How Come? Spiders can walk on a sticky web

Some spiders catch insects just by being sneaky, waiting patiently in a trap-doored burrow for a bug to bumble by. Others chase their prey on foot, the spider version of big cats. And some, like most orb weaver spiders, build webby traps. Usually spun under cover of darkness, an elaborate web can take hours to finish.

Orb weavers, who create the most intricate (and beautiful) webs, use several different kinds of silk. Dry threads are used to create the web's scaffolding. Think of a web as a bicycle wheel. The nonsticky threads form the hub and the spokes. Once the dry framework is in place, the spider adds a spiraling network of sticky threads, also known as snare silk.

Both varieties of thread come from specialized glands in the spider's abdomen. A single web may be woven of seven different kinds of silk, two sticky varieties and five nonsticky threads. Each has its own combination of stretchability, strength, and durability. (The dragline thread a spider uses to drop from the ceiling is only one-tenth as wide as a human hair. But it is five times stronger than steel of the same weight.)

When the web is finished, an unsuspecting insect (like a fly) may land among the sticky strands. But within a matter of seconds, the fly may have wiggled his way out again. So a hungry spider usually stays close by to pounce on her struggling prey. That's why you'll often see a spider hanging head down (and very still) at the center of a large web. Some spider species quickly wrap their prey in silk before administering the first venomous bite.

If even a large moth can't always get free from a web, then what prevents a spider from getting caught in its own sticky trap?

Some have claimed that spider feet secrete a kind of anti-stick oil, but no studies confirm this idea. Biologists do know that spiders seem to prefer navigating the drier, less-sticky parts of their webs, avoiding the gluey bits.

However, spiders can and do walk across the supersticky threads. That's because such spiders evolved with feet made for web-walking.

How does it work? The web-builders have two serrated claws designed for walking easily across grass and leaves. But the spiders also have a third claw, rather like a dog's dew claw. As a spider walks across a sticky strand, its third claw grabs the strand, and pulls it back against some spiny hairs on the leg. The claw then yanks the spines back, as an archer would draw back a bow and arrow. Finally, the claw "lets go," and the spines spring back into place, throwing off the sticky strand. So with each step, a spider automatically detaches her feet.

To find out more about the how a spider's foot works, visit: microscopy-uk.org.uk/mag/art

97b/benspid.html.