Arachnophobes often cite the spiders’ unpredictable movement as the basis of their fear, pointing out how each spindle-shaped leg appears to lift, bend, and examine with a threatening degree of autonomy.
Perhaps worrying is that research by my colleagues and I has shown that each leg of a spider actually enjoys some brain independence – especially when it comes to the complex task of webbuilding.
Our study showed that spider legs “have their own thoughts” and build webs without monitoring the spider’s brain. This has important implications for the field of robotics, which may be inspired by this example of decentralized intelligence to build similarly autonomous robot limbs.
To reach our conclusions, we observed the common garden spider, Araneus diadematus, a creature we are all familiar with – both in our backyards and as a heroine on the pages of the children’s book Charlotte’s Web.
Spider webs serve many functions. They provide a safe home, but are also an invisible and highly dynamic trap that was set up to trap and then hold insects that come too close.
To perform this function, webs use a strong structural framework of radiating spokes with a so-called “capture coil” built on top, soft and sticky, and using an extremely clever microscopic rolling mechanism to pull in a spider’s prey.
The spiral trap not only uses static electricity to catch a fly, it also has a complex adhesive to hold it in place and a specific elasticity that makes the net too stretchy for the leg of an unfortunate insect to fight for freedom could push against it.
The analogy of the internet as “web” is good: Since at least five different silks are used in the spider web, the way in which they intersect and network creates a kind of information filter capacity – with tiny vibrations that are always detected by the listening legs of a spider.
Given the incredible complexity of spider webs, we have to wonder how such a small animal – with an obviously tiny brain – can design and build this advanced structure. Modern technology has helped us understand how spiders handle such a complex task.
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By filming and following the movements of its eight legs, we were able to follow the web structure of a spider down to the smallest detail and reveal the construction process as a kind of dance around a central node with a precise choreography of replicable rules.
These rules are surprisingly simple. Each step and thread manipulation follows a fixed pattern of action, with one of the spider’s legs measuring an angle and distance in order to place one thread with a quick adhesive strip and then connect it to another – always with impeccable accuracy and spacing. Many years ago we programmed a virtual spider called Theseus to demonstrate how this works.
The apparent complexity of the structure is the result of a long series of thousands of small steps and actions, each of which builds on the previous steps and actions. In this iterative process, the network invests in “emergent properties” – special properties that result from the cooperation of different components – which in turn offer outstanding architectural and technical functions.
The complexity of the task (or rather the foot) of building a web seems to have required spider brains to outsource the work to the eight legs. In other words, spider legs build up semi-autonomous webs – eight phantom limbs perform their dance in local, closed feedback loops.
We discovered this after studying spiders in our laboratory that built webs within frames. In some experiments we cut threads of a network to be created. In other cases we have turned the train like a ferris wheel. This research wasn’t done to annoy the spider: it was meant to help us understand the rules that govern website building.
Using a set of rules – including rules that will help spiders keep building a troubled web – we taught Theseus, our virtual spider, these.
The new rules we taught Theseus – based on the dances of real spiders that we observed in our laboratory – have shown that each leg is actually doing many web building actions as an independent agent. This in turn helped us solve the mystery of how spiders build perfect webs after losing a leg.
If a spider’s leg gets pinched, it is thrown away, and a shorter leg will regenerate the next time the spider melts its exoskeleton. This replacement is not only half the size of a normal leg, but also has a different shape with different hair and sensors. Yet somehow spiders with regenerated legs continue to build perfect webs.
An ordinary garden spider with regenerated legs on the left. Author provided
Evolution has ensured that spider legs can in a certain way think for themselves, which means that the different properties of regenerated legs do not affect the structure of a web.
This relieves the brain of micromanaging eight legs performing complicated activities and gives it the freedom to focus on survival actions like looking for predators. This efficient, decentralized system is remarkably relevant to modern robotics, who often draw inspiration from nature in their artificial designs.
Spiders aren’t alone in decentralizing tasks from the brain – in fact, most animals do so to some extent, like the continuous beating of a human heart. But with their webs, spiders offer us a tangible, observable and fascinating means of measuring and understanding how this decentralization works.
That neat trick is to have a spider embed a simple task calculation into the structure of its limbs. Robotics call this morphological computing and only discovered its efficiency relatively recently. It turns out the humble garden spider has been using it for well over 100 million years.
This article by Fritz Vollrath, professor emeritus at the Zoological Institute, Oxford University, is republished from The Conversation under a Creative Commons license. Read the original article.
Published on February 11, 2021 – 12:41 UTC