Most spiders spin a variety of different silks that are utilized for
diverse ecological functions (Blackledge and Hayashi, 2006).
Dragline silk, produced from major ampullate silk glands, evolved
early in the diversification of spiders, perhaps in the Jurassic
(Vollrath and Selden, 2007), and it was the first silk to be spun as
discrete structural threads, rather than sheets of fibers. The origin
of dragline silk is a hypothesized key innovation in spiders'
evolutionary success (Bond and Opell, 1998). More than 40,000
species of spiders now use dragline silk for a variety of functions
such as lifelines and the frames of webs.
In addition to its clear evolutionary importance, dragline silk
exhibits many desirable qualities that make it a focus of biomimetic
research (Hakimi et al., 2007; Vollrath and Porter, 2006b). Dragline
silk is mechanically impressive. It combines high tensile strength
and elasticity in a low density fiber, achieving a strength to weight
ratio ~5 greater than steel and a toughness ~3 greater than Kevlar
(Booth and Price, 1989; Gosline et al., 1986; Guan, 2007; Vollrath,
2000; Vollrath and Knight, 2001; Vollrath and Porter, 2006b).