Life Cycle of a Silkworm

Mulberry silk is the most common kind of silk in the world . It makes up 90% of the silk supply in the world. Mulberry Silk is produced by the silkworms which are fed from the mulberry leaves (hence its name). Mulberry silkworm is a silk-secreting insect that was domesticated by the ancient wild mulberry silkworm. It feeds on mulberry leaves, so it is also called the silkworm. Due to the long-term selection and cultivation of the silkworm, the silk glands in the silkworm body are very developed, and the silk secretion ability is far stronger than that of the wild silkworm with its homology. The silk quality is good, and it has extremely high economic value. The quantity and yield of cocoon silk are far more than other silkworms. The scientific name of the silkworm is Bombyx mori. Silkworms can be divided into different types according to their chemical properties, dormancy, cocoon color, and place of origin. The position of silkworm in animal taxonomy is as follows: Arthropoda — Insecta — Lepidoptera — Heterocera — Bombysidae — Bombyx — Bombyx mori.

The life cycle of a silkworm is a fascinating example of complete metamorphosis, a biological process where an insect undergoes distinct physical changes through several stages. This cycle, crucial for the production of silk, has been cultivated by humans for thousands of years. The silkworm, the larva of the domestic silk moth Bombyx mori, goes through four main stages: egg, larva (caterpillar), pupa (chrysalis), and adult moth.

Egg Stage

The life cycle begins with the laying of tiny eggs by the female silk moth. These eggs are typically about 1 mm in size and resemble poppy seeds. In commercial sericulture (silk farming), the moths are allowed to mate, and the females lay between 300 to 500 eggs. These eggs are carefully collected and incubated under controlled conditions of temperature and humidity, usually in the spring to coincide with the availability of fresh mulberry leaves, the silkworms’ primary food source. The eggs are initially adhesive and stick to the surface they are laid on. They are usually grayish or yellowish in color and darken slightly as the embryo develops. This incubation period lasts about 10-14 days, depending on the environmental conditions.

Larva (Caterpillar) Stage

Once the eggs hatch, tiny larvae emerge. These are the silkworms, also known as caterpillars. This stage is marked by rapid growth and several molting phases. The silkworm’s primary function during this period is to eat and grow. They have a voracious appetite, feeding almost constantly on mulberry leaves. This continuous eating is essential for accumulating the necessary energy and resources for the next stages.

The larval stage is divided into five instars, separated by four molts. An instar is the period between each molt. Molting is the process of shedding the old exoskeleton to allow for growth. As the silkworm grows, its skin becomes too tight, triggering the molting process. Before each molt, the silkworm stops eating and becomes lethargic. It then sheds its old skin, revealing a new, larger one underneath. Each instar lasts for several days, with the entire larval stage lasting about 24-32 days, depending on factors like temperature and food quality.

During the larval stage, the silkworm undergoes significant changes in size and appearance. It grows from a few millimeters at hatching to about 7-8 centimeters by the end of the final instar. Its color also changes from a pale yellowish-white to a more creamy white. The silkworm also develops specialized silk glands that produce the liquid silk, which will be used to spin its cocoon.

Pupa (Chrysalis) Stage

After the final molt, the silkworm enters the pupa stage. This is a transitional stage where the larva transforms into a moth. The silkworm stops eating and begins to spin its cocoon. It secretes a liquid silk from its silk glands, which hardens upon contact with air to form a fine thread. The silkworm moves its head in a figure-eight motion, carefully constructing a protective shell around itself. This cocoon is typically oval or elongated in shape and is made of a single continuous silk thread, which can be up to 1 kilometer long if unwound.

The pupa stage lasts for about 10-14 days. Inside the cocoon, the silkworm undergoes metamorphosis, transforming its larval tissues into the adult moth form. This process involves significant changes in the insect’s internal and external structures.

Adult (Moth) Stage

Once the metamorphosis is complete, the adult moth emerges from the cocoon. To escape, the moth secretes an enzyme that softens one end of the cocoon. It then pushes its way out, leaving behind the empty pupal case. The adult silk moth is a relatively small, white, furry moth with a wingspan of about 4-5 centimeters. Unlike many other moths, the adult Bombyx mori is flightless due to domestication.

The primary purpose of the adult moth is reproduction. The moths mate shortly after emerging from their cocoons. The female then lays her eggs, completing the life cycle. Adult silk moths do not have functional mouthparts and therefore cannot eat. They rely on the energy reserves accumulated during the larval stage. The adult stage is short-lived, lasting only about 7-10 days. After mating and laying eggs, the moths die.

The silkworm’s life cycle is intricately linked to sericulture, the practice of raising silkworms for silk production. In commercial sericulture, the cocoons are harvested before the moths emerge. This is because the emerging moth damages the continuous silk thread, reducing its value. The cocoons are then processed to extract the silk fibers, which are used to create various silk products. The life cycle of the silkworm is a remarkable example of nature’s ingenuity and has played a significant role in human history and culture. From the tiny egg to the silk-producing larva and the final transformation into a moth, each stage is essential to this fascinating process.