Moths, specifically their larval stage known as the caterpillar, produce silk, which is a protein fiber spun from specialized glands. This substance is a complex protein polymer consisting primarily of fibroin, which provides strength, coated with a gummy protein called sericin that acts as an adhesive. This natural fiber is extruded as a liquid through a spinneret located near the mouth, solidifying immediately upon contact with air. The primary purpose of this silk varies widely across the Lepidoptera order, ranging from a simple safety tether to the construction of elaborate, multi-layered shelters.
The Silkworm and Commercial Textiles
The common association of silk with textiles is centered almost entirely on one species, the domesticated silkworm, Bombyx mori. This moth has been selectively bred over approximately 5,000 years, making it completely dependent on humans for survival and reproduction. The larva of the silkworm feeds exclusively on mulberry leaves, a diet that contributes to the purity and high quality of its silk fiber.
The silkworm spins its silk to create a single, continuous strand that forms its cocoon for pupation, a protective case measuring between 300 to over 900 meters in length. For commercial sericulture, the pupa must be killed with heat or steam before the moth can emerge. If the adult moth were allowed to chew its way out, the continuous filament would be broken into many smaller pieces, making it unsuitable for reeling into fine thread.
This practice allows for the harvesting of the unbroken silk, which is then processed to remove the sericin coating, yielding the lustrous fiber. This fiber accounts for roughly 90% of the world’s silk production. Other species, like the wild silkworms that produce Tasar or Eri silk, are also used for textiles but are less commercially viable due to the non-continuous or less-uniform nature of their fibers.
The Biological Purpose of Silk in Other Moths
For the vast majority of moth species, silk serves ecological functions rather than commercial ones, acting as a survival tool throughout the larval stage. A common use is the dragline, a single thread extruded by caterpillars to anchor themselves to a plant. This lifeline allows a caterpillar to drop quickly to escape a predator or disturbance, then climb back up the strand when the danger has passed.
Gregarious species, such as tent caterpillars, use silk collectively to construct large, communal shelters or bivouacs. These tents are multi-functional, providing a safe space for molting and protection from enemies, while also aiding in thermoregulation. Caterpillars also utilize silk to create an organized environment for foraging, spinning trails often marked with chemical pheromones that guide nest mates to food sources. Finally, like the silkworm, many other moth larvae spin cocoons for the pupal stage, sometimes mixing the silk with debris or hair for added camouflage and protection.
How Moth Silk Compares to Spider Silk
Moth silk and spider silk are both protein-based fibers, but they possess distinct mechanical properties reflecting their divergent biological functions. Spider silk, particularly the dragline silk used for the structural frame of a web, has superior tensile strength and elasticity compared to typical moth cocoon silk. Spider silk needs to be dynamic, absorbing the shock of a flying insect, while moth silk is designed for static, protective structures.
The difference in function is evident in the final product: moth silk is primarily used to build a fixed shelter, such as a cocoon or a communal tent. Spider silk, by contrast, is used for dynamic structures like webs, snares, or as a safety line. Researchers have found that while silkworm silk is weaker, it is produced in massive, easily harvestable quantities, whereas spiders are territorial and cannot be farmed efficiently for their stronger silk.