Spiders co-ordinating using simple rules

Watch a colony of Anelosimus eximius spiders and you will soon notice something fascinating...

These small arachnids, native to the Lesser Antilles in the Caribbean, live in colonies numbering thousands of individuals. Their net-like webs over plants form large horizontal sheets, but, surprisingly for spiders, they're not sticky. This means that when prey falls in, the spiders must move quickly to grab it.

But, weirdly, they don't all make a mad dash for dinner: instead, in a bizarre ballet of stuttering sprints, they move in unison towards the prey, pause periodically, and then set off again.

This behaviour has some logic behind it. Spiders live in a world of vibrations, but they can't hear and track down their prey if there are too many fellow spiders moving at the same time. So, by pausing, they have the opportunity not just to get their own bearings but those of the prey so they can close in.

Until recently, the mechanism of the strange coordination has been a mystery: how do the spiders all stop at once? Now, researchers at the Centre de Recherches sur la Cognition Animale in Toulouse, France, think they've found a few simple rules that each spider follows, the net result of which is this co-operative behaviour.

In effect there are two rules, as Violette Chiara explains: when you run, always run for the same duration each time; and when you stop, wait until the noise of the prey is louder than the noise of your neighbours before setting out again.

This simple system, devoid of direct communication, allows for a very flexible hunt with spiders moving faster for larger prey. It also results in a large number of spiders arriving at the prey at once, critical when taking down morsels severeral hundred times the bodyweight of one spider.

Chiara hopes that, apart from shedding light on our arachnid friends, this research may help with other distriubted systems. She imagines flying drones or swarming robots powered by similar simple rules, forming something much larger than the sum of their many parts.