Turning Leaves Into Silk

Since we use clothing and fabrics every day, it’s easy to take them for granted. However, many fabrics come from a rather unexpected origin and have a fascinating history. One amazing example is silk; not only is it created by an insect, the silkworm, but these creators have been domesticated for millennia.

The silkworm (Bombyx mori) was native throughout Asia, and domesticated in China over 5,000 years ago to make silk. Legend has it that around 2700 B.C., empress Xi Ling-Shi observed caterpillars eating the local mulberry trees, when one of the cocoons fell into her hot tea. The cocoon unraveled in the boiling water, turning into a long strand of silk. While there is no way of knowing whether this legend is historically accurate, archaeological excavations have found 5,000-year-old silk products in China, revealing that the domestication of the silkworm had already taken place. It would have been hard to predict that these little caterpillars would create a thriving economic export; silk production is now a multi-billion dollar industry, with around 150,000 tons of silk created every year.

Silkworms today are biological silk-producing machines that are the product of thousands of years of careful breeding. Because they have such short life spans, and one individual can produce hundreds of caterpillars, silkworms were quickly and easily bred for certain desirable traits over many generations. At around 77 degrees Fahrenheit, a fertile silkworm egg will hatch after only seven to ten days. The newborn is very tiny (around 1/8th of an inch long) and covered with black hairs, but as the caterpillar matures it becomes white, hairless, and quite soft. After three to four weeks of constant eating and growing, a silkworm can reach a plump three inches in length, increasing its mass an astonishing 10,000 times since hatching.

At this point, the silkworm starts to spin a yellow or white cocoon, which takes around four days to complete. Two to three weeks later, the white silkworm moth emerges. These moths have been bred to have a large body and small wings, with a wingspan of one-and-a-half to two inches, and consequently domesticated silkworm moths cannot fly. After the female moth mates and lays about 300 eggs, it dies, but the cycle starts anew with the eggs.

Just as monarch caterpillars depend upon milkweed, silkworms eat mulberry tree leaves (trees of the genus Morus). The mulberry is a large, deciduous tree, that can have black, red, or white edible berries, or it can be fruitless. While mulberries live in many different warm temperate regions throughout the world, most mulberry species are native to Asia, home to the silkworms. Research reported in 2009 by Dr. Kazushige Touhara at the University of Tokyo found that silkworms are attracted to mulberries by a chemical fragrance the leaves release (specifically called cis-jasmone). The silkworms’ relationship with the mulberry tree is so close that the domestic species is named after the mulberry genus. Indeed, without the mulberry the silkworm could not amazingly increase its mass 10,000-fold, and we would be greatly lacking in silk kimonos.

We know that silkworms accomplish an amazing feat: They turn leaves into silk. But how? To create the silk, silkworms secrete a fluid, from modified salivary glands, that is pushed through special openings, called spinnerets, on their mouthparts. The fluid hardens and turns into a thread of silk, which the silkworm uses to make the cocoon around its body. The entire cocoon is made by a single thread, which can be around 3,000 feet long. Silkworm farmers boil the cocoon (with the caterpillar inside) to loosen the silk and harvest the long, single silk thread by unwinding it. The thread is very thin and it actually takes around 2,000 to 3,000 cocoons to make just one pound of silk.

(For those of you who don’t like the idea of boiling living insects for the creation of clothes: There are some silk farmers that create silk without killing the silkworm. The difficulty is that when the moth emerges from its cocoon, it destroys the long, single thread of silk, creating many shorter fragments of lesser quality. One company advocating to let the moth live and still harvest silk is “Peace Silk.”)

For thousands of years, the secrets of silkworm rearing (called “sericulture”) were closely guarded in China. The penalty for releasing any information to the outside world—from what created the silk, to how to culture the silkworms, and how to process the raw silk—was death. However, eventually silkworm domestication spread from China; Chinese immigrants took the silkworms and their secrets with them in the 200s B.C. to Korea. From Korea, the knowledge of silk production made its way to Japan around 200 A.D.

Around 550 A.D., silkworms arrived in the Byzantine Empire and were presented to Emperor Justinian. Although China had been trading silk since 200 B.C. along the Silk Road, no secrets had left Asia until then. While it’s unclear whether the silkworms were smuggled by monks or merchants, either way, the silk industry began to get a foothold outside of Asia. In Byzantium, the silk industry developed slowly under tight government regulation. It was not until between the 700s and 1000s A.D. that silkworms and culturing information made their way to Europe through Spain, Italy, and France. Silk culturing became so ingrained with European life that silkworms arrived in North America in the 1600s with colonists.

Because silkworms were domesticated and easy to manage early on in scientific history, early scientific discoveries were made using these critters. In 1835, Italian entomologist Agostino Bassi identified a microorganism (the soil-dwelling fungus Beauveria bassiana) as causing a fatal disease in silkworms. Microorganisms had not been previously identified as being responsible for diseases. Around 1870, French chemist and biologist Louis Pasteur discovered that disease in silkworms could also be caused by other microscopic organisms: bacteria. Again, bacteria were not previously suspected to cause health problems until spotted in the caterpillars.

It was becoming apparent that thousands of years of inbreeding silkworms might not have been such a great idea. While the silkworms were selected for producing large amounts of silk, many strains were quite susceptible to disease. In 1905, Dr. Kametaro Toyama, of the University of Tokyo, carried out detailed genetics studies of silkworms in Thailand to deal with this problem. Toyama bred hardy, wild silkworms (Bombyx mandarina, predecessors of the domesticated silkworm), which had low silk production but were more disease-resistant, with established, inbred Japanese strains. The resultant hybrid strain was hardier and produced three times the silk. Although Japan had previously only used long-established “purebred” strains, Toyama convinced Japan to use these improved, hybrid strains. In 1911, the Sericultural Experiment Station was founded by the Japanese government to regulate the strains of silkworms used by silkworm farmers, creating official policies and distributing the improved silkworm eggs to farmers.

After adopting the hybrid strains, the silkworm industry in Japan continued to improve. As a large amount of space is required for the production of the mulberry leaves, many efforts have focused on finding space or utilizing space in such a way as to make silk production cost effective. Out of these efforts came the creation of an artificial silkworm diet that contains much less mulberry than a normal diet. Because space is still an issue, though, much of the industry has spread to tropical regions, specifically India, Thailand, Vietnam, and Brazil, where space is less limited and mulberry trees grow quite well. Today, over 30 countries are involved in silk production. China, however, still produces the majority of silk.

In addition to their economic importance in the silk industry, silkworms have been the focus of recent biological studies, from investigating the proteins and enzymes involved in making silk, to pheromone experiments to see how moths select their mates. Because over 1,000 inbred strains exist throughout the world, the silkworm is also an ideal model for studying genetics and mutations between different strains; this was emphasized by its entire genome being sequenced in 2008. They also make perfect elementary school pets, as they are easy to maintain and a great example of insect metamorphosis.

Their latest claim to fame is talk of sending them to space, but not necessarily as test animals; as food. This may sound strange, but the boiled, protein-rich silkworm pupae (which are killed in the cocoon for silk production) are often eaten in China and Korea already. Even the silk can be turned into a kind of jam with proper treatment. For now, I might stay with raspberry spread on my toast.

For more on silkworms, see Wikipedia’s article on “Bombyx mori,” a study on silkworm domestication revealed through their genetics, a study on how cis-jasmone is used to lure silkworms to mulberry leaves, an argument for using silkworms in space for food, experience some silkworms first-hand by purchasing some from Carolina Biological Supply, or visit the University of Arizona’s to read about silkworm rearing.

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at science@independent.com.