In the life cycle of the silkworm (Bombyx mori), at which stage is the silk fibre of commercial importance obtained from the cocoon formed around the developing insect?

Introduction / Context:
Sericulture, the rearing of silkworms for silk production, is an important agro based industry in many countries. The silkworm, Bombyx mori, passes through several stages in its life cycle: egg, larva, pupa, and adult (imago). The commercially valuable silk fibre is obtained from the cocoon that the larva spins around itself before entering the pupal stage. Many examinations ask at which stage of the life cycle the silk of commercial use is obtained, making this a common general knowledge and biology question.

Given Data / Assumptions:

• The organism is the silkworm, Bombyx mori.
• We must identify the stage when the silk fibre used in industry is harvested.
• The options list egg, larva, pupa, adult, and embryonic stages.
• We assume knowledge of the basic life cycle and cocoon formation.

Concept / Approach:
The larva of the silkworm feeds on mulberry leaves and grows through several moults. When it is fully grown, it secretes a protein rich fluid from its silk glands and spins a protective cocoon around itself. Inside this cocoon, the larva transforms into a pupa. From a commercial viewpoint, silk filaments are unwound from this cocoon. Practically, silk producers boil or steam the cocoon while the insect is in the pupal stage to kill the pupa and soften the sericin, then unwind the continuous silk filament. Although the cocoon is spun by the larva, the standard exam answer is that the silk fibre is obtained from the cocoon at the pupal stage.

Step-by-Step Solution:
Step 1: Recall the silkworm life cycle: egg → larva (caterpillar) → pupa inside cocoon → adult moth. Step 2: Understand that the larva spins a cocoon of silk around itself before becoming a pupa. Step 3: Recognize that farmers collect the cocoons and process them while the insect is in the pupal stage inside the cocoon. Step 4: The silk thread that we use is derived by reeling and unwinding the cocoon as a continuous filament. Step 5: Therefore, the exam focused life cycle stage associated with obtaining silk fibre is the pupal stage inside the cocoon.

Verification / Alternative check:
To verify, think of standard textbook diagrams that show trays of cocoons collected from rearing houses, followed by boiling and reeling. The insect inside these cocoons is described as a pupa. If allowed to complete its development, the adult moth emerges by breaking the cocoon, which damages the silk filament and reduces its commercial value. In commercial sericulture, emergence is prevented so that the long continuous fibre can be preserved, confirming that harvesting occurs at the pupal stage in practice.

Why Other Options Are Wrong:
Larval stage, when the caterpillar spins the cocoon: The larva spins silk to create the cocoon, but the silk fibre is harvested from the completed cocoon that contains the pupa. Egg stage, before any visible body segments form: No silk fibre is produced at this stage; it is simply a dormant stage. Adult or imago stage, after emergence from the cocoon: If the adult emerges, the cocoon is broken and the silk fibres are cut, which is not desirable for commercial silk production. Embryonic stage within the egg shell: This is part of the egg stage and has no role in silk fibre production.

Common Pitfalls:
Many students confuse the act of spinning silk with the stage from which silk is harvested. Since the larva spins the cocoon, it is tempting to choose the larval stage. However, in commercial practice the cocoon remains intact and is processed while the enclosed insect is at the pupal stage. Remembering the full sequence of events and connecting silk harvesting with the intact cocoon containing the pupa helps to avoid this common mistake in exams.

Final Answer:
Thus, from a commercial sericulture point of view, the silkworm yields silk fibre at the Pupal stage, enclosed within the cocoon.