Silkworm — The silkworm is the larva or caterpillar of Bombyx mori (Latin: “silkworm of the mulberry tree”), the domesticated silkmoth. A moth in the family Bombycidae, it is very important economically as the producer of silk. It is entirely dependent on humans for its reproduction and no longer occurs naturally in the wild. Silk culture has been practised for at least 5,000 years in China (Goldsmith et al. 2004). A silkworm’s preferred food is White Mulberry leaves. It is native to northern China.
Its nearest wild relative is Bombyx mandarina which is able to hybridize with the domestic taxon (Goldsmith et al. 2004), and which ranges from northern India to northern China, Korea and Japan. It is not known when the domestic silkmoth diverged from its wild relatives, only that the domestic population originated from inland Chinese rather than Japanese or Korean stock (Maekawa et al. 1988, Arunkumar et al. 2006). Molecular clock studies suggesting an age of many millions of years cannot be taken seriously, as they assume that both species have evolved with constant speed since their divergence.
This is not correct however due to the domestication process having accelerated the pace of evolution (a similar problem affects the attempt to resolve the phylogeny of domestic Western honey bee subspecies). In fact, the domestic silkworm has undergone such strong artificial selection that it is completely unable to survive in the wild for any length of time. It is probably the most heavily domesticated animal known apart from domestic hybrids such as mules. Regardless whether the domestic silkworm is derived from a wild species that has since gone extinct, or from a stock of Bombyx mandarina that was taken into human care some 4,600 years ago (Yoshitake 1968), breeding of silkworms cannot have originated before the Neolithic as the tools necessary to make use of the silk thread on a large scale only have become available since then.
Sometimes, the Wild Silkmoth is considered a subspecies of Bombyx mori (the older specific name is used as per ICZN rules) as they are theoretically capable of full hybridization. However, due to the domesticated moth’s requirement for human care to survive, gene flow is all but nonexistent and thus, despite its apparently recent origin, the domestic animal is generally treated as a distinct monotypic species today.
Eggs take about ten days to hatch. Silkworms have a strong appetite, as do all lepidopteran larvae. They eat day and night, preferring White Mulberry but not being strictly monophagous they also take other species of Morus and some other Moraceae. Hatchlings and second-instar larvae are called kego (“hairy silkworm”) in Japan, or chawki in India. They are covered with little black hairs. When the color of their heads turns darker, it means that it is time for them to molt. Later instars are white, nude, and have a horn on the back.
After they have moulted four times (i.e., in the fifth instar), their bodies turn slightly yellow and their skin becomes tighter. The larvae enclose themselves in a cocoon of raw silk produced in the salivary glands that provides protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera produce cocoons, but only a few large Bombycidae and Saturniidae have been exploited for fabric production.
The cocoon is made of a single continuous thread of raw silk from 300 to 900 meters (1000 to 3000 feet) long. The fibers are very fine and lustrous, about 10 micrometers (1/2500th of an inch) in diameter. About 2,000 to 3,000 cocoons are required to make a pound of silk. Based on 1 kilometer (about 1100 yards) per cocoon, ten unravelled cocoons could theoretically extend vertically to the height of Mt Everest. At least 70 million pounds of raw silk are produced each year, requiring nearly 10 billion pounds of mulberry leaves. According to E. L. Palmer (Fieldbook of Natural History 1949), one pound of silk represents about 1,000 miles of filament. The annual world production represents 70 billion miles of silk filament, a distance well over 300 round trips to the sun.
If the animal is allowed to survive after spinning its cocoon, it will release proteolytic enzymes to make a hole in the cocoon so that it can emerge as a moth. This would cut short the threads and ruin the silk. Instead, silkworm cocoons are boiled. The heat kills the silkworms and the water makes the cocoons easier to unravel. Often, the silkworm itself is eaten.
The adult phase (the moth) cannot fly. The silkmoths have a wingspan of 3-5 cm (1.5 – 2 inches) and a white hairy body. Females have about twice to three times the bulk of males (for they are carrying many eggs), but are similarly coloured. Adults in the Bombycidae have reduced mouth parts and do not feed.