How Do Spiders Make Silk?
Though often feared, most spiders are harmless and actually quite impressive considering the strength and durability of the tangled webs they weave. The silk that spiders produce is five to six times stronger than high-grade steel by weight, and is stronger than any known natural or synthetic fiber on Earth.
Spider silk is more durable and elastic than the strongest man-made fiber, Kevlar, which is used to fill bulletproof vests, said Fritz Vollrath, an evolutionary zoologist at the University of Aarhus in Denmark and an expert on spider silk, in an interview with Discover magazine.
It's also extremely pliable – spider silk is able to stretch up to 140 percent of its length without breaking, according to John Lienhard, professor emeritus of mechanical engineering at the University of Houston.
For such a mighty material, spider silk is remarkably lightweight: a strand long enough to circle the Earth would weigh less than a bar of soap, according to "The Book of Animal Ignorance: Everything You Think You Know Is Wrong" (Random House, 2007).
The fine protein fiber spun by spiders, also called gossamer, serves many purposes. When a baby spiderling first hatches from an egg, it releases a lone stand of silk, patiently waits to be carried away by the wind and is dispersed to a new location by the breeze, a process known as "ballooning."
When a spiderling lands, it will secrete silk to build webs to capture other insects for food. Some species of spiders prefer to make tiny webs between their legs and then pounce on the insect they hope to make their dinner, wrapping the web around the prey in order to overcome it. This costs the spider much less time and energy than building a large, elaborate web, according to Linda Rayor, an assistant professor of entomology at Cornell University.
Some spiders also use gossamer to make protective nests or cocoons, and some suspend themselves on silk strands so they can travel distances of at least several feet, carried by the wind. And some even recycle – they eat their own web once it has served its purpose.
Scientists have long tried to understand the mechanisms that spiders use to make their silk, and have so far been unsuccessful in attempts to recreate the silk in laboratories. What makes the silk so difficult to artificially recreate is the make-up of its complex protein molecules and repetitive DNA sequences.
However, scientists do understand how the silk is hardened within spiders. Vollrath discovered that spiders harden their silk by acidifying it, a method similar to the process used in the manufacturing of industrial fibers such as nylon.
By examining the ducts inside spiders through which the silk flows before exiting the spider, Vollrath determined that before entering the duct, the silk consists of liquid proteins. When the liquid enters the duct, cells draw water away from the silk proteins and hydrogen is pumped into another part of the duct, creating an acid bath, according to Vollrath's paper published in Nature.
In this way, the unspun silk transforms from a gel into a final solid fiber as it is pulled through the spider's acidic silk glands, called the spinneret glands. Spiders can have two to eight spinnerets, which are usually in pairs, depending on the species. These spinnerets excrete different types of silk, from sticky to non-sticky to extra fine, depending on what the spider requires at the time.
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