Silkworms (also spelled silk worms) are the larval form of the domesticated silk moth, Bombyx mori. Native to northern China, the silk moth was domesticated from its progenitor species Bombyx mandarina about 3500 BC.
The fabric we call silk is made from long thin fibers made by the larval stage insect to create a cocoon for its transformation into the moth form. The cocoons of the silk moth are spun by the larva themselves—and the silk farmers simply unravel the fibers, each cocoon producing between 100-300 meters of fine, very strong thread.
The history of the use of the cocoons of the silkworm species Bombyx to produce cloth suggests that it was in use at least as early as the Longshan period (3500-2000 BC), and perhaps earlier. Evidence of silk for this period is only known from a few remnant textile fragments recovered from well-preserved tombs. Textual evidence for silk production is found in the Shi Ji, and art depictions of garments.
The chemistry of silk production involves the synthesis and secretion of fibroin and sericin (silkworm cultivation is often called sericulture) by the insects' glands, which is pushed forward and then pulled out of the spinnerets to form the cocoon filament. The caterpillar feeds exclusively on mulberry leaves, which has a latex with very high concentrations of alkaloid sugars, which are toxic to other caterpillars. Silkworms are completely dependent on humans for survival, a direct result of artificial selection. Other characteristics bred into the domestic caterpillar are a tolerance for huma proximity and handling as well as for excessive crowding. More than 1,000 inbred strains are kept worldwide.
Silkworms and Jin Brocades
The Western Zhou Dynasty (11th-8th centuries BC) saw the development of the famous Jin brocades, as evidenced by excavations at Wulipai. Many silk textile examples have been recovered from archaeological excavations of Mashan and Baoshan sites, dated to the Chu Kingdom (7th century BC) of the later Warring States period.
By the Han Dynasty (206 BC-AD 9), silk production was so important to international trade that the roadways used to connect Chang'An with Europe were named The Silk Road. The secret of the production for silk remained unknown outside China until the 6th century AD.
Sequencing the Silkworm
Two draft genetic sequences of Bombyx were published in 2004, one in China (Xia et al.) and one in Japan (Mita et al.); and the entire sequence was published in 2008 by the International Silkworm Genome Consortium. The insect has 28 chromosomes, 18,510 genes and over 1,000 genetic markers. Bombyx has an estimated 432 Mb genome size, much larger than fruitflies, making the silkworm an ideal study for geneticists, particularly those interested in the insect order Lepidoptera. Lepidoptera includes some of the most disruptive agricultural pests on our planet, and geneticists hope to learn about the order to understand and combat the impact of silkworm's dangerous cousins.
In 2009, an open access database of the silkworm's genome biology called SilkDB was published (see Duan et al).