🐞 Silkworm Biology and Life Cycle

Silkworm life cycle knowledge is essential for scientific rearing, cocoon productivity, and post-cocoon quality management.

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Introduction to Bombyx mori

The mulberry silkworm, Bombyx mori (Lepidoptera: Bombycidae), is the most commercially important silk-producing insect in the world. It is a completely domesticated species that has been reared by humans for over 5,000 years, originating in China. Unlike its wild ancestor Bombyx mandarina, the domesticated silkworm cannot fly, has lost its natural camouflage, and is entirely dependent on human care for survival. India is the second-largest silk producer after China, with Karnataka, Andhra Pradesh, Tamil Nadu, Jharkhand, and West Bengal being the leading sericulture states.

Life Cycle Stages

Bombyx mori undergoes complete metamorphosis (holometabolous development) with four distinct stages: egg, larva, pupa, and adult.

Egg Stage

The female moth lays 300-500 eggs on a flat surface shortly after mating. Eggs are initially pale yellow, turning slate grey within 2-3 days if fertilized (due to serosa pigmentation). The egg stage lasts 10-12 days under normal conditions. In univoltine races, eggs enter a state of dormancy called diapause, which is broken by cold treatment (acid treatment with HCl or refrigeration at 2-5 degrees Celsius for 40-60 days).

Larval Stage

The larva (caterpillar) passes through five instars separated by four moults. The total larval period is 25-30 days. Larvae feed exclusively on fresh mulberry leaves (Morus spp.) and grow from approximately 3 mm at hatching to 7-8 cm at maturity. The fifth instar alone accounts for about 80% of total leaf consumption. The mature fifth-instar larva becomes translucent, stops feeding, and begins searching for a site to spin its cocoon.

Pupal Stage

The mature larva spins a cocoon of continuous silk filament from its spinneret (modified labium) using secretions from a pair of silk glands. Each silk gland produces fibroin (the core protein), while the sericin (gum protein) coating is added as the filament exits the spinneret. A single cocoon contains 1,000-1,500 metres of continuous filament. Pupation occurs inside the cocoon and lasts 10-14 days.

Adult Stage

The adult moth emerges by secreting an alkaline enzyme (cocoonase) that dissolves one end of the cocoon. Adults do not feed (vestigial mouthparts), and their sole purpose is reproduction. Mating occurs within hours of emergence, and the female begins oviposition shortly after. The adult lifespan is only 5-10 days.

Voltinism and Races

Silkworm races are classified by the number of generations per year. Univoltine races produce one generation per year, lay diapausing eggs, and are reared in temperate climates. They produce superior silk quality. Bivoltine races produce two generations per year and yield high-quality silk suitable for reeling. Multivoltine races produce 4-6 generations per year, are adapted to tropical climates, are hardy and disease-tolerant, but produce coarser silk of lower quality. In India, bivoltine x multivoltine hybrids are widely reared to combine quality with tropical adaptability.

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Summary Cheat Sheet

• Bombyx mori has egg, larva, pupa, and adult stages.
• Larval growth has five instars and drives cocoon yield.
• Cocoon silk filament is produced by paired silk glands.
• Voltinism type affects adaptability and silk quality.

References

1 source • [1]

[1]

Standard sericulture biology references used for lesson preparation.