Silkworms coerced to make better silk. (Spinning Fine Threads).

The caterpillars that spin commercial silk can make much tougher or more elastic threads, depending on how fast they're forced to spin.

If this research finding is translated into a marketable process for obtaining silk, the fibers could rival those of widely acclaimed but commercially impractical spider silk, says Fritz Vollrath of Oxford University.

Many scientists hold that spider silk is the ultimate material--strong and tough, yet elastic. If produced in large quantities, spider silk could replace synthetic materials in surgical sutures, seat belts, or even carpeting, suggests Carl Michal of the University of British Columbia in Vancouver.

But harvesting spider silk is labor intensive. Spiders must be tied down and their silk reeled out using a small motor. What's more, "if you put many spiders in a box together, you tend to end up with only one big one," says Michal. "So, you have to store them in different cages."

In contrast, even children can easily harvest silk from the caterpillars known as silkworms, says Vollrath. The silkworms construct thumb-size cocoons, each made from a single thread nearly a kilometer long. More than 4,000 years ago, cloth makers in China began collecting, washing, and unraveling these cocoons. Silkworms yield 80,000 to 100,000 tons of commercial silk annually, says Vollrath.

Now, he and a coworker have found that fibers directly reeled out of silkworms' bodies can have improved characteristics. In the Aug. 15 Nature, Vollrath and Zhengzhong Shao of Fudan University in Shanghai, China, report that slow reeling--at 4 millimeters per second--produces silk with the same elasticity as spider silk. Fast reeling--at 13 mm per second--doesn't improve elasticity but increases silkworm silk to three-fourths the toughness of spider silk. The silkworms normally spin at speeds varying from 4 to 20 mm per second.

Although not quite as good as spider silks, the improved silkworm silk could give them "a run for their money," the researchers argue. Moreover, Vollrath suggests, silk producers might breed caterpillars that naturally spin silk faster or slower than normal and thus produce better silk threads in their cocoons.

The new work "is clever," says David Kaplan of Tufts University in Medford, Mass.

"It really emphasizes the importance of the fiber-formation conditions," notes Michal. Yet, he points out, "there is something important in the genes of the various organisms that plays a role in determining the properties of the different materials."