Explaining how cobwebs are constructed
I am writing this ‘Material Matters’ in early November when, unseasonably, it is warm and the sun is shining. A warm period at the beginning of November is traditionally called ‘St Martin’s summer’ and early November is also the time when geese are customarily eaten. Hence a St Martin’s summer became a ‘goose-summer’, which in turn evolved into ‘gossamery’ because at this time of year gossamer, the silken webs of small spiders, covers the fields and floats in the air.
The uniqueness of spiders
This brings me to my subject, the magic and science of gossamer, that is, of cobwebs. But first a word or two about spiders. There are at least 35,000 species occupying every continent except Antarctica and varying in body length from just under one millimetre to 90mm. They have up to eight eyes, and each of their eight legs has the same number of segments. Should a spider lose a leg it can usually grow another.
The males are smaller than the females, and mating so exhausts them that they often die afterwards and are unceremoniously eaten by the female. Nearly all spiders eat insects and those that produce webs have up to six silk glands in their abdomens, each producing a slightly different silk thread. The silk is a protein called fibrion.
Solving the problem of elasticity in cobwebs
The silk threads in a web must withstand the impact of an unsuspecting insect, but if they deformed elastically, the prey would rebound as from a trampoline and escape. One solution is to make the silk sticky so that the prey is trapped. Another solution relies on some silk threads that have much of their lengths in reserve in the form of coils trapped inside watery beadlets strung along the length of the filaments. On impact, these windlasses unwind and the filaments extend up to 10 times their apparent initial length. Little elastic recovery can occur and the prey gets tangled and caught in this additional material.
How cobwebs are constructed
The difficult task of constructing the first thread across a gap within which a web is to be created is solved by some spiders extruding a single silk filament and constructing a tiny, flattened silk sail or kite at its end, which they make sticky. This is floated in the air and hopefully reaches the other side where it sticks, bridging the gap. This silk ‘bridge’ may not be the right length to form part of the final web so the spider severs it by biting through it while keeping hold of the cut end with her feet.
She then crosses the gap supported by the cut end in front, which she gradually eats while extruding new thread from her rear, becoming a living link in her own bridge. She extrudes new silk slightly faster than she consumes the silk bridge so the new thread ends up just the right length to form a ‘V’ which will be the first spokes of her web. The spider then creates the third spoke by attaching a thread from the point of the ‘V’ downwards to the lowest anchor point, thus creating a ‘Y’. Subsequently, 25-30 spokes of sturdy non-sticky silk form the framework. Finally, a spiral of slender sticky thread is put in place, starting from the centre and working outwards. To assist in laying down this spiral, a temporary ‘scaffolding’ of another type of (non-sticky) silk is employed and afterwards consumed.
Much of the information given here, and much more besides, appears in the second chapter of Climbing Mount Impossible by Richard Dawkins. Dawkins, who has been described as the Chief Disciple of Darwin on Earth, will no doubt be celebrating the 200th anniversary of Darwin’s birth on 12 February.