🐛 Life Cycle of Silkworms (Bombyx mori)
The silkworm (Bombyx mori) is a fully domesticated insect that has been cultivated for thousands of years for its ability to spin luxurious silk. It belongs to the order Lepidoptera, which also includes butterflies and moths. The life cycle of the silkworm is a complete metamorphosis, consisting of four major stages: egg, larva, pupa, and adult moth.
Understanding the life cycle of Bombyx mori is essential in the field of sericulture, which is the practice of rearing silkworms for silk production. Each stage plays a vital role in determining the quality and yield of silk.
🥚 1. Egg Stage
The life cycle of Bombyx mori begins with the egg, laid by the female moth after mating. Each female moth can lay approximately 300 to 500 eggs during its short lifespan. The eggs are tiny, about the size of a pinhead, and are usually pale yellow when freshly laid. As they mature, they turn greyish. The eggs can be of two types:
Diapause eggs – which undergo a resting phase, usually in temperate regions.
Non-diapause eggs – which hatch quickly, common in tropical climates.
Under optimal environmental conditions (22–25°C temperature and 80–85% relative humidity), the eggs hatch in about 10 to 14 days, giving rise to larvae. This stage is crucial for scheduling rearing cycles in commercial sericulture operations.
🐛 2. Larval Stage (Caterpillar)
After hatching, the silkworm enters its most active and longest phase—the larval stage. The larvae are tiny, black, and hairy when they first emerge, but they grow rapidly by feeding exclusively on fresh mulberry leaves (Morus alba), which are rich in nutrients. The larval stage lasts for 20 to 30 days, during which the silkworm passes through five instars, or molting phases. After each molt, the larvae shed their old skin to accommodate their growing bodies. During this period, the caterpillar increases its body weight nearly 10,000 times and becomes creamy white, soft, and plump. As the larva matures, it stores silk proteins in its silk glands, preparing to spin a cocoon. This stage is the most critical for silk production, as the quality of feeding directly affects the strength and fineness of the silk thread.
🧵 3. Pupal Stage (Cocoon Formation)
Once the larva reaches maturity, it stops feeding and prepares for pupation. The larva begins to spin a protective cocoon around itself using a continuous silk filament secreted from its specialized salivary (silk) glands. The spinning process takes about 2–3 days, during which the larva extrudes a silk thread composed of fibroin and sericin, forming a dense, fibrous cocoon. The silk filament can be as long as 600 to 900 meters in a single cocoon. Inside the cocoon, the larva transforms into a pupa, a stage of dormancy and internal reorganization. This stage is economically significant because commercial silk is harvested at this point, by boiling or steaming the cocoon to kill the pupa and preserve the silk filament intact.
🦋 4. Adult Moth Stage
After undergoing metamorphosis, the adult moth emerges from the cocoon. The adult moth secretes a proteolytic fluid to soften the cocoon, enabling it to break through and emerge. However, this breaks the silk thread, making such cocoons unsuitable for silk reeling. The adult moth is creamy white, with a thick body and feathery antennae. Females are generally larger than males. Adult moths do not have functional mouthparts and hence do not feed. Their sole purpose is reproduction. Within 4–5 days, the moths mate, and the females lay eggs, thus completing the life cycle. To ensure continuous silk production, breeders often prevent the moth from emerging by stifling the pupa and preserving the unbroken silk filament for reeling.
🔄 Types of Silkworm Breeds
Depending on the number of generations produced per year, Bombyx mori strains are classified into:
Univoltine: One generation per year; suited to temperate regions.
Bivoltine: Two generations per year; common in subtropical areas.
Multivoltine: More than two generations; suitable for tropical climates like southern India.
📝 Summary Table: Life Cycle of Bombyx mori
|
Stage |
Duration |
Description |
|
Egg |
10–14 days |
Tiny, laid by female moth; hatches
into larva |
|
Larva |
20–30 days |
Caterpillar stage, feeds on
mulberry leaves, passes through 5 instars |
|
Pupa |
10–14 days |
Resting stage inside cocoon;
metamorphosis occurs |
|
Adult Moth |
4–5 days |
Cream-colored moth, does not feed,
reproduces and lays eggs |
The life cycle of the silkworm (Bombyx mori) is a fascinating example of biological transformation. From a tiny egg to a silk-spinning larva and finally an adult moth, each stage is vital for successful silk production. Knowledge of this cycle is essential in sericulture practices to optimize silk yield, cocoon quality, and economic returns.
📚 References
-
Ganga, G. (2003). Comprehensive Sericulture: Vol. 1 - Silkworm Rearing. Oxford & IBH Publishing.
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Srivastava, P.N. (2012). Textbook of Sericulture. Kalyani Publishers.
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Central Silk Board India – http://csb.gov.in
-
FAO Sericulture Manual. (1999). http://www.fao.org
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Kumar, A. (2016). Economic Zoology. Rastogi Publications.
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