The Story of Silk (eBook)

2

Silk in Nature

2:1 Dew covered spider’s webs on my garden hedge.

In summer 2003 a four millimetre (1/6 inch) long thread with droplets of glue spaced along it was found near Jazzine in southern Lebanon. It looked as though it had come from the web of a modern orb web spider, but it was in a blob of tree resin that had solidified to amber 130-135 million years ago, according to popular legend at the time when the dinosaurs ruled the world. One of the plant eating forms may even have fed on the leaves of the tree. It is the oldest silk known. Its sticky droplets separated out then, as they do now, when the spider disturbed the viscous coating of glue on the thread by tweaking it with its hind leg.

Silk is one of the most amazing products of nature. Many insects, spiders, mites, centipedes, millipedes and even some marine shrimp-like animals (all, you will note, animals with jointed legs and no backbones) depend on it for survival at least some time in their lives. They may use it in their young stages, on special occasions, or daily, and they use it for a wide variety of purposes.

2:2 A hand-sized tarantula wandering from its lair in Peru.

As I mentioned earlier silk first made its appearance when spineless sea creatures started to invade the land. Among them were the ancestors of our spiders, and after 400 million years of development and innovation spiders have become the most familiar silk producers. They originally used their silk to wrap up their eggs and stop them drying out, a vital measure to ensure their survival on land. Later they used it to line their burrows and make trip wires to detect possible prey. Silk-lined burrows still feature in the lives of some descendants of these early ground-dwellers, including trapdoor spiders and this 13 cm (5 inch) legspan pink-toed tarantula Avicularia that I encountered in the Amazonian jungle. Since those early days animals have come to use silk in a wide variety of ways related to food gathering, self preservation, reproduction and dispersal.

2:3 A lacewing takes a rest after laying a batch of eggs on a plant.

Many creatures rely on silk for protection from their enemies. Latrodectus hesperus, a close relative of the notorious American Black Widow spider Latrodectus mactans repels mice and other unwelcome intruders into its territory by smearing sticky silk on their heads. Unprotected eggs are especially at risk and we see a variety of ways for keeping them safe from predators.3 Lacewings for example, as shown here (Fig. 2:3) lay their eggs on the ends of long stalks made of a relatively inflexible silk where they are beyond the reach of many potential foe, while the common garden Wolf Spider Pardosa hunts with its eggs concealed in a silk sack attached to the rear of its abdomen. Nursery-web spiders of the family Pisauridae also carry their egg sacks around while hunting, clasping them beneath their bodies between their fangs and palps (Fig. 2:4). When the female detects the vibrations that tell her that her young are about to hatch out she puts the sack in a silken mesh over which she stands guard until her offspring can fend for themselves.

2:4 The Nursery-web spider Pisaura mirabilis with her silken egg sac.

2:5 Guarding her newly hatched brood in their web – she can just be seen to the right of the silk mesh.

2:6 Moth caterpillars feeding communally in their nest, Ancona, Lake Maggiore, Italy.

Pisaura and its relatives are not alone in ensuring the safety of their brood in silken shelters. Some sawfly caterpillars take refuge in silken webs, as do tiny relatives of wasps, lacewings and various other insects and spiders – usually during the most vulnerable stages of their life cycles. Honeybees line their pupal cells with silk, while some insects such as leaf-rolling caterpillars and a wingless cricket with the delightful name of the Carolina Leaf Roller (Camptonotus carolinensis) live in tubes made by binding leaf edges together with silk strands. Others, including caterpillars of ermine moths of the genus Yponomeuta feed communally beneath silken tents for protection and in some years they are so abundant as to shroud whole trees in their canopies of white silk. Outbreaks of the European Apple Ermine moth Yponomeuta malinellus and the Bird-cherry Ermine moth Yponomeuta evonymella (Figs. 2:7-2:10) can be spectacular, sometimes hitting newspaper headlines.4

Left to right 2:7, 2:8 & 2:9. The Bird-cherry Ermine moth defoliating trees in Northumberland. Its silken nests (centre) protect the caterpillars and their silken cocoons (right).

