Understanding the Growth Stages of Mulberry Silkworms

Silk – one of the world’s oldest and most coveted natural fibers – begins its journey not in a loom, but inside the body of a small caterpillar. The mulberry silkworm, known scientifically as Bombyx mori, has been domesticated for over 5,000 years and remains the backbone of the global silk industry. To rear these insects successfully, understanding each stage of their growth is not optional – it is essential. From a pinhead-sized egg to a spinning larva to a cocooned pupa, every phase has its own biology, timeline, and management needs.

The life cycle of Bombyx mori: an overview 🔗

Bombyx mori undergoes complete metamorphosis, passing through four distinct stages: egg, larva, pupa, and adult moth. According to the Government of Assam’s Department of Handloom, Textiles and Sericulture, the entire life cycle is completed in 45 to 55 days. The exact duration varies with race type and climatic conditions. Multivoltine races in tropical regions like India complete the cycle faster – in as few as 42 to 54 days – while univoltine and bivoltine races in temperate zones take slightly longer. This biological flexibility is what makes Bombyx mori suitable for sericulture across diverse geographies.

Stage 1: The egg 🔗

The life cycle begins with the egg. After mating, a female moth deposits her eggs – typically around 350 to 500 eggs per laying, depending on the race. Each egg is tiny, measuring just 1 to 1.3 mm in length and 0.9 to 1.2 mm in width, and weighs approximately 0.55 to 0.6 mg. Freshly laid eggs appear pale yellow or white; as development progresses, they darken to a grayish or purplish hue due to changes in serosal pigmentation. This color change is a useful indicator of embryonic progress for sericulturists.

The egg’s outer covering – the chorion – has a micropyle at the anterior pole through which sperm enters for fertilization, and numerous funnel-shaped respiratory canals distributed across its surface that allow gas exchange. Under optimal conditions (around 25°C and 75-85% relative humidity), incubated eggs hatch within 11 to 14 days in univoltine and bivoltine races, and within 9 to 12 days in multivoltine tropical races.

Stage 2: The larva – the heart of silk production 🔗

Once the egg hatches, the larva – commonly called the silkworm – emerges. This is the most critical stage for silk production. The larva’s sole function is to feed and grow, accumulating the protein reserves that will eventually be spun into silk. Newly hatched larvae are only about 0.3 cm long, dark brown to black, and densely covered in bristles. As they grow and their cuticle stretches across successive instars, they become smoother and lighter in color.

The larval body is divided into head, thorax, and abdomen. The thorax carries three pairs of true legs used mainly for gripping leaves, while the abdomen bears four pairs of fleshy prolegs (on the 3rd to 6th segments) and one anal proleg. The 8th abdominal segment has a distinctive caudal horn on its dorsal side – a useful identification feature for this species.

The five larval instars 🔗

Larval growth is not continuous – it is punctuated by four molts, which divide the larval stage into five distinct phases called instars. Each instar consists of a feeding phase followed by a molting phase. The first three instars are referred to as “young ages,” and the fourth and fifth as “late ages.” This distinction matters practically: young-age larvae require finely chopped, tender mulberry leaves and more controlled environmental conditions, while late-age larvae can consume whole leaves and are more resilient.

The table below summarizes the approximate duration of each instar for multivoltine and uni/bivoltine races:

The molting process 🔗

Before each molt, the larva stops feeding, raises its head, and becomes still – a behavior sericulturists informally call “going to sleep.” The skin becomes loose, translucent, and wrinkled. The larva secretes a small amount of silky material to anchor itself to a dry surface. The entire molting period typically lasts around 20 hours. Once the old cuticle is shed, the larva “wakes up” with a new, larger exoskeleton and resumes feeding. Disturbing silkworms during this sensitive window can disrupt uniform molting and reduce larval health.

The fifth instar: the critical growth phase 🔗

The fifth instar is the longest and most important phase of the larval stage. By this point, the larva has reached approximately 10,000 times its hatching weight – a remarkable feat achieved within just 20 to 25 days. The silk glands, which are modified labial glands, become so enlarged during this stage that they account for nearly 40% of the larva’s body weight and are visibly translucent through the body wall. This transparency is a key sign that the worm is “ripe” and ready for mounting.

When fully mature, the larva stops feeding, excretes soft, moist feces, and begins restlessly raising its head – actively seeking a surface to spin on. This behavioral shift is the practical cue for sericulturists to transfer worms to the mounting frame.

Identifying sex during the larval stage 🔗

One of the most practically useful aspects of silkworm biology is the ability to distinguish male from female larvae – particularly during the fourth and fifth instars, when the sexual markings are most clearly visible. These markings appear on the ventral side of the abdominal segments.

