The mulberry silkworm, Bombyx mori, is a remarkable insect with a unique connection to human civilization. This domesticated moth larva has been cultivated for millennia, primarily for its ability to produce luxurious silk fibers. Its journey from egg to silk-spinning marvel has shaped economies and cultures globally. Understanding this creature offers insight into one of the oldest forms of biological engineering.
The Mulberry Silkworm: An Overview
Bombyx mori is the scientific name for the mulberry silkworm, a domesticated insect in the family Bombycidae. Selectively bred over thousands of years, this moth is largely dependent on human care for survival. The “mulberry” in its name refers to its specialized diet. The silkworm exclusively consumes leaves of the white mulberry tree, Morus alba, from which it derives proteins for silk production. This strict diet influences the silk’s quality, strength, and luster.
Domestication of Bombyx mori began in ancient China, with cultivation evidence dating back around 5,000 years. This long history of human intervention has resulted in a creature significantly different from its wild ancestors. Adult mulberry moths have lost the ability to fly and are largely unable to feed, existing solely to reproduce. Their reliance on mulberry leaves highlights the connection between the insect and its food source.
Life Cycle of the Silkworm
The life cycle of the mulberry silkworm unfolds through four distinct stages: egg, larva, pupa, and adult moth. The cycle begins with eggs, typically laid in clusters by the female moth. These tiny eggs remain dormant before hatching. Under suitable conditions, the eggs hatch into larvae.
The larval stage, commonly known as the caterpillar, is the longest and most active phase, lasting approximately 20 to 33 days. During this period, the silkworm consumes vast quantities of mulberry leaves, growing rapidly and shedding its skin four or five times in a process called molting. Each molt marks the beginning of a new instar, with the larva increasing significantly in size. Once fully grown, the larva stops feeding, its body becomes translucent, and it begins to search for a suitable place to spin its cocoon.
The pupal stage occurs within the protective silk cocoon spun by the mature larva. The silkworm extrudes a continuous, double filament of fibroin protein, coated with sericin gum, from specialized glands in its mouth. This process takes about 3 to 8 days, creating an oval-shaped cocoon that hardens upon exposure to air. Inside this cocoon, the larva undergoes complete metamorphosis, transforming into an adult moth. If allowed to emerge, the adult moth will secrete an alkaline fluid to dissolve a portion of the cocoon, creating an exit hole and breaking the single silk filament.
From Cocoon to Silk: The Process
The transformation of cocoons into usable silk thread is a process known as sericulture. Once silkworms complete their cocoons, they are harvested. To preserve the long, continuous silk filament, the pupae inside the cocoons must be killed, a step called stifling. This is achieved through methods like hot air drying, steam, or boiling, which prevent the emerging moth from breaking the silk thread. Stifling ensures the single filament remains intact for high-quality silk production.
Following stifling, the cocoons undergo reeling. Cocoons are immersed in hot water to soften the sericin gum that binds the silk filaments together. Workers locate the outer end of the silk filament from several cocoons and combine them to form a single, stronger thread. These combined threads are then wound onto reels, creating raw silk skeins. A single cocoon can yield a continuous filament ranging from 300 to 900 meters in length.
After reeling, the raw silk still contains sericin, which makes it stiff and dull. Degumming, a washing process using soap and hot water, removes the sericin, revealing the silk’s natural luster, softness, and sheen. The degummed silk can then be spun into yarn, dyed, and woven or knitted into various fabrics. This process, perfected over centuries, has made silk a prized material, establishing the mulberry silkworm as a foundation of textile history and global commerce.