An orb web is a two-dimensional, circular web recognized by its distinct wheel-like shape. Crafted by orb-weavers, these intricate structures are a classic example of animal architecture. They are designed to efficiently capture flying insect prey, and their construction and materials showcase natural engineering.
Anatomy of an Orb Web
An orb web exhibits several distinct structural components. Non-sticky frame threads create the outer boundary, anchoring the web to surrounding objects. Radial lines extend inward from the frame, resembling spokes on a bicycle wheel and converging towards the center. These radial threads provide the web’s structural support and act as pathways for the spider.
At the web’s center lies the hub, an irregular network of threads where the spider often rests and monitors for vibrations. Spiraling outwards from the hub, interwoven with the radial lines, is the sticky capture spiral. This spiral is coated with a glue-like substance designed to ensnare insects.
The Step-by-Step Construction
The building of an orb web begins with the spider releasing a single bridge line, a silk thread carried by the wind until it snags on another support. Once this initial line is secure, the spider reinforces it by laying down additional strands. From this reinforced bridge, the spider forms an initial Y-shaped structure, establishing the first three radial lines and a central point for the hub.
The spider then constructs the outer frame, connecting the Y-shape ends to create a polygonal boundary. Additional radial lines are then spun, extending from the hub outwards to the frame, filling in the wheel-like pattern. Next, the spider lays down a temporary, non-sticky auxiliary spiral, working from the hub outwards. This temporary spiral provides a scaffold and allows the spider to move across the web while it constructs the final capture threads.
Finally, the spider replaces the temporary auxiliary spiral with the permanent, sticky capture spiral, working from the outside of the web inward towards the hub. As the sticky spiral is laid, the spider often removes the non-sticky auxiliary spiral. This methodical process, which can take approximately an hour to complete, results in a functional trap.
The Engineering of Spider Silk
Spider webs are composed of various types of silk, each possessing distinct properties for specific roles. Dragline silk, also known as major ampullate silk, forms the framework of the web, including the frame threads and radial lines. This silk exhibits high tensile strength, comparable to steel, and effectively absorbs kinetic energy from impacts, preventing breakage.
The stretchy, glue-coated viscid silk, derived from flagelliform and aggregate glands, is used for the capture spiral. This silk is highly elastic, with a breaking extension ratio significantly greater than that of frame silk, allowing it to stretch considerably when prey strikes. Its surface is covered with sticky droplets that ensnare insects. Minor ampullate silk is sometimes used for the temporary auxiliary spiral, providing elasticity.
Web Variations and Features
Some orb-weaving spiders incorporate unique additions or modifications to their web designs. One notable feature is the stabilimentum, a dense, zigzag pattern of silk within the web. These structures can appear in various forms, such as crosses, spirals, or lines. Spiders from the genus Argiope, like the garden spider, are well-known for creating these decorations.
The purpose of the stabilimentum remains a topic of scientific discussion, with several theories. Some researchers suggest it acts as a visual warning to birds, preventing web damage. Another hypothesis proposes that the highly UV-reflective silk of the stabilimentum attracts insects. Other theories include camouflage for the spider, a platform for molting, or a way for the spider to expend excess silk.