How silk works - the orb web
Despite its light and delicate appearance spider silk has amazing structural properties. These properties vary considerably between silk types and usage.
The functional use of silk is well illustrated by the orb web. Catching a fast flying insect in an orb web can be likened to snaring a jumbo-jet in a scaled-up web with silk lines only a few millimetres thick. How is this done?
The typical orb web consists of outer frame lines to which radial (spoke-like) lines are attached, providing support for the characteristic spiral sticky line that occupies most of the web's surface. Dragline silk from the major ampullate silk glands (which also provide the spider's safety line) is used to make the frame and radial lines of the web. Its tensile strength is greater than that of mild steel and almost all artificial fibres (kevlar is the exception). By contrast, the spiral line silk (from the flagelliform glands) is weaker but very much more stretchy (elastic) than the framework silk. The use of these two very different silks gives the orb web the required strength and stretchiness to cope with the impact of fast flying insects and the struggles of captured prey. The absorption of an insect's impact energy by the stretching silk means that the insect neither breaks through nor simply bounces off the web surface - such webs would not make good trampolines! And finally, of course, the droplets of sticky silk (from the aggregate silk glands) on the spiral line stick the prey onto the web.
The orb web framelines have another property - when moistened in humid air they contract, a characteristic that may help keep the web under tension and in shape after deformation by wind, rain or prey. The relative stiffness of the frame and radial lines also makes then excellent transmitters for signalling the twitches of struggling prey back to the spider at the web's centre.