2:10 An Ermine moth emerged from one of the cocoons shows us its long coiled tongue or proboscis.

The tropical weaver ants (Oecophylla) that I have seen in Africa, southern Asia and Australia make some of the most impressive nest shelters, and they make them in a most unusual way. Lining up side by side at the edge of a leaf, the adults clamp their jaws on a nearby leaf and pull it towards them. If this proves too difficult they summon help from their companions, and together they form parallel chains to bridge the gap between the leaves and draw them closer together. Meanwhile other ants bring their larvae to the scene and wait on the sidelines until the leaves overlap. Then at a given signal they move forward with their young in their jaws, and stroke them gently with their antennae. If they hold their offspring the right way round, that is with their heads pointing forwards, they respond by releasing sticky silk. The adults then use the grubs like living shuttles to glue the leaves together – almost as though they were minute tubes of superglue.

2:11 A weaver ant’s nest with its white silk threads binding leaves together. Sal tree forest, Bangladesh.

The result of their joint efforts is a rainproof nest as big as a melon in which the ants can rear their brood in safety. Any disturbance to their abode evokes an immediate response by the colony’s soldiers, which rush around in an agitated fashion, jaws agape, preparing to fend off intruders (Fig. 2:12). And although they do not sting, like Captain Cook’s men who set shore on the east coast of Australia in 1789 I can vouch for their ability to inflict painful bites, made all the more unpleasant for several days by their habit of squirting a corrosive spray of formic acid into the open wounds.

2:12 On the warpath: weaver ants on an Australian ‘T’ shirt.

One curious incident exercised my powers of observation some years ago. I had received a bag of beans imported from China with a request to identify the pest that had damaged them. About half of them had 2-3 mm (1/10 inch) diameter holes with microscopic bite marks round their edges and traces of faeces in the cavities. Although the culprits were long gone, I was convinced that they were insects, yet I searched in vain for their cast skins among the seeds. Eventually I came across a 2 mm diameter bundle of tightly intertwined fibres wedged in one of the holes. When teased apart, I found one relatively long grey thread, but most were short, 3 mm or so, and red or blue. Here was the clue; the pest was one of two moth caterpillars that feed in legume seeds in Far East Asia and make their cocoons with coloured silk. They have fascinating names – the Adzuki Pod Worm, Matsumuraeses phaseoli and the Soybean Pod Borer, Leguminivora glycinivorella.

Many spiders and caterpillars have adopted a wily way of protecting themselves against enemies with a form of bungee jumping that can be practiced anywhere at a moment’s notice. When threatened they fix a stout silk dragline to a nearby perch and launch themselves into thin air, letting out more line as they drop out of harm’s way (Figs. 2:13 and 2:14). Their threads can be extended at will, baffling those enterprising predators that try to reel them in as though they were caught on fishing lines. That theirs is an even more upmarket version of the sport is attested by the fact that when danger is past the creatures simply climb back up the threads, a practice that has not to my knowledge been repeated by bungee jumpers. On other occasions web-making spiders and various caterpillars emulate rock climbers and use their draglines abseil fashion to make a more leisurely and controlled descent to the ground.

2:13 & 2:14 Emperor moth caterpillars and spiders suspended on silk threads, from a nineteenth century natural history book.

Butterfly and moth caterpillars spin their silk through a small tube that projects from their mouthparts. It was originally a duct leading from a salivary gland. Spiders such as the common garden diamond back Araneus diadematus on the other hand have a much more complex set of silk producing organs arrayed on several lobes known as spinnerets situated on the underside of their abdomen, as we see in Figs. 2:14 and 2:15.

2:15 Spinnerets of a spider, with the hairlike tubes through which it secretes its silk.

2:16 Silk thread anchorage.

Each thread from the spinnerets is com-posed of hundreds of individual fibres and when the spider fixes them to a substrate it spreads their ends out over the surface to make a firm, broad based anchorage (Fig. 2:16). Several such anchor threads are combined to make the final support for a web (Fig. 2:17).

2:17 Nine composite anchor threads merging to form a strong multi-stranded support for the web of the common garden spider Araneus...