Worms’ Work

Aarathi Prasad’s Silk encompasses millennia. Silkworm caterpillars 125 million years old have been found sealed in amber from Lebanon, and in amber from northern Myanmar a spider, “flanked by fifteen strands of silk threads, stood frozen at the moment it attacked a wasp, before both were drowned in resin.” One fascinating string of the book follows the evolution of different silk-spinning creatures and their “highly specialized system of genes, proteins, and glands.” Another charts the human exploitation of these spinners, from Neolithic China to the laboratories of today. Describing the repeated efforts to produce the fine, shimmering threads of silk—at times more valuable than gold—Prasad ranges beyond the familiar mulberry silkworm to the wild moths of India and South America, to the hairlike threads of the huge mollusk Pinna nobilis, and to an array of spiders, from the common European garden variety to the mammoth species of South America, whose webs are strong enough to knock off a man’s hat. Over thousands of years, Prasad notes,

wherever delicate and beautiful threads were seen emerging from the bodies of other animals that surrounded humans—anchoring them to the floors of shallow seas, draped like nets across forests, egglike cocoons from which winged insects emerged—these were sometimes harvested gently; at other times aggressively extracted; and, nearly always, someone, somewhere, would attempt to corral them into farms.

The oldest piece of evidence of the weaving of silkworm threads comes from carved caterpillars and fragments of silk found in funeral urns in China’s fertile Henan province and dating to around five thousand years ago. Before that time a long process of domestication and breeding had produced silkworms whose cocoons were ten times heavier than those of the local wild moths and could be unwound to produce a thread stretching for a kilometer. This long thread, however, needed human intervention. In the wild, the moth’s pupa secretes a substance that dissolves a small patch of its cocoon so that it can fly free. But since this breaks the threads, early Chinese farmers learned to kill the pupa inside the intact cocoon.

A second early innovation was the use of an alkali to strip away the sericin, the gluey substance that envelops the strands, so that the threads became glossy and could take colored dyes. After these early developments, different peoples farmed and wove silk, as the practice spread west along the route later known as the Silk Road from Central Asia to northern India, Turkmenistan, Iran, Syria, Iraq, and the Mediterranean. Around the sixth century CE the silkworm reached Europe, and by the late thirteenth century widescale silk production was well established, particularly in Italy and France.

Prasad is an accomplished popularizer of science. With a Ph.D. in molecular genetics and training in bioarchaeology, she is now part of a team analyzing DNA from ancient funerary sites, and her previous books, Like a Virgin: How Science Is Redesigning the Rules of Sex (2012) and In the Bonesetter’s Waiting Room: Travels Through Indian Medicine (2016), illustrate the breadth of her concerns. In Silk, to marshal the mass of scientific and historical material, she follows individual Western researchers from the seventeenth century to the present. Each chapter opens with a dramatic scene: Maria Sibylla Merian setting off from Amsterdam to Suriname, Genghis Khan sacking the great silk city of Merv (now in Turkmenistan), or the gunfight at the O.K. Corral in Tombstone, Arizona, where a silk handkerchief slowed down a bullet. This sounds formulaic, but the human stories are cleverly designed to pull in readers who might balk at immediate immersion in technical detail.

Prasad presents her own engagement, too, opening the book with her exploration of the tall gray filing cabinets in the Natural History Museum in London. Here she finds the specimen of the silk moth that Carl Linnaeus named Bombyx mori in 1758, from Bombycidae, its family, and mori, for the mulberry tree that provides its food. This is enough to pique her interest, but silk, we learn, was also part of her childhood in South India, where her mother and aunts dressed the statues of their gods in silk for religious festivals. Curious, she buys Bombyx mori eggs and watches them hatch into caterpillars and weave the layers of their cocoons, finally emerging as small moths with wings but no legs or mouths, only to mate and die.

