How do flies manage to appear to defy gravity and walk upside down

A.� The fly can stroll along our ceilings by virtue of its sticky feet. Two flaps of membrane, called pulvilli, cover their feet and allow them to cling to any surface. The combination of the hairs creates a kind of suction and the fly sticks to the surface, even upside-down.

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Additionally the hollow hairs, called retineriae, secrete a viscous substance that gives the fly extra sticking power by excluding air that would impede the effectiveness of suction.

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Q.� What do pulvilli look like

A.� They're tiny hooked adhesive hairs.

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Q.� That sounds like Velcro

A.� That's because the inventor of this modern miracle got his inspiration from the fly's foot. One side of the Velcro is made up of tiny hair like hooks which latch onto the opposing surface of tiny loops.

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Q.� But my ceiling isn't obligingly covered in tiny hooks for the fly to latch onto

A.� No the flies tiny hooks can latch onto to microscopic cracks that you can't see on your ceiling. The hairs can even 'stick' to minute fissures in glass that are invisible to the naked human eye.

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Q.� Having secured itself so effectively, how does the fly actually walk

A.� It usually has to lift one of its six feet at a time. It tips up the membranous flaps, which release the suction, allowing the insect to put one foot in front of another.

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Q.� Do the hairs on the fly's feet do anything else

A.� Yes, the hairs on its foot pads also serve as sensory organs. When the fly lands on a surface the feet start looking our for sugars, water and anything that has an unusual texture, basically they're scouting for food.

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by Lisa Cardy