Orb weaver spiders (family Araneidae) construct the iconic spiral-wheel webs often seen in gardens and forests. Many species, particularly those in the genus Argiope, weave a dense, zigzagging band of silk into the web’s center. This conspicuous, often bright white structure presents an evolutionary puzzle because it contradicts the strategy of building a cryptic, nearly invisible trap. The function of this unusual addition has been extensively investigated, with evidence suggesting it serves multiple, sometimes contradictory, biological purposes.
What is the Stabilimentum
The decorative silk structure is formally known as a stabilimentum, a name originally given under the mistaken assumption that its purpose was solely to reinforce the web. Stabilimenta typically appear as thick, bright white zigzags, discs, or an “X” shape, radiating from the hub of the web where the spider rests. Unlike the web’s sticky capture threads, this non-adhesive silk is structurally different.
Spiders in the genus Argiope, such as the black and yellow garden spider, are the most recognized builders of these decorations. The conspicuous nature of the stabilimentum is due to its remarkable ability to reflect high-intensity light. The silk reflects ultraviolet (UV) light in particular, a spectrum highly visible to insects and birds but invisible to humans. The physical characteristics of the stabilimentum, including its shape and size, can vary depending on the spider’s age and species, suggesting a flexible behavior linked to environmental pressures.
The Competing Theories: Prey Attraction Versus Predator Warning
The two primary hypotheses regarding the stabilimentum’s function are opposed: prey attraction and predator warning.
Prey Attraction
The prey attraction hypothesis posits that the stabilimentum functions as a visual signal to flying insects. Many insects use UV light to locate flowers and food sources. The silk’s high UV reflectivity mimics a potential food resource, effectively luring prey directly into the web’s sticky threads.
Experimental evidence supports this theory, showing that webs with intact stabilimenta capture more prey than webs where the decoration has been removed. This suggests a foraging benefit that compensates for the web’s high visibility. This visual baiting mechanism is effective for diurnal spiders, like Argiope, which hunt during daylight hours when UV light is abundant.
Predator Warning
The predator warning hypothesis suggests the stabilimentum’s purpose is to make the web visible to animals that would otherwise destroy it. Birds and other larger vertebrates frequently fly through or walk into webs, tearing the structure and forcing the spider to spend time and energy rebuilding. This hypothesis is supported by experiments using artificial stabilimenta, which have shown a significant reduction in the frequency of web damage caused by flying birds.
The visibility of the silk serves as a warning sign, diverting larger creatures away from the delicate trap. This function is relevant for spiders that construct large, semi-permanent webs, as rebuilding an entire orb is energetically expensive. The presence of a highly visible decoration suggests a trade-off where the spider accepts a less cryptic web in exchange for protection from catastrophic damage.
Structural Role and Web Dynamics
The stabilimentum also plays a mechanical role in the web’s physical dynamics, though the initial idea that it added tensile strength has been dismissed. A more recent structural function relates to the web’s ability to transmit vibrations.
The dense silk may act as a physical conduit, modulating how vibrations travel across the web. When a struggling insect strikes, vibrations are transmitted along the threads, allowing the spider to locate its prey. Studies indicate that webs containing stabilimenta enhance the transmission of these vibrations, particularly those generated by prey impact.
This improvement in signal clarity allows the spider to more quickly and accurately pinpoint the captured insect, reducing the chance of escape. The stabilimentum’s placement at the central hub is consistent with enhancing information transfer, as this is where the spider typically rests. This localized function contrasts with the larger signaling roles of prey attraction and predator deflection.
The Hidden Costs of Building a Stabilimentum
Building a stabilimentum is not universal among orb weavers and presents a clear evolutionary trade-off due to significant costs.
Energetic Cost
The first major cost is energetic, as producing the dense, non-capture silk requires a considerable investment of metabolic resources. Orb weavers that build stabilimenta expend more energy and material than those that do not. This expense must be balanced by an equivalent increase in foraging success or a reduction in web destruction.
Spiders adjust their building behavior based on physical condition and food availability. Well-fed spiders tend to build more elaborate decorations, while starved spiders reduce the size or frequency of the structure.
Predation Risk
The second cost is the increased risk of predation for the spider itself. While the highly visible stabilimentum may deter birds from destroying the web, it can attract the spider’s own predators, such as wasps or other visually hunting animals. The bright, reflective silk draws the eye to the web’s center, where the spider often sits in full view. This increased conspicuousness turns the structure into a beacon for a hungry predator.