A similar combination of awe and curiosity, sometimes nearing obsession, characterizes many of the researchers she describes. Merian, for example, began her entomological diary in 1660 when she was thirteen, and made her name as a flower painter—selling her work to the king of Denmark, among others—before she could concentrate on her primary interest and publish her two influential volumes of The Wondrous Transformation and Particular Food Plants of Caterpillars (1679, 1683), each with fifty ravishing plates.

For Merian, growing up in Frankfurt with its flourishing silk trade, the silkworm was the model for understanding the metamorphosis of all similar moths. Breeding had made the silkworm caterpillar “so docile, prevalent, and immobile that it would also quite seamlessly become the focus of intense scientific study”—and of commercial interest, too. In 1668, at the request of the Royal Society in London, the Italian physician and anatomist Marcello Malpighi, known for completing William Harvey’s work on the circulation of blood, read his study of silkworm anatomy, describing the way that the long, tubular silk glands extend forward and backward through the caterpillar. The society’s interest followed King James I’s attempt forty years earlier to encourage silkworm cultivation in England. Capitalism and nationalism, as well as scientific curiosity, drove their inquiries.

From the seventeenth to the twentieth century, Western greed for profit also involved the exploitation of indigenous peoples and their knowledge. In Suriname, where Merian examined the huge wild moths with their jeweled colors, wingspans wider than a hand, and great cocoons, she stayed on Dutch plantations notorious for horrific brutality. Enslaved indigenous Americans and Africans cleared her path through the jungle, brought her samples, and told her the names of plants and their medical uses. At the same time, in Ambon in Indonesia, the elderly naturalist Georg Eberhard Rumpf, who had gone there with the Dutch East India Company in the rush for cloves and nutmeg, discovered a similar species. Their lives were linked. After Rumpf’s death in 1702 Merian engraved his collection of shells. Their moths were linked, too, classified in 1841 in two different “tribes” in the family Saturniidae, Merian’s in the Attacini tribe and Rumpf’s in the Saturniini.

Like that from the silkworm, silk from wild moths has an ancient history. During twentieth-century excavations of the great Bronze Age sites of the Indus civilization, archaeologists unearthed tiny steatite beads strung together with strands of silk and a copper bangle and necklace containing woven threads, all dating from between 2450 and 1900 BCE—nearly 4,500 years ago. The long single threads, all from Saturniidae moths, showed that the people of this culture, too, had learned to kill the pupa before it broke free.

This silk, named “tasar”—from the Sanskrit trasara, a weaver’s shuttle—and unwound from cocoons “harvested like fruit” from the local bair, or “ber,” trees, was still being spun when the British East India Company arrived in the eighteenth century. Spotting a commercial opportunity, William Roxburgh, the superintendent of the company’s botanical garden in Calcutta, suggested to Joseph Banks, the president of the Royal Society, that this “durable, coarse, dark-coloured silk, commonly called Tusseh silk” could be useful “to the inhabitants of many parts of America and the south of Europe, where a cheap, light, cool, durable dress, such as this silk makes, is much wanted.” Around forty years later the naturalists Mathilde Pauline des Granges and her husband, Johann Wilhelm Helfer, encouraged by the Asiatic Society of Bengal, recommended the use of the wild silk of Bengal in European silk production as an alternative to Bombyx mori, whose inbreeding had made it vulnerable to parasites and infections.

Two other types of silk recommended for Western markets were “muga” silk, from the delicate yellow cocoons of the huge red and orange wild moths of Assam, and the finer, softer, and warmer “eri” silk of the Bengal Samia moths. And although the traditional Bombyx mori was still the source of the cocoons and silk from many countries on display at the Great Exhibition of 1851, interest in wild Indian silk grew. Wild moth cocoons were exhibited at the Paris Exposition of 1855. Frederic Moore, the curator of the Indian Museum in South Kensington, published his Silkworm Moths of India three years later. When Thomas Wardle, a textile manufacturer and associate of William Morris, solved the difficulties of unreeling the tangle-prone threads and dyeing the naturally brown tasar silk, Indian silk began to be imported, first to France and then to Britain. Visitors to London’s Colonial and Indian Exhibition in 1886 could watch Indian artisans working with tasar, muga, and eri silks. Yet this display was, as Prasad says, a “very British circus of smoke and mirrors.” The silk workers—exotic exhibits themselves—were not “authentic” villagers but prisoners from the Agra jail who had been trained in the craft.

