It's hard to believe that the intricate patterns of fine silk we call cobwebs are each constructed by a single spider. They diligently design and build homes strong enough to stop an insect in its tracks. A team of researchers at Johns Hopkins University, USA, led by Andrew Gordus, set out to characterize the way orb-weavers, Uloborus diversus, construct such exquisite webs by closely analyzing the spiders' movements while they build.
To observe the spiders' skills up close, the researchers outfitted a chamber with a video camera looking down onto a plexiglass box. They then annotated a portion of video, frame by frame, labelling the body, legs and joints so that they could adapt algorithms to predict where the legs and body would be in the rest of the videos. Using this approach, the researchers characterized each spider's movements as they spun their webs.
It turns out that web assembly is made up of four previously defined construction phases: proto-web, radii, auxiliary and capture. In the initial stage of a web the spider builds a proto-web, which serves as scaffolding and looks like a messy tangle. The spiders paused most often while building the proto-web, suggesting that they use this time to assess the integrity of their composition. During the radii stage the spiders removed much of the proto-web and anchored silk between the edges and the center hub. Following the radii stage, the spiders built an auxiliary web, which is a single spiral that sweeps out from the hub to the edge of the web. And finally, the spiders removed the auxiliary spiral during the final capture stage, when the spider spun silk around the radii spiralling in tightly toward the center.
By tracking the spiders' limbs, the research team distinguished behaviors such as walking, moving right or left legs, and pulling silk quickly or slowly. When they organized these behaviors by construction phase, they saw that particular behaviors are more likely to occur at some stages than others, such as leg sweeps during capture and increased pausing while building the proto-web. By tracking the position of the spider, they predicted when the spider was building radii by the straight walks from the center to the edge and back again. They then analyzed transitions between behaviors by calculating the probability that one behavior will follow another. They found that although each stage is made up of similar sets of behaviors, the probability that one will follow another is different for each stage. For example, during the capture stage, a fast silk pull is likely to follow multiple different behaviors, whereas during the radii stage, slow silk pulls are more common.
Finally, the team used a mathematical model to test if the spiders' motions were distinct enough to predict the stage based on the probability of movements and transitions between movements. They found that their model successfully predicted different stages except for the proto and radii stage, which the model did not differentiate well. The researchers interpreted this outcome to mean that spiders move similarly while building the proto-web and the radii but the different patterns of web construction result from where the spider is on the web.
A close and careful investigation of orb-weavers honing their craft shows that even a complex masterpiece like a spiderweb is ultimately made by an individual's flexible execution of a limited set of repeated movements.