A spider web is a highly organized biological structure constructed from silk, a natural polymer extruded by the spider’s spinnerets. The web serves as an external sensory organ, transmitting vibrations from trapped insects to the spider, while also providing shelter and a secure pathway. Its complex architecture and remarkable material properties allow it to absorb the energy of fast-moving prey without breaking.
The Protein Blueprint: Composition of Spider Silk
Spider silk is a polypeptide fiber composed mainly of large proteins called spidroins. These proteins are characterized by highly repetitive amino acid sequences, with glycine and alanine being the most abundant components. The structure includes poly-alanine regions that stack together to form pleated beta-sheets, which are stiff crystalline structures providing the silk’s incredible strength.
Silk proteins are manufactured and stored in specialized glands, such as the major ampullate glands, as a concentrated liquid solution known as dope. This liquid is transformed into a solid fiber through a spinning process as it travels through a long, narrow duct to the spinnerets. The process involves the extraction of water, a decrease in pH (acidification), and the application of mechanical shear forces.
These changes trigger the spidroin molecules to align and self-assemble, causing the liquid crystalline dope to solidify rapidly into an insoluble fiber. The final silk thread is a semi-crystalline polymer, featuring hard crystalline regions interspersed with softer, disordered glycine-rich regions. This precise molecular architecture gives the finished silk its unique combination of performance characteristics.
Architectural Diversity: Major Web Structures
Spider webs exhibit significant architectural diversity, reflecting the hunting strategies of different species, but most webs built for prey capture fall into three major structural categories. The most recognizable is the orb web, a roughly circular, two-dimensional structure featuring radial threads that radiate outward like spokes and a sticky capture spiral wound between them. The radial and frame threads are made of non-sticky, high-tensile-strength dragline silk, which acts as the primary mechanical support to absorb impact energy.
Another common design is the sheet web, which consists of a flat, horizontal sheet of silk, often woven close to the ground or vegetation. Prey that falls onto the sheet is typically attacked from below by the spider, which remains hidden in a silken retreat or under the structure. These webs sometimes have a separate, irregular tangle of non-sticky silk threads suspended above the sheet, which serves to knock down flying insects onto the flat surface.
The third main category is the tangle web, also known as a cobweb, typically built by spiders in the family Theridiidae. These are irregular, three-dimensional structures of crisscrossing, non-oriented silk threads that lack the geometric precision of the orb web. The tangle-web captures prey through a trip-line mechanism; the silk is often coated with a sticky layer that adheres to a surface, and when an insect breaks the line, the elastic tension pulls the prey upward into the main body of the web.
Material Science: Exceptional Physical Properties
The finished spider silk fiber possesses an exceptional combination of tensile strength and elasticity, making it one of the toughest materials known in nature. Tensile strength, the material’s resistance to breaking under tension, can reach 1.6 gigapascals in some dragline silks, comparable to high-grade steel. The silk’s true advantage lies in its strength-to-density ratio, as it is about six times less dense than steel, making it far superior on a per-weight basis.
The silk’s elasticity allows it to stretch up to four times its original length without yielding, giving the web a remarkable capacity to absorb the kinetic energy of a flying insect. This high toughness results from the molecular arrangement where flexible, amorphous regions of the protein stretch, while the strong crystalline beta-sheets resist fracture. This unique balance ensures the web can deform significantly under impact without snapping.
The capture spiral of the orb web is made sticky by a coating of specialized glue droplets arranged in a “beads-on-a-string” morphology. This adhesive is a viscoelastic substance composed of glycoproteins and low molecular weight compounds, which is highly hygroscopic. The stickiness is a synergistic effect, combining the glue’s adhesion with the elasticity of the underlying flagelliform silk fiber, allowing the droplets to stretch and dissipate energy as the prey struggles.
Beyond the Trap: Other Uses for Spider Silk
Spiders employ silk for many critical functions throughout their life cycle beyond prey capture. A constant application is the dragline, a safety line made of the strongest major ampullate silk that the spider trails behind it. This line serves as a lifeline, anchoring the spider to a surface and providing a means to rapidly descend or escape danger.
Female spiders use silk from the tubuliform glands to construct a robust, multi-layered egg sac to protect their offspring. This silk is typically the stiffest type a spider produces, providing a tough shell against predators and environmental stresses. The females of some species carry the sac, while others suspend or hide it until the spiderlings hatch.
Small spiders use a technique called ballooning for aerial dispersal to colonize new areas. The spider climbs to a high point, raises its abdomen, and releases fine silk threads, often called gossamer, that catch air currents or utilize electrostatic fields. This passive flight mechanism allows them to travel distances ranging from a few meters to hundreds of kilometers.