🐛 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.
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| Life Cycle of Silkworm (Bombyx mori) – 4 Stages |
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.
Sericulture and Economic Importance
- Silk is obtained from cocoons
- Important cottage and commercial industry
- Provides employment and income
Conclusion
The life cycle of silkworm is a well-organized biological process essential for silk production. Understanding each stage is important for both academic studies and practical applications in sericulture.
FAQs
1. What are the 4 stages of silkworm life cycle?
The silkworm undergoes complete metamorphosis with four distinct stages:
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Egg (Ovum) 🥚
- The life cycle begins when the female moth lays eggs.
- Eggs hatch into larvae under suitable conditions.
-
Larva (Caterpillar) 🐛
- The feeding and growth stage.
- Silkworms eat mulberry leaves and grow rapidly.
-
Pupa (Cocoon Stage) 🟤
- The larva spins a silk cocoon around itself.
- Inside the cocoon, it transforms into a pupa.
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Adult Moth 🦋
- The adult emerges from the cocoon.
- Its main function is reproduction.
2. What is the larval stage in silkworm?
The larval stage is the active feeding and growth phase of the silkworm.
- It begins after the egg hatches.
- The larva (caterpillar) feeds mainly on mulberry leaves.
- It undergoes several molts (instars) as it grows.
- Silk glands develop during this stage.
- This stage ends when the larva starts spinning a cocoon.
Key point: This is the most important stage for silk production.
3. What is cocoon in silkworm?
A cocoon is a protective covering made of silk that the larva spins around itself.
- Formed during the transition from larva to pupa.
- Made of a single continuous silk thread.
- Protects the developing pupa inside.
- It is the source of commercial silk in sericulture.
Key point: The cocoon is harvested before the moth emerges to obtain silk.
What is larval stage in silkworm?
The larval stage is the feeding stage where the silkworm eats mulberry leaves, grows rapidly, and produces silk for cocoon formation.
What is cocoon in silkworm?
A cocoon is a protective silk covering spun by the larva around itself, inside which it transforms into a pupa.
Quick Revision (Exam Tip)
- 4 stages: Egg → Larva → Pupa → Adult
- Larva: Feeding + silk production stage
- Cocoon: Silk covering around pupa
Multiple Choice Questions (MCQs)
1. The life cycle of silkworm (Bombyx mori) is an example of:
a) Incomplete metamorphosis
b) Simple metamorphosis
c) Complete metamorphosis
d) No metamorphosis
Answer: c) Complete metamorphosis
2. Which stage of silkworm is most important for silk production?
a) Egg stage
b) Larval stage
c) Pupal stage
d) Adult stage
Answer: b) Larval stage
3. The silk thread in silkworm is produced from:
a) Malpighian tubules
b) Silk glands (salivary glands)
c) Midgut
d) Fat bodies
Answer: b) Silk glands (salivary glands)
4. The cocoon in silkworm is formed during:
a) Egg stage
b) Larval stage
c) Pupal stage
d) Adult stage
Answer: c) Pupal stage
5. How many instars are present in the larval stage of silkworm?
a) 3
b) 4
c) 5
d) 6
Answer: c) 5
References
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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
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FAO Sericulture Manual. (1999). http://www.fao.org
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Kumar, A. (2016). Economic Zoology. Rastogi Publications.
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