Non-stick spiders take delicate steps

Orb-weaving spiders avoid sticking to their own webs because of the way they move, say scientists. Naturalists first investigated the puzzle of how spiders move around their webs a century ago. The spiders' non-stick abilities were originally attributed to a special coating on their legs. But researchers in Costa Rica have now used modern imaging technology to record close-up footage that fully explains the ability. Dr William Eberhard and Dr Daniel Briceno published their findings in the journal Naturwissenschaften. "Orb-weaving spiders are abundant in many parts of the world, and their ability to build and utilise adhesive traps without becoming entangled themselves has been a source of puzzlement for more than 100 years," Dr Eberhard said. Observations in the early 1900s suggested that the spiders coated their legs with an oil from their mouth but without a close-up examination, this remained a theory. More recent studies found that spiders "tip-toed" around their webs on fine hairs to avoid too much contact with the sticky surface. However, during web construction the spiders are known to forcefully thrust their legs on to the threads but still, somehow, they do not stick. To solve this puzzle, Dr Eberhard and the team combined a video camera and a microscope to record the spiders in minute detail. They identified three factors that combined to stop the spiders from sticking: leg hairs that decreased the surface area available to stick; a chemical coating on the hairs that reduced the adhesion and the delicate way the spiders move their legs. "Spiders reduce their adhesion to the sticky lines in their webs by moving their legs carefully so as to allow the sticky lines to slide off easily," Dr Eberhard told BBC Nature. Under the microscope, the researchers saw that when a spider made contact with a sticky line the adhesive droplets were transferred to its leg hairs. Then, as the spider withdrew its leg from the web, the droplets slid down these non-stick hairs and dripped off the fine point at the end.The scientists compared this "drip tip" to the way water easily runs off the pointed leaves of tropical plants. "We were pleased to be able to confirm the conclusions made long ago from very rudimentary experiments by two pioneering naturalists," said Dr Eberhard who named the early researchers Henri Fabre and Maj R W G Hingston as his "scientific heroes".