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Silk
and the mulberry-tree.
The official definition of the word ‘silk’ is ‘yarn reeled from the
cocoons spun by the caterpillars of silk-producing insects’. The
domesticated silkworm,
Bombyx mori, is reared systematically and its production
accounts for almost all raw silk production. However, there are numerous
varieties of wild silkworms (see next page). Strictly speaking the
silkworm is not a worm but a caterpillar. It feeds exclusively on
mulberry-leaves and thus can be reared only in those regions where
mulberry-trees can be grown. The mulberry is now cultivated in shrub form,
to facilitate leaf harvesting and reduce labour costs.
Eggs. The eggs of the silkworm, also known as ‘grains’ are
laid by the
Bombyx mori moth. The eggs are cold-stored to arrest their
development and control the hatching period.
Larva or caterpillar. The eggs are prepared for hatching at
a temperature of about 22°C (71 - 72°F) to coincide with the appearance
the shoots of the mulberry leaves which will feed the newly-hatched
larvae. Each egg, the size of a pin-head, produces a tiny caterpillar
about 3 mm. long, endowed with a ferocious appetite for mulberry-leaves.
After three or four weeks, during which they will shed their skin four
times, the caterpillars are fully developed. The caterpillars are now 8-10
centimetres long and weigh 10,000 times as much as when newly hatched.
They stop eating and start looking for a place to spin their cocoon.
Cocoon.
In the past, rearing-trays were covered with sprigs of heather or other
plants to give the caterpillars a firm support for spinning their cocoons.
Nowadays the trays are fitted with flat, bottomless boxes divided into a
large number of compartments. The caterpillar begins by stretching a silk
thread between the walls of its individual compartment then starts to spin
the outer envelope of its cocoon. With rhythmic movements of its head it
lays the silk filament in figure-ofeight form. In this way the caterpillar
builds up 20 to 30 concentric layers of thread. Gradually the caterpillar
disappears from sight inside its cocoon, which remains porous to allow it
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Silk
glands. The caterpillar produces the silk filament thorough a tiny
hole in its lower lip called the spinneret, which is connected to the two
silk glands running the length of the caterpillar’s body. The semi-liquid
silk solidifies into a continuous filament on contact with the air.
Cocoon filament. The thread (or bave) is made up of two
filaments extruded from each of the silk glands and bonded together after
passing through the spinneret. The bave is composed of the two main
ingredients : fibroin (75 - 80% of the weight) and sericin 25 - 20%). It
is this sericin, a natural gum secreted by the silkworm, which bonds the
two brins together in a protective sheath. (This is why the cross-section
of silk thread is like an electric cable). This gum is later partially or
totally removed by a process called boiling-off (or degumming) but it
performs the function of protecting the fibroin during the mechanically
stressful operations of twisting and weaving. The diameter of the bave is
too fine for it to be used alone. For a silk thread to be used as a
textile fibre it must be composed of at least four baves, i.e. the produce
of four cocoons. The operation which consists of assembling the baves is
known as reeling.
Chrysalis (pupa). When it has finished spinning its cocoon
the caterpillar changes into a chrysalis and after that into a moth, if
the normal life-cycle is allowed to continue. The moth emits a brownish
liquid to soften one end of the cocoon and enable it to emerge by pushing
its way through the threads. These cocoons cannot be used for reeling. The
chrysalis is stifled by hot air before it can reach maturity (except for
those kept for reproduction). The surviving male and female moths are
mated and almost immediately after the female lays 350 - 500 eggs, which
are preserved in cold storage until the incubation period when the whole
cycle starts again. The Bombyx mori moth is blind, it cannot
fly and it has no digestive tube. It is doomed to die after3 or 4 days.
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