Commercial silkworm fibers are currently harvested from the finished cocoon of the silk moth, Bombyx mori. This product is less than a third as strong as a spider's dragline silk and only half as stretchy. But if the threads are extracted directly from the silkworm, these properties can be enhanced dramatically, report Zhenghong Shao of Fudan University in Shanghai and Fritz Vollrath of Oxford University. The determining factor, it seems, is the speed at which the silk is spooled as it comes out of the caterpillar's silk glands. Slow spinning produces silk that matches the stretchiness of dragline silk, whereas fast spooling makes it tougher, though not quite as tough as the spider's strands.
The researchers propose that silkworms could be a viable source of strong silk, with fewer modifications than the arachnid variety requires. (The biotech firm Nexia is currently experimenting with goats genetically engineered to produce dragline silk in their milk.) "We suggest that silkworms might be able to produce threads that compare well with spider silk by changing their spinning habits," they write, "rather than having their silk genes altered."