A spider web is a silken structure created by spiders. These natural constructions are primarily composed of protein fibers. While widely recognized for trapping prey, spider webs also function as shelters, sensory extensions, and protective enclosures for offspring.
The Building Blocks of Silk
Spider silk is primarily a protein fiber, a natural polymer known as fibroin. It is largely composed of amino acids, particularly glycine and alanine, which often constitute around 75% of the total amino acids present. Other amino acids are also present, contributing to the silk’s specific characteristics. The unique arrangement of these amino acids, with glycine-rich regions providing elasticity and alanine-rich regions contributing to strength, underpins the material’s exceptional properties.
How Spiders Spin Their Webs
Spiders produce silk as a liquid within specialized silk glands in their abdomen. Most spiders possess multiple gland types, each synthesizing a specific kind of silk. The liquid silk is then extruded through spinnerets at the tip of the abdomen.
As it passes through narrow ducts, it transforms into a solid fiber. This hardening involves water removal, a pH change, and protein molecule alignment. The spider controls the thickness and speed of the extruded silk, creating diverse thread types.
Diverse Designs and Their Functions
Spider webs exhibit a wide array of designs, predominantly for capturing prey. Orb webs, for instance, are classic circular structures featuring a sticky spiral for snaring insects, supported by non-sticky radial threads. These designs can vary significantly, with over 130 known shapes reflecting spider species and their hunting strategies.
Beyond orb webs, other common designs include funnel webs, which are sheets of silk narrowing into a funnel-shaped retreat. Prey touching the sheet triggers the spider to ambush from the funnel. Sheet webs consist of horizontal or inclined silk sheets, often reinforced, designed to entangle flying insects. Tangle webs, also known as cobwebs, are irregular, three-dimensional structures typically found in undisturbed areas, serving as effective traps for crawling or flying prey. Spiders also use silk for purposes beyond prey capture, such as building shelters, creating protective egg sacs, or for dispersal through “ballooning.”
Remarkable Properties of Spider Silk
Spider silk possesses an array of exceptional physical characteristics that contribute to its effectiveness. It is renowned for its strength, often considered stronger than steel by weight and comparable to high-grade alloy steel. The tensile strength of spider silk can reach approximately 1.3 gigapascals (GPa).
In addition to its strength, spider silk exhibits remarkable elasticity, allowing it to stretch considerably before breaking. Certain types, like flagelliform silk, can extend up to 300% or two to four times their original length. This combination of strength and elasticity results in high toughness, meaning the silk can absorb a substantial amount of energy before fracturing.
For example, the silk of the Darwin’s bark spider is recognized for its extraordinary toughness, surpassing that of Kevlar. The stickiness of capture silk, crucial for trapping prey, is achieved by a glue-like substance, typically composed of a glycoprotein core surrounded by a viscous, aqueous coating, which often forms droplets along the silk strands. The inherent elasticity of the silk fibers further enhances this adhesive quality.