From the Agra jail, Silk turns back to eighteenth-century naturalists, to ancient history, and to a very different creature, the enormous mollusk dissected and described by René-Antoine Ferchault de Réaumur in 1717 and named Pinna nobilis—the “noble pen”—by Linnaeus. Living up to fifty years in the warm Mediterranean seagrass, the Pinna, whose oval brown shell is covered with layers of razor-sharp spikes and lined with mother-of-pearl, can grow up to a meter long. Flowing from a muscle near the shell’s narrow foot is “a shock of long, fine filaments disturbingly like the auburn locks of human hair, but up to three times finer.”

As soon as the larvae of the Pinna begin to create their shells, liquid secretions form in their glands and turn to silk on contact with the water. Composed of “highly aligned helixes of globular proteins,” the threads end in sticky pads that anchor the young mollusk permanently to the sea floor. Washed, dried, combed, and plunged into an acid that turned the color to gold, this was the precious “sea silk” that Réaumur believed to have been used in the ancient world. Warily, conscious of semantic pitfalls, Prasad wonders whether this might be the fragile and expensive material known as byssus, generally considered to be fine cotton or linen but identified by Aristotle with the Pinna. It proves impossible to tell. A small amount of supposed sea silk discovered in Pompeii in 1941 turned out to come from a sponge, while a fragment unearthed in 1912 in a wealthy woman’s burial tomb in a Roman colony in Hungary vanished during World War II. Prasad cites the persuasive report of a contemporary expert, the plant histologist Francis Hollendonner, and cites references to the Pinna’s silk in classical, Byzantine, and Arabic sources. But the oldest sea-silk article in existence—possibly the oldest knitted object in Europe—is a small gold and brown cap from the fourteenth century found at Saint-Denis near Paris.

In Sant’Antioco in southwest Sardinia—an ancient Phoenician colony—the weaving of sea silk continued. In 1804 Admiral Horatio Nelson gave Lady Hamilton a pair of sea-silk gloves “made only in Sardinia of the beards of mussels.” A century later an entire robe, interwoven with linen, was created for the statue of Saint Francis in Assisi. But although dedicated craftsmen have tried to uphold the tradition, in the past decade Pinna nobilis and related species have died by the thousands across the Mediterranean. Reduced by overfishing and parasitic disease, they are now listed as critically endangered.

The silk of the Pinna, whose threads can vanish when exposed to the light, remains a mystery of the past. By contrast spider silk—much stronger than any other kind—offers tantalizing hints of potential use in the future.

Unlike moths, which produce silk only as larvae, arachnids create it all their lives, with several different types for different functions, from weaving a web to depositing sperm. Entomologists and others have long been determined to prove its utility—and profitability. In the early eighteenth century the prospect of collecting cheap thread from the “silk-wrapped eggs” of ordinary garden spiders excited the French Academy until, after many trials, Réaumur worked out that it would take more than 55,000 spiders, all kept in separate compartments to protect them from their cannibalistic siblings, to produce a single pound of silk. The next step, he thought, would be to look abroad, to larger spiders like the bird-attacking species Merian had found in Suriname. And look they did. The huge spiders turned out to be everywhere, from China to Australia. In Paraguay the Jesuit priest Ramón María Termeyer reared enormous orb-weaving spiders, Aranea latro, on his pomegranate plantation. In Vanuatu the great golden light-reflecting webs of the Nephila genus were turned into cloaks and ritual headdresses. On Mauritius and Madagascar Jacob Paul Camboué bred Nephila, collected by young Malagasy girls in bamboo cages, and watched them spinning their webs “at the rate of a centimeter every second.”

