The Story of Silk

Acknowledgements

1

Introduction

1:2 VERMIS SERICUS, or THE SIKWORM

Plate 1 of a series illustrating silk production.

This print comes from a set of six illustrating silk production. They were published in Antwerp in 1589 from designs by Johannes Stradanus (1523-1605), a Flemish emigrant who settled in Florence and became Court Artist to the Medici. Five of the drawings that he made for the engraver – one of them is long lost – are in the Collection of HM Queen Elizabeth II in the Royal Library, Windsor Castle. I have described the drawings and the prints elsewhere, ¹, ² hinting at what may be one of the first recorded cases of industrial espionage, discovering the remarkable transformation from caterpillar to moth of the only insect that has been completely domesticated, and revealing, through the eyes of the sixth century Byzantine Emperor Justinian I how silkmaking was spread to the West.

I first saw silk being made in Bangladesh in 1990, and since then I have cultured my own silkworms and travelled more than 13,000 kilometres (8,000 miles) over ancient silk roads ranging from Turkey across central and southern Asia to the Pacific coast of China. In my experience the processes involved in silkmaking in these countries show remarkable resemblances to those shown in the prints that were published more than four centuries ago in Europe.

Nowadays silk production – otherwise known as sericulture – can be practiced wherever silkmoths can be reared. But it was not always so. For thousands of years it was confined to the country of its origin, China, and there the process was shrouded in secrecy. Eventually the practice spread beyond its borders to other countries, notably India and Persia, but as far as the western world was concerned the Chinese monopoly lasted until the sixth century AD, when silkmaking was introduced to Asia Minor. It was to be another 400 years before it became widely known in Western Europe.

So what is silk, and why is it so special? The subject is vast: when I entered the word ‘Silk’ in Google in February 2011 the computer came up with 126 million hits. ‘Spiders’, whose lives depend more than any other creature’s on silk brought up 30 million, ‘moths’ 5 million, five hundred thousand and ‘silkmoths’ 340 thousand articles. Clicking Silk in Google in February 2015 produced 246 million hits. Clearly there is no shortage of information about silk. But this is a personal account. It relies as much on my travels through Asia as on my vocation as an entomologist, added to which I must declare the interest in engravings that set me off on this long trail, for it was in Lindisfarne Castle on Holy Island off the coast of Northumberland that I first saw a set of the silkworm prints.

But first we will find out what kinds of animals make silk, how they make it and how they use it in their daily lives. We will describe the features that explain its unique properties, and look at silk production and processing. We will accompany merchants trading silk along ancient caravan trails from China across the deserts, plains and high mountain passes of central Asia and Asia Minor to the shores of the Black Sea and Mediterranean and from there by sea to western Europe and finally to the Americas.

As I mentioned above our account begins millions of years ago. I have devoted almost 26 years on and off to this subject – although my wife would tell you it seemed a great deal longer – and I still feel as though I have barely scratched its surface. Be that as it may, we are in for some surprises along the way as we weave our thread through the biology, history, geography, art and economics of this amazing product of nature. I hope you enjoy reading about silk as much as I have enjoyed finding out about it.

John Kershaw

1:3 Dressed in their finest silks the bride, bridesmaids and musicians arrive for a wedding in the grassy uplands of the Ili valley in northwest China, some 2,900 km (1,800 miles) from the bustle of Beijing. The yurts in the background were brought up by camels from the valley below and provided accommodation for the guests. I still have my tape recording of the festivities.

1:4 Brought up in the saddle: the master of ceremonies arrives with the fathers of the bride and bridegroom.

1Kershaw, J. Age-old Secrets of the Silkmaker’s Art, The Northumbrian. No.11. 2010.

2Kershaw, J. From Flanders to Florence: The Story of the Lindisfarne Silkworm Prints. Dolman Scott, 2015.

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,