to build, maintain, and clean their webs. They cooperate in capturing prey, and dine together when they snare a large feast.
The females feed their offspring by vomiting up food for them, just like mother birds. They even regurgitate food for juveniles other than their own. In other words, they work together to care for the youngest in the colony. It is a sort of spider day-care.
Nearby colonies tend to be related to each other, because new colonies form in one of two ways.
Sometimes, a large web is broken in half, perhaps by heavy rainfall or falling branches, or even a falling monkey. In its place, two smaller colonies are left behind.
My students know more about social spiders than your average arachnologist
Other times, individual females will strike out on their own, setting up a new web. Eventually other dispersing females will join the group; perhaps ones who tried to set up their own webs but failed. Survival is more likely in groups, after all.
These two processes together help to explain why A. eximius colonies tend to be found in clusters, sometimes as many as 40 related colonies within just a few kilometres.
A. eximius is perhaps the best known of the social spiders, but that is not saying much.
Gregarious arachnids are not much studied – perhaps because so few of them exist in the first place. "My students know more about social spiders than your average arachnologist," says Linda Rayor, an entomologist at Cornell University in Ithaca, New York, who studies Australia's social huntsman spiders.
Why was the bee subject to such an awful fate?
"Most animals are solitary," she tells me, "whether you're looking at mammals, or birds, or whatever. Social behaviour in otherwise solitary animals is cool."
Cool indeed, if somewhat creepy.
As I watched the social spiders go about their day deep in the jungle, a bee found itself stuck in the giant web. Suddenly, dozens of the eight-legged predators descended en masse onto the struggling black-and-yellow striped insect. It was impossible to see through the writhing mass of tiny spider legs, but one thing was certain: that bee was done for.
If death by one spider seems bad enough, it must be nothing compared to an attack by a whole swarm of them.
Why was the bee subject to such an awful fate? Or, to put it another way, how and why did a few spiders become social when the overwhelming majority of their fellow species are solitary?
The first spider, the proto-spider, was probably solitary. So the spiders that gave rise to today's truly social species must have been too. But then they did something remarkable: they turned sub-social: they learned to tolerate each other, at least for a time – even if they did not exactly enjoy hanging out in groups.
As they mature, they become intolerant of each other
That early tolerance could have come about as a result of parental care. In some spiders alive today, that is just defending an egg sac, but in others – the ones that are poised to evolve sociality – it looks more familiar to our mammal brains.
In those species, females provide food and protection for their developing offspring, and occasionally even to other juveniles. In some species, mothers even make the ultimate sacrifice, allowing themselves in their last moments to become a meal for their brood. Sub-social spiders can still be cannibals, after all.
"They're the spiders that already have cooperation as juveniles," says arachnologist Jonathan Pruitt of the University of California, Santa Barbara. "But as they mature, they become intolerant of each other."
In other words, juvenile tolerance alone is not enough to turn spiders fully social. The environment in which the spiders live also has to be right. Researchers have discovered three ecological elements that often lead to cooperative living among arachnids.
Social spiders tend to feed on bigger prey, for one thing.
Solitary spiders living in places where it is difficult to subdue large or more profitable prey alone may eventually figure out that it is in their interest to work together. It is a smaller step from tolerance t