Silk is an extremely precious textile filament derived from the secretion of an insect known as the silkworm.
The best-known silkworm is the bombyx, larva of the Bombyx mori moth, that is bombyx of the mulberry tree, belonging to the order of Lepidoptera and the family of the Bombycidae.

The Life Cicle

The complete life cycle of the insect lasts approximately 45-50 days and passes through the following stages: hatching, growth of the larva through the five stages, spinning of the cocoon, emergence of the moths, mating and laying of eggs.

In the course of a year, the silkworm appears in four distinct forms:
• embryo or egg, from May to the following April,
• larva or caterpillar, for about 30 days,
• chrysalis, for about 15 days,
• moth, for about 10 days.

Embryo or Egg

During the winter period, when the mulberry tree is in a phase of vegetative repose, the embryo of the silkworm is inside the egg in a state of hibernation known as “diapause”.

In spring, the eggs begin to hatch when the leaves of the mulberry tree reach the ideal state of maturity for the nourishment of the newborn larvae, that is between the end of April and the beginning of May.
Upon hatching, the eggs turn white and the larvae are born, about 3 mm long.

Larva or Caterpillar

When the eggs hatch, the farmer transfers the small caterpillars onto horizontal trays covered with mulberry leaves; the larvae, about half a centimetre long, are small and dark; after about ten days they become a whitish colour.

Not all eggs produce a cocoon; on average 10% are lost through the death of the embryo or of the larva.

The larvae are insatiable and feed voraciously on the leaves of the white mulberry “morus alba”. 1,000 kilos of leaves are required to nourish 28 grams of eggs.

Their development is huge: the caterpillar grows to 40 times its original length and 8,000 times its weight.

During growth, the larvae shed their skins four times, each of these changes is called a moult. The moults take place around the 5th, 9th, 14th and 21st day from hatching, and divide the larva period into five stages known as “instars”.

During moults the caterpillar does not eat and breathes with difficulty, while during the instars it feeds day and night.

After about six weeks the silkworm is completely developed and begins to spin the cocoon: at the beginning, it attaches to a twig several stands of saliva to create a sort of web or net, which becomes the outer covering (floss), then moving its head in all directions, it begins to perform the actual spinning, secreting a sticky filament and winding it round and round.

During this phase, which lasts 3-4 days, the caterpillar stops eating and diminishes in weight and size.

Chrysalis

Having completed the cocoon, the caterpillar “pupates” to transform itself into a coriaceous insect, an immobile brownish-coloured chrysalis.

After about two weeks the chrysalis is transformed into a moth and, using a small gland which secretes a special liquid, it opens a passage between the filaments of the cocoon and emerges.

The moth will then mate, setting off a new life cycle.

Moth

The adult moth is usually yellow or whitish yellow in colour, with a squat, hairy body.

In the course of its short life (10 days), it does not feed, and it dies immediately after having laid the eggs, which are yellow, which it attaches to the chosen surface using a gum-like substance which it secretes.

A moth usually lays between 300 and 500 eggs which, if they are fertilised, turn brown shortly afterwards and hatch within about ten days. The eggs are small, lenticular in shape and brown.
They are collected by special institutes which conserve them and distribute them to the farmers on gauze trays. 28 grams correspond to approximately 55,000 eggs, which will become approximately 70 kg of cocoons.

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