ENHS Nature Trails May 2001 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Spinning a Silky Tale

Since antiquity fabrics made of silk have been the most valued and sought after in the world. The Silk Road, for centuries the world's greatest East-West trade route from China through Persia and Byzantium to Rome, is but one indication of the high demand for silk products that existed thousands of years ago and still persists to this day. The high commercial interest in silk then and now is the result of amazing biological and mechanical processes underlying silk and silk production.

Silk is an invention of and unique to arthropods. It is commercially harvested primarily from the classic silk moth Bombyx mori and a few other related silk moths. Interestingly, numerous other insects also produce silk for a variety of purposes including bees and wasps (Hymenoptera) who embed simple silk fibers into the wax of their nests for strength, fleas (Siphonaptera) who use silk in their nests and cocoons, lacewings (Neuroptera) who employ silk to cement eggs on stalks, caddisfly larvae (Trichoptera) who use silk to make underwater webs, and glow worms (Coleoptera) who use sticky silk lines to catch flies. Silk production is, of course, not limited to insects; arachnids also generate silk. Pseudoscorpions and mites use silk for nest building, and spiders use silk in all aspects of life including web construction.

Insect and spider silk are each formed from the same building blocks; proteins made from non-essential amino acids which are put together end-to-end into long fibrous chains. The spinning of silk in insects and spiders is the result of a very complex production process. Insect silk is produced in paired silk glands with ducts that open onto the edges of the mouth. The silk glands create a fibrous protein, silk fibroin, formed in a liquid state known as silk dope. The dope solution travels through several specialized regions of silk gland where it is coated with three layers of a second protein, sericin. Each coating is very different but all contain sericin plus a mixture of several unique proteins and sugars. After the three coatings are completed, the silk, still in a liquid state, is extruded through the duct and drawn into a long filament as the caterpillar waves its head side to side. This head waving also joins together the two filament strands being produced simultaneously from each of the bilateral silk glands. The two strands are glued together by sticky secretions from a specialized gland and molded by the muscular silk press valve and the mouth. During the cocooning process, the silk glands of an individual Bombyx mori caterpillar will weigh about 20% of the animals total body weight and will produce about 800 meters (_ mile!) of silk fiber. Insect species produce a wide range of silks with varied composition but web spiders are unique in that each individual is able to produce many different types of silk from a battery of specialized silk glands.

The complexities of silk production, much of which is still unknown, results in a silk that is exceptionally strong and extensible, making it a very tough fiber when spun. The toughness of spider silk is equal to that of the synthetic polyaramid nylon filaments, which, as the benchmark of modern polymer technology , are used in materials ranging from radial tires to bulletproof clothing. Unlike polyaramid fibers, spider and moth silks are spun at room temperatures, low pressures and using water instead of petrochemicals as a solvent. Spiders alter the mechanical properties of their silk by changing the chemical composition of the dope solution and by altering the spinning conditions, including adjusting the spinning speed, or rate of pulling the fiber with their legs or by the time of day the silk is produced. Spider silk is light as well as strong, with tensile strengths approaching that of high tensile engineering steel despite being 1/5th the density. Thus, silk, on a weight basis, is much stronger than steel.

Silk is the ultimate natural fiber-- its ratio of strength to weight is unsurpassed. Silk's outstanding mechanical properties are achieved through an as yet poorly understood process. Many companies and researchers have been trying to produce synthetic silk, however, their efforts have been less than successful. Perhaps this is not completely surprising since insect and spiders have been perfecting their spinning skills for over 300 million years - quite a head start, wouldn't you say?

Postscript: This is my final President's column as I am stepping down from the post in June. I thank David Wagner for being an excellent 1st Vice-President and Nature Trails editor, Melody Clarkson for being a superb 2nd Vice President and trusted pal, Herb Wisner for being our long standing treasurer, and all the members of the ENHS Board for giving so much to the Society. Finally I wish to thank the entire ENHS membership for being such a wonderful group of warm hearted and dedicated people, full of spirit and vitality. It has been a true honor and a deep pleasure working with and meeting all of you.

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