The properties of spider silk, including high tensile strength and elasticity, make it stronger by weight than steel. These characteristics originate within specialized internal organs known as silk glands.
Different glands produce unique silks, each tailored for a specific function. This internal system creates fibers with distinct mechanical properties that enable spiders to build webs, ensure their safety, and protect their offspring.
Anatomy and Location of Silk Glands
The silk glands are housed within the spider’s abdomen. These glands are sac-like or elongated tubular structures that can occupy a significant portion of the abdominal cavity, especially in prolific web-building species. The size, number, and shape of the glands vary among different spider families and species.
Each gland is connected to an external, mobile appendage called a spinneret via a narrow, winding duct. Spiders have three pairs of spinnerets, although this number can range from one to four pairs. The silk, stored as a liquid, travels through the duct to microscopic, nozzle-like openings on the spinnerets called spigots.
The arrangement ensures that silks from different glands can be deployed independently or in combination. Fluid from multiple glands can even lead to the same spinneret, allowing the spider to produce composite threads with specific properties tailored to an immediate need.
Diversity of Silk Glands and Their Specialized Silks
Most web-weaving spiders possess an array of up to seven different types of silk glands, each producing a silk with distinct properties for a specialized purpose. The sticky capture spiral of an orb-web is a composite material created by two glands working in concert. The flagelliform glands secrete the elastic core fibers, while the aggregate glands produce the adhesive droplets that coat them.
Other glands serve purposes beyond web construction.
- Major ampullate glands: Produce the strong, non-sticky dragline silk for a web’s outer frame and the spider’s lifeline.
- Minor ampullate glands: Create a weaker silk used for temporary scaffolding during web construction.
- Tubuliform glands: Produce the tough silk used to construct protective egg sacs.
- Aciniform glands: Make the swathing silk used for subduing captured prey and lining retreats.
- Pyriform glands: Create a cement-like silk for making attachment discs that anchor threads to surfaces.
The Silk Synthesis and Spinning Process
Silk production begins within the epithelial cells lining the walls of each gland. These cells synthesize silk proteins, known as spidroins, and secrete them into the gland’s central cavity, or lumen. Here, the proteins are stored in a highly concentrated aqueous solution called the silk dope.
The transformation from liquid dope to solid fiber occurs as the material is drawn through a long, narrowing S-shaped duct connecting the gland to the spinneret. As the dope travels through this duct, it is subjected to physical shear forces that stretch and align the spidroin molecules. Simultaneously, specialized cells lining the duct actively remove water from the solution, accompanied by changes in ion concentration and a drop in pH.
These combined physical and chemical changes induce a phase transition in the spidroins. The protein molecules unfold and lock together, forming the ordered structures that give the final silk fiber its strength and elasticity. The final extrusion occurs through a spigot on the spinneret, where a valve controls the flow and diameter of the thread.
Composition of Silk Precursors within Glands
The raw material for silk is a liquid precursor inside the glands composed primarily of proteins called spidroins. This silk dope is a highly concentrated solution, with spidroin concentrations reaching up to 50% by weight. Each type of silk gland synthesizes its own unique set of spidroins, which determines the mechanical properties of each silk type.
The molecular architecture of spidroins is responsible for silk’s dual characteristics of strength and flexibility. These proteins are defined by long, repetitive sequences flanked by non-repetitive terminal domains. The repetitive regions consist of glycine-rich segments that form amorphous, coil-like structures, which impart elasticity, while interspersed poly-alanine sequences align into crystalline structures known as beta-sheets, which provide strength.
The spidroins remain soluble within the gland’s lumen because they are stored in a specific conformation stabilized by the surrounding chemical environment. This state prevents the beta-sheets from forming until the dope enters the spinning duct. This carefully controlled liquid crystalline state is essential for preventing premature solidification and ensuring the spider can spin a thread on demand.