UNIT 2 - Biology of Silkworm - Life Cycle of Bombyx mori, Silk Gland Structure and Secretion

Oct 28, 2026

UNIT 2 explains the biology of Bombyx mori, focusing on its systematic classification, development, and silk production. The life cycle is described as a complete metamorphosis with four stages: egg (ova), larva, pupa, and imago, including key events like molting, cocoon spinning, and transformation. The silk gland section details the modified labial gland structure, its layered organization, regional specialization, and how gland regions secrete silk proteins such as fibroin. Manufacturing processes for commercial silk are also addressed.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages10

Reading Time25 min

AccessFree to download

Life cycle of Bombyx mori

Egg (ova)

Larva

Pupa

Cocoon

Imago

Silk gland

Modified labial glands and distribution

Structure of gland layers

Regions and secretion (fibroin)

UNIT 2: BIOLOGY OF SILKWORM- LIFE CYCLE OF BOMBYX-MORI STRUCTURE OF SILK GLAND AND SECRETION OF SILK Systematic Position Class: Insecta Order: Lepidoptera Family: Bombycidae Genus: Bombyx Species: B. mori The Silkworm, Bombyx mori produces the silk of commercial importance. It is the caterpillar of a moth whose cocoon is used to make silk. This insect is also called the silkworm-moth and the mulberry silkworm . Male and female moths are flightless and lack functional mouth parts. The moths differ in morphological features. The female has a larger abdomen whereas the male has a much larger pair of antennae. The life cycle of Bombyx mori demonstrates the most advanced form of metamorphosis. The serial progressions of four distinct stages of development complete one generation of the species; egg (ova), larvae, pupa and imago. Ova: Egg is the first stage of a silkworm’s life cycle. The female moth lays an egg about the size of an ink dot during summer or the early fall. The egg remains in dormant stage until spring arrives. The warmth of the spring stimulates the egg to hatch. The egg of Bombyx mori is a very small and hard structure; about the size of a pin head and resembling a poppy seed. The egg shell provides a protective covering for embryonic development. When first laid, an egg light yellow. A fertile ovum darkens to a blue-gray within a few days. Larva: The larva is the vegetative stage where growth takes place. The larva of Bombyx mori, commonly called a silkworm, is host specific to mulberry. During growth, the larva molts 4 times. The period between successive molts is called an instar. The silk worm, upon hatching, is about 1/8th of an inch and extremely hairy. Young silkworms can only feed on tender mulberry leaves. However, during the growth phase they can eat tougher mulberry leaves as well. The larval stage lasts for about 27 days and the silkworm goes through five growth stages called instars, during this time. During the first molting, the silkworm sheds all its hair and gains a smooth skin. Pupa: As the silkworm prepares to pupate, it spins a protective cocoon. About the size and color of a cotton ball, the cocoon is constructed from one continuous strand of silk, perhaps 1.5 km long (nearly a mile) . The silk cocoon serves as protection for the pupa. Cocoons are shades of white, cream and yellow depending on silkworm genetics. After a final molt inside the cocoon, the larva develops into the brown, chitin covered structure called the pupa. Metamorphic changes of the pupa result in an emerging moth If the silkworms are allowed to mature and break through the cocoon, the silk would be rendered useless for commercial purposes. So the encased insect is plunged into boiling water to kill the inhabitant and dissolve the glue holding the cocoon together. The end of the silk is then located and the cocoon unwound onto a spindle to be made into thread. Cocoon: Cocoon is the stage in which the larva spins silk threads around it, to protect itself from its predators. The larva traps itself inside the cocoon in order to pupate. The color of the cocoon varies, depending upon what the silkworm eats. It can range from white to golden yellow. The second molting occurs inside the cocoon, when the larva turns into a brown pupa. It takes about 2-3 weeks for the pupa to metamorphose into an adult moth. Imago: The adult stage completes the life cycle of Bombyx mori. It is the reproductive stage where adults mate and females lay eggs. Moths are flightless and lack functional mouth parts, so are unable to consume the food/nutrition. SILK GLAND The silk thread of the cocoon is secreted by a pair of silk glands which are actually modified labial glands. These dermal glands are well differentiated in the fourth and fifth instar larvae. They lie below the alimentary canal and are so large in the fully grown final instar larva that it occupies most of the body cavity ventral to the alimentary

FAQs

What are the main developmental stages in the life cycle of Bombyx mori?

Bombyx mori undergoes complete metamorphosis with four stages: egg (ova), larvae, pupa, and imago. Each stage represents a distinct progression of development that completes one generation.

Why is a cocoon spun, and what happens if the silkworm breaks through it?

The larva spins a protective cocoon to shield itself from predators. If the silkworm matures and breaks through the cocoon, the silk becomes useless for commercial purposes, so the cocoon is processed to unwind the filament.

How are the silk glands structured and how does secretion relate to silk formation?

Silk is secreted by a pair of silk glands, which are modified labial glands with three layers. The posterior region secretes the major silk protein fibroin as a precursor that becomes fibroin after extrusion, while the middle region acts as a reservoir and supports maturation during storage.

Oct 28, 2026

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