Turning spider silk into usable thread was not easy. The prime difficulty was the need to immobilize the spider, which quickly cut the thread with its back legs when it felt the silk being extracted. Different solutions were tried over the years with little success, but around 1900 Camboué and his colleague M. Nogué built a machine that twisted together the threads of twelve spiders imprisoned in separate “guillotines.” With this they created a great bed canopy displayed in Paris in 1900, its fabric, reported Le Matin, “of paradoxical lightness and thinness, vaporous, almost unreal.”

Spider cannibalism, too, remained a problem. In an early-twenty-first-century project in Madagascar led by Simon Peers and Nicholas Godley, women filled baskets with countless spiders, only to open them and find that “only one, very happy-looking spider remained.” Yet in the end Peers and Godley managed to produce shimmering golden capes, exhibited in London and New York in 2012.

Spider silk would never, as its advocates hoped, be the profitable silk of the future, but its other attributes drew attention. Astronomers, including William Herschel in 1782, had long made the crosshairs in their telescopes from spider threads, which were valued for their consistency as well as their fineness, “being as good as weightless but almost impossible to break.” During World War II Nan Songer from Tennessee became famous for supplying spider threads that were barely 1/500,000th of an inch thick and were used as crosshairs for telescopes, bombsights, and gunsights. (Conventional silk—which is lightweight and water- and fire-resistant—was also used during the war to make parachutes for the German invasion of Crete.)

Silk could prevent wounds, too. The Mongol soldiers of Genghis Khan’s army, following a Chinese example, wore tightly woven silk undershirts: if their leather armor was pierced by a spear or arrow, the silk wound itself around the point, slowing it down, as well as making arrows easier to pull out. In Tombstone in 1887, the doctor George Emory Goodfellow (whose wife, with a nice irony, came from the Colt family) discovered silk’s ability to slow missiles and reduce blood flow when he examined a gambler who had been shot at close range and found that the bullet had been slowed by the folds of a silk handkerchief in his breast pocket. The next decade saw the invention of an effective silk bulletproof vest.

The deep structure of the chains of fibroin protein from silk moths gives their silk unparalleled elasticity and strength. Spider silk is even stronger, “three times as tough as the heat-resistant aramid fibers used in aerospace, in military applications, and as fabric for ballistic-rated body armor,” and offers an irresistible promise of bulletproof clothing for soldiers and police. The drawback, of course, is the difficulty of obtaining it in sufficient quantity. This was the challenge that provoked Randy Lewis’s radical bioengineering project in Utah in the 2010s, which involved placing modified spider genes into the milk-producing DNA of goats and harvesting the silk proteins from the milk. Although this was, astonishingly, partially successful, the goat silk was far weaker than the original, and so was that produced by introducing spider genes into silkworms. Another route is to create biosynthetic materials, as is already being done on a small scale by the Bolt team in California, producing “Microsilk” by fermenting spider DNA with yeast.

The thread of silkworms, evolved to protect the pupa in its cocoon, also contains antiviral and antibacterial proteins. In early Chinese medicine, silk was used for sutures and dressings, with the sticky sericin that coats the protein fibers removed lest it cause too extreme an immune reaction, and its medical application is the subject of diligent new research today. Spider silk, which lacks potentially dangerous sericin, is already being used for nerve repair.

Silk thus links a distant past with a tentative future. The only gap that mars this engrossing account, with its wealth of extraordinary detail, is the total lack of bibliography or notes—we do not even learn the titles of the many memoirs, biographies, histories, and reports that are quoted so freely. It’s a puzzle, as the discourtesy to other writers and scholars is in striking contrast to the warm tone and the sympathy with scientific endeavor that make Prasad’s book so enjoyable, as well as marvelously informative. Certainly, after reading it, I will never look at a silk shirt or a spider spinning its web in the same way again.