks. 



But flies do something even more 

 active and interesting to get a grip, as 

 Gorb's group found. By flash-freez- 

 ing surfaces where flies had been 

 walking, the researchers highlighted 

 the sweaty little footprints that flies 

 leave wherever they go. Under pres- 

 sure, the flies' footpads secrete an 

 emulsion that, like ice cream, is a 

 mixture of sugars and oils; the goop 

 coats the tenent setae and creates a 



28 



NATURAI IIISIORY February 2006 



