The silkworm, Bombyx mori, is renowned for its ability to produce silk, a natural fiber with exceptional properties. This biological process transforms digested food into a strong, continuous filament. Understanding this involves exploring its specialized internal structures and the intricate steps from liquid protein to a solid, protective cocoon.
The Silkworm’s Internal Machinery
The silkworm’s capacity for silk production stems from a pair of specialized silk glands, also known as sericteries. These modified salivary glands are located along much of the silkworm’s body, ventral to its digestive system. In a fully grown larva, these glands are large, occupying a significant portion of its internal cavity.
Each silk gland is a long, tubular structure divided into three distinct regions, each with a specific role in silk synthesis. The posterior region produces fibroin, the core structural protein of silk. The middle silk gland, the most prominent and widest part, acts as a reservoir for fibroin and synthesizes sericin, a gummy protein that coats the fibroin strands. The thin anterior region serves as a duct, transporting processed silk proteins to the spinneret, an extrusion organ near the silkworm’s mouth.
From Liquid to Fiber
The transformation of raw nutrients into a silk fiber is a complex biochemical and physical process occurring within the silk glands. After consuming large quantities of mulberry leaves, the silkworm converts these digested nutrients into two primary silk proteins: fibroin and sericin.
As the fibroin moves into the middle silk gland, it is stored there while sericin is secreted around it. Sericin, a natural macromolecular protein, acts as an adhesive, binding two fibroin filaments together. The liquid silk, composed of fibroin surrounded by sericin, is drawn through the narrow anterior silk gland and through the spinneret. During this journey, changes in pH, tension, and water removal cause the proteins to undergo a transformation. The liquid proteins solidify into a continuous, strong filament upon contact with air.
The Art of Cocoon Spinning
Once ready to pupate, the silkworm begins constructing its cocoon, a protective enclosure made from secreted silk filament. It anchors initial silk threads to a surface and meticulously builds the cocoon by moving its head in a continuous figure-eight motion. This rhythmic movement ensures even distribution and layering of the silk.
It continuously secretes a single, unbroken silk strand, which can measure hundreds of meters to over a kilometer. This filament is layered systematically to create the cocoon, often taking two to three days to complete. The cocoon serves as a secure environment for the silkworm to undergo metamorphosis into a moth, providing protection during this vulnerable stage.