Ishiwata’s glands in females 🔗

In female larvae, two pairs of milky white spots appear on the ventral side of the 8th and 9th abdominal segments. These are the Ishiwata’s Fore Gland (on the 8th segment) and Ishiwata’s Hind Gland (on the 9th segment). Their presence in pairs – one on each segment – is the distinguishing marker of female larvae.

Herold’s gland in males 🔗

Male larvae, by contrast, display a single small milky white body – the Herold’s gland – located centrally on the ventral side between the 8th and 9th abdominal segments. Unlike the paired spots in females, Herold’s gland appears as one compact structure. This difference in number and position allows experienced rearers to sex the larvae with reasonable accuracy during the later instars.

Larval sex identification is valuable in selective breeding and hybrid silkworm programs, where controlling the sex ratio and pairing specific strains improves cocoon quality and silk yield. It also supports research into the unique ZZ/ZW sex determination system of Bombyx mori, where females are ZW and males are ZZ – the reverse of the XY system seen in mammals.

Stage 3: The pupa – transformation inside the cocoon 🔗

Once the mature larva is mounted, it begins spinning a cocoon within 48 to 72 hours. Spinning is a continuous process in which the larva moves its head in a figure-eight pattern, extruding a single unbroken silk filament from its spinneret. This filament can measure up to approximately 1.5 km in length and forms the structural basis of commercial silk. The silk itself consists of two proteins: fibroin, the tough inner core secreted by the posterior segment of the silk gland, and sericin, a water-soluble protein from the middle segment that binds and coats the fibroin fibers.

Inside the completed cocoon, the larva transforms into a pupa – the transitional stage during which the entire larval body plan is reorganized into an adult moth. The pupal period lasts 12 to 15 days in univoltine and bivoltine races, and 10 to 12 days in multivoltine races. This is a largely inactive but biochemically intense phase: muscles break down, wings develop, and reproductive organs form. Maintaining stable temperature (24-26°C) and humidity (70-80%) during this phase is critical for cocoon integrity and adult development.

In commercial sericulture, most cocoons are harvested before adult emergence. The pupae are killed – typically by heat – to prevent the adult moth from secreting the enzyme cocoonase, which breaks the long silk filaments and reduces the reelable length of the thread. This biological detail has significant ethical implications and has drawn criticism from animal welfare advocates worldwide.

Stage 4: The adult moth 🔗

If allowed to complete development, the adult moth emerges by secreting cocoonase – an alkaline protease – that softens and dissolves a section of the cocoon shell. Bombyx mori adults are completely domesticated; they cannot fly and lack functional mouthparts, so they neither feed nor disperse. Adult life lasts only 3 to 10 days, during which the sole purpose is reproduction. Females are larger and more sluggish; males are smaller and more active. Mating occurs soon after emergence, after which females lay their eggs and the cycle begins again.

Multivoltine females lay approximately 400 eggs per laying, while univoltine and bivoltine females typically lay 500 to 600. The number of generations per year varies significantly by race: multivoltine tropical races can yield up to seven to eight generations annually, making them particularly suited to continuous rearing in countries like India and Thailand.

Why understanding growth stages matters in sericulture 🔗

Each growth stage of Bombyx mori requires different environmental conditions, feeding regimes, and management practices. Young larvae in the first three instars need temperatures around 27°C, high humidity (80-85% RH), and finely chopped tender mulberry leaves. Late-age larvae in the fourth and fifth instars need slightly lower temperatures (22-26°C) and more robust leaf supplies. The pupal stage demands stable thermal conditions to protect cocoon quality. Any deviation in these parameters at the wrong stage can trigger disease, abnormal molting, or poor silk yield. This is why sericulturists study larval biology closely – not just for academic interest, but because it directly determines the economic outcome of every rearing cycle.

What do you think? Given that the fifth instar alone accounts for the majority of a silkworm’s silk gland development and leaf consumption, how should rearing facilities prioritize resources across the five instars? And with larval sex identification possible as early as the fourth instar, how might early sexing improve the efficiency of hybrid silkworm breeding programs?

References

1. https://en.wikipedia.org/wiki/Bombyx_mori
2. https://hts.assam.gov.in/frontimpotentdata/life-cycle-of-mulberry-silkworm
3. https://www.dacollege.org/uploads/stdmat/zoology-unit2.pdf
4. https://infonet-biovision.org/animal-species/mulberry-silkworm
5. http://www.celkau.in/Agrienterprises/enerprise/26.%20Sericulture/5.%20Mulberry%20Silkworm.pdf
6. https://hbmahesh.weebly.com/uploads/3/4/2/2/3422804/life_cycle_and_morphology_of_bombyx_mori._word.pdf
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC6021594/