How can bugs climb walls

This form of attraction, based on the positive and negative charges of individual molecules, acts only when molecules of opposite charges are within a few nanometers of one another. The triangular-tipped setules on spiders' feet are perfectly designed to take advantage of the van der Waals force because they form hundreds of thousands of flexible contact points.

Because there are many small contact points, spiders can adjust the number of contacts needed for different surfaces, whether vertical, horizontal, smooth or rough. Though the total van der Waals force on the spider's feet is strong, it is really just the sum of many small attractive forces on each setule.

That makes moving its foot easy; the spider just lifts each setule one at a time, rather than trying to lift all at once. And unlike many types of glue, the van der Waals force is not affected by the surface or the surrounding environment.

This allows for an unusually high degree of adhesion on wet or oily surfaces. Like spiders, insects have evolved with their own climbing strategies, including claws and clamp-like devices on their feet. In addition, insects secrete an oily liquid that gives them extra adhesion. But in spite of these adaptations, most insects have only a fraction of spiders' ability to climb.

This is why flies tend to leave small, greasy footprints wherever they go. Each foot has a pair of large claws the fly uses to remove his feet from the wall or ceiling, enabling him to walk even though his feet are stuck to the surface.

To lift the foot, the fly presses down with the claws and twists the foot a little to remove it from the surface. Each step taken involves this process, which can be accomplished so rapidly, the insect doesn't even appear to pause while walking.

Insect feet often have microscopic bristle-like hairs, that are stiff enough to find the small inconsistencies in surfaces. Most ceiling and wall materials such as wood, drywall, ceiling tile and even concrete and glass, have very tiny pits and holes, making them porous. Insects can feel the holes and pits in the surface and grab into them with the stiff hairs on the bottoms of their feet.

This keeps the insect tethered to the surface. Walking upside-down requires a careful balance of adhesion and weight, and specialized trekking tools to combat the constant tug of gravity.

Each fly foot has two fat footpads that give the insect plenty of surface area with which to cling. The adhesive pads on the feet, called pulvilli, come equipped with tiny hairs that have spatula-like tips.

These hairs are called setae. Scientists once thought that the curved shape of the hairs suggested that flies used them to grip onto the ceiling. In fact, the hairs produce a glue-like substance made of sugars and oils.