Life Cycle of the Mulberry Silkworm: From Egg to Moth

Bombyx mori, the mulberry silkworm, is one of the most economically significant insects in the world. Sericulture – the practice of rearing silkworms for silk production – has been practiced for over 5,000 years, originating in ancient China and spreading across Asia and beyond. At the heart of this industry is a tightly orchestrated biological process: a complete metamorphosis that takes the silkworm from a pinhead-sized egg to a silk-producing larva, through a transformative pupal stage, and finally to an adult moth. Understanding each stage of this cycle isn’t just academic – it is the foundation of every successful silk farm.

Overview: complete metamorphosis in four stages 🔗

The development of Bombyx mori includes a complete metamorphosis consisting of four stages: egg, larva, pupa, and adult. This type of development – where each stage looks and behaves entirely differently from the others – is called holometabolous development. The entire cycle, from egg to adult, takes approximately six to eight weeks depending on race, temperature, and humidity. In tropical sericultural regions like India and Thailand, warmer conditions allow multivoltine races to produce as many as seven to eight generations per year, while in temperate zones, rearing is mostly seasonal.

Stage 1: the egg 🔗

The life cycle begins with the egg. A female moth lays between 300 and 500 tiny eggs that resemble poppy seeds, requiring specific temperature and humidity to hatch in approximately 10 to 14 days. When first laid, the eggs are pale yellow; fertile eggs darken to gray or purple within a few days, while infertile ones remain white – a key quality indicator for sericulturists.

In univoltine and bivoltine races, eggs laid at the end of a season enter a dormant state called diapause – a physiological pause in development that allows them to survive winter conditions. Diapause can be significantly influenced by photoperiod, temperature, and humidity. In commercial sericulture, eggs are kept in cold storage and brought out in spring when fresh mulberry leaves become available, ensuring larvae always have a food source upon hatching.

Stage 2: the larva (silkworm) 🔗

The larval stage is the most critical phase in silk production. This is where the silkworm does all of its feeding and accumulates the raw material – silk protein – it will later use to spin its cocoon. The larval stage lasts about 27 days, during which the silkworm passes through five growth stages called instars.

Newly hatched larvae 🔗

The silkworm larva when newly hatched is black or dark brown, with a large head and a body densely covered with bristles, giving it the appearance of a hairy caterpillar. At hatching, it measures roughly 3 mm in length. Young larvae can only feed on tender mulberry leaves. As they grow through successive instars, they can handle tougher, more mature leaves.

The five instars and molting 🔗

The caterpillars molt four times, passing through five larval instars over a period of 23 to 28 days, growing to about 3 inches in length – approximately 10,000 times their size at hatching. An instar is simply the stage between two successive molts. Before each molt, the larva stops feeding and becomes relatively inactive. Its skin stretches taut and eventually splits, revealing a new, larger exoskeleton underneath. During the first molt, the dense hair covering is shed, leaving the larva with smoother, lighter-colored skin.

Each instar is progressively longer as the larva eats more and grows faster. By the fifth and final instar, the silkworm is at its maximum weight. The silk glands are so enlarged at this point that they account for nearly 40 percent of the larva’s body weight, and the thorax and body segments appear visibly translucent. This translucency is a practical indicator to rearers that the larva is mature and ready to spin its cocoon.

Feeding and the role of mulberry leaves 🔗

Bombyx mori is essentially monophagous – its preferred and primary food is the leaves of white mulberry (Morus alba). The larvae do most of their eating during the larval stage, and the quality of the mulberry leaves directly influences cocoon size and silk quality. Leaves must be fresh, pesticide-free, and supplied regularly. In commercial rearing, larvae are kept in bamboo or plastic trays, fed multiple times daily, and monitored for disease.

Stage 3: the pupa (cocoon) 🔗

Once the fifth instar is complete, the larva stops eating and begins the transition to the pupal stage by spinning a cocoon around itself. Just before cocooning, the silkworm appears translucent and yellowish and excretes a runny fluid to clean out its system. It then anchors itself and begins drawing out a single, continuous filament of silk by moving its head in a repetitive figure-eight pattern.

