1. Life Cycle of Bombyx mori
The silkworm, Bombyx mori, undergoes complete metamorphosis, a process involving four distinct stages: egg, larva, pupa, and adult moth. This cycle typically lasts for about 45-50 days under optimal conditions.
The Egg Stage
The life cycle begins with the egg. Female moths lay a large number of tiny, spherical eggs, usually around 300-500. These eggs are laid in clusters and are typically pale yellow when laid, turning darker as development progresses.
Definition: Diapause
Some silkworm eggs enter a state of dormancy called diapause, usually in unfavorable conditions like winter, to ensure survival. Non-diapausing eggs hatch within 9-12 days.
- Hatching: Under favorable conditions (temperature, humidity), the eggs hatch into larvae.
- Size: Very small, resembling mustard seeds.
- Appearance: Initially pale, darkens over time.
The Larval Stage (Silkworm)
The larva, commonly known as the silkworm, is the feeding and growing stage. This stage is crucial for silk production as the larva consumes large quantities of mulberry leaves to accumulate energy and silk proteins.
- Feeding: Silkworms are voracious feeders, primarily consuming fresh mulberry leaves. They grow rapidly in size.
- Moulting: The larval stage involves four moults (shedding of old skin) and five instars (growth stages between moults). Each instar ends with a moult.
- Duration: This stage lasts about 20-30 days, depending on environmental factors and food availability.
- Appearance: The larvae are segmented, whitish, and have a small horn-like structure at the posterior end.
- Silk Gland Development: Throughout this stage, the silk glands grow significantly, becoming ready for silk production in the fifth instar.
Observation: Fifth Instar
The fifth instar is the longest and most active feeding stage. Towards the end of this instar, the larva stops feeding, changes color (becomes translucent), shrinks slightly, and begins searching for a suitable place to spin its cocoon. This is a critical indication that it is ready for pupation.
The Pupal Stage
After the larval stage, the silkworm enters the pupal stage. This is a non-feeding, transitional stage during which the larva transforms into an adult moth. The larva first spins a protective silk cocoon around itself.
- Cocoon Formation: The larva secretes silk from its mouth and spins a continuous silk filament around itself, forming a cocoon. This process takes about 2-3 days. The cocoon provides protection during metamorphosis.
- Pupation: Inside the cocoon, the larva transforms into a pupa. The pupa is oval-shaped and initially soft, hardening and changing color as it matures.
- Duration: The pupal stage typically lasts about 10-14 days.
The Adult Moth Stage
The final stage is the adult moth. The moth emerges from the cocoon, usually by secreting an alkaline fluid to dissolve a portion of the silk at one end.
- Emergence: The moth cuts through one end of the cocoon and emerges. In sericulture, cocoons are usually stifled (heated) before emergence to prevent damage to the silk filament.
- Appearance: Adult Bombyx mori moths are creamy-white, have a hairy body, and cannot fly effectively due to centuries of domestication. They also lack functional mouthparts, meaning they cannot feed.
- Reproduction: The primary purpose of the adult stage is reproduction. Moths mate soon after emergence, and the female lays eggs, completing the life cycle.
- Lifespan: Adult moths have a very short lifespan, typically only 5-7 days.
2. Structure of Silk Gland and Secretion of Silk
The silk gland of Bombyx mori is a highly specialized organ responsible for producing the silk filament. It is a pair of long, tubular glands located laterally in the body of the larva, extending almost the entire length of its body.
Structure of the Silk Gland
Each silk gland is a long, transparent tube that can be divided into three main functional regions:
- a. Posterior (Secretory) Region:
- This is the longest and widest part of the gland, located towards the rear of the larva's body.
- It is responsible for synthesizing and secreting the core silk protein, fibroin.
- The epithelial cells in this region are large and highly active, packed with rough endoplasmic reticulum and mitochondria for protein synthesis and energy.
- b. Middle (Reservoir) Region:
- This region is narrower than the posterior region and serves as a storage area for the fibroin and for the secretion of sericin.
- It consists of three sub-regions: anterior, middle, and posterior parts, each contributing to sericin secretion.
- Sericin is a gummy protein that coats the fibroin filaments, holding them together.
- Fibroin, synthesized in the posterior region, passes into this middle region, where it gets enveloped by sericin.
- c. Anterior (Duct) Region:
- This is the narrowest part of the gland, leading to the spinneret.
- It acts as a common collecting duct where the two silk glands (left and right) merge into a single tube.
- It is responsible for drawing out the silk liquid into a fiber and consolidating it as it passes through.
- The spinneret is a tube-like structure located on the larva's labium (lower lip), through which the silk is extruded.
Secretion of Silk
The process of silk secretion is a complex biochemical and physical transformation:
- Fibroin Synthesis: In the posterior region, amino acids from the silkworm's diet (mulberry leaves) are utilized by the gland cells to synthesize fibroin, a long-chain protein. This fibroin is stored in the lumen of the posterior region.
- Sericin Coating: As the fibroin passes into the middle region, it is coated by sericin, another protein secreted by the cells of the middle region. Sericin acts as a gummy binder, cementing the fibroin strands together. At this stage, the silk is in a liquid, viscous form called silk gel or liquid silk.
- Passage through Anterior Duct: The liquid silk, now a fibroin-sericin complex, flows into the anterior duct.
- Extrusion and Solidification:
- As the silkworm begins to spin, muscles around the anterior duct contract, forcing the liquid silk out through the spinneret.
- The physical stress, stretching, and exposure to air cause the protein molecules to align and crystallize.
- The water in the liquid silk evaporates, and the fibroin undergoes a conformational change from a random coil to a β-sheet structure, leading to its solidification into a strong, continuous filament.
- The sericin coating helps to hold two fibroin filaments together as a single strand of raw silk.
Important Note: Raw Silk Composition
A single strand of raw silk consists of two fibroin filaments cemented together by sericin. Fibroin provides the strength, while sericin gives the raw silk its gummy texture. Sericin is usually removed during processing to obtain lustrous, soft silk.