Silk, a natural protein fiber, is known for its strength, flexibility, and lightweight nature. While often linked to a few creatures, silk production is diverse across the animal kingdom. This versatile material serves many functions, from building structures to providing protection and aiding dispersal.
Silk from Arachnids
Spiders are well-known silk producers, using it for many purposes beyond catching prey. A single spider can produce up to seven different types of silk from specialized glands. Dragline silk, from major ampullate glands, forms the strong, non-sticky radial lines of orb webs and acts as a safety line, supporting the spider’s weight. This silk is remarkably strong, with tensile strength comparable to steel and five times stronger by weight.
Spiders also produce flagelliform silk for the sticky capture spiral of orb webs, designed to be highly elastic and dissipate prey’s kinetic energy. Other silk types include aciniform silk for wrapping prey and lining egg sacs, tubuliform silk for stiff egg cases, and pyriform silk for attaching threads. Beyond webs, spiders use silk for shelters, mating structures, and dispersal via “ballooning.” In ballooning, smaller spiders release fine silk threads that catch wind currents or utilize the Earth’s electric field to travel vast distances.
Silk from Insects
Insects also produce silk with diverse applications. The domesticated silkworm, Bombyx mori, is famous for producing silk for its cocoon during metamorphosis, historically significant for human textiles.
Many other insects produce silk:
Caddisfly larvae use silk from salivary glands to construct protective cases from debris like sand, twigs, or plant fragments. Some also weave silken nets to filter food from water currents.
Webspinners create silk tunnels and galleries for shelter using glands on their forelegs.
Sawfly larvae spin cocoons, often in soil or leaf litter, where they overwinter before pupating.
Ant larvae may spin cocoons for pupation, while honeybee larvae use silk to reinforce the wax cells of their nests.
Silk from Marine Animals
Marine animals produce specialized silks despite aquatic challenges. Mussels, for example, create strong, adhesive byssal threads to firmly anchor themselves to rocks and other surfaces in turbulent waters. These threads have a central stem with radiating collagenous filaments, providing strength and extensibility to withstand wave action.
Each byssal thread has a proximal region that unravels to absorb force and a stiffer distal region, culminating in an adhesive plaque that bonds to the substrate. This structure allows mussels to remain securely attached, even in strong currents, and contributes to intertidal zone stability. Some marine worms also produce silk-like materials to construct tubes, providing shelter and protection.
How Silk is Made
Silk production across animal groups shares underlying biological principles. Silk is primarily composed of proteins: fibroin, which forms the structural core, and sericin, a gummy protein that often coats the fibroin fibers. These proteins are initially stored as a liquid solution within specialized glands.
Liquid protein transforms into solid fiber as it passes through a narrow spinning duct. During this process, changes in pH, ion concentration, and shear forces cause proteins to align and solidify. Silk’s molecular structure, particularly beta-pleated sheets, gives it strength and elasticity. Tightly packed beta-sheets form crystalline regions for tensile strength, while disordered, amorphous regions contribute to flexibility and toughness.