Cocoon construction 🔗

The larvae spin cocoons using a single, continuous silk thread that can be over 1,300 meters long, and the process takes two to three days. The silk filament is produced by a pair of large salivary glands – modified into silk glands – that secrete a protein called fibroin, coated with a sticky protein called sericin. When this fluid contacts air, it solidifies instantly into a fine, strong thread. The resulting cocoon is ovate, roughly the size of a cotton ball, and comes in shades of white, cream, or yellow depending on the silkworm’s genetics.

Transformation inside the cocoon 🔗

The pupal stage is a transitional phase during which the larval body and its internal organs undergo a complete change and assume the new form of the adult moth. Soon after pupation the pupa is white in colour and soft but gradually turns brown to dark brown as the pupal skin hardens. This is not passive resting – it is an intensive period of biological restructuring. The pupal period typically lasts 8 to 14 days.

In conventional sericulture, this is the stage at which cocoons are harvested. The cocoons are boiled in water to kill the silkworm inside, which also makes the cocoons easier to unravel – since a moth allowed to emerge naturally would secrete enzymes that break the silk filament into short, unusable segments.

Stage 4: the adult moth 🔗

When metamorphosis is complete, the adult moth emerges from the cocoon. The transition from pupa to adult moth, known as eclosion, typically begins after about two weeks in the pupal stage. The moth’s feet, wings, and abdomen darken, and its body rhythmically stretches until cracks appear on the pupal skin, allowing it to push free.

Physical characteristics and limitations 🔗

An adult silkworm moth has a wingspan of 40 to 50 mm and a thick, bristly body. Its mouthparts are reduced or absent – meaning it cannot eat – and it cannot fly either. These traits are the result of thousands of years of selective breeding; the wild ancestor Bombyx mandarina could fly, but domesticated B. mori has been bred solely for silk production, losing flight capacity in the process. The adult moth lives only a few days.

Mating and egg laying 🔗

The adult’s single biological purpose is reproduction. Female moths attract males by releasing a pheromone called bombykol – one of the first pheromones ever identified in any organism. When females secrete this pheromone, males begin a “flutter dance” that helps them locate the female. Mating is brief, and females begin laying eggs shortly after separation from the male.

After fertilisation, each female moth lays about 300 to 400 eggs, which are placed in clusters and covered by a gelatinous secretion that glues them to the surface. The ideal conditions for mating and egg laying include a stable temperature of around 24°C with relative humidity of 70% to 80%, and a dark or dimly lit environment – conditions that sericulturists carefully maintain to maximize yield. Once the eggs are laid, the female dies, and the cycle begins again.

Why this cycle matters for silk production 🔗

Every gram of silk begins with this biological cycle. One ounce of silkworm eggs contains around 40,000 eggs. Those worms will consume approximately 1,500 kilograms of mulberry leaves and spin cocoons that collectively yield around 8 kilograms of silk thread. Understanding the precise duration and requirements of each life stage helps rearers time their rearing seasons, manage disease, optimize leaf supply, and improve cocoon quality. Only the largest and best cocoons are selected to allow moths to hatch for breeding, while the rest are harvested for silk reeling.

The mulberry silkworm is one of the most thoroughly domesticated organisms on Earth. Domestic silk moths are entirely dependent on humans for reproduction as a result of millennia of selective breeding, and the insect no longer exists in the wild. This mutual dependence – humans providing food, shelter, and mating assistance; silkworms providing silk – has underpinned one of humanity’s oldest and most enduring agricultural industries.

What do you think? Given that Bombyx mori cannot survive without human intervention at every stage of its life cycle, how does that level of domestication compare to other agricultural animals you know? And considering that harvesting silk typically ends the silkworm’s life in the pupal stage, do you think there is a viable path toward scaling up ethical “peace silk” production – where moths are allowed to complete their full life cycle – without compromising the silk industry?

References

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