From the spiders of Mirkwood to the Acromantulas of Hogwarts’ Forbidden Forest, colonies of giant arachnids - usually with a taste for human (or Hobbit) flesh - have long stalked the worlds of fantasy and horror.
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Happily for arachnophobes, the vast majority of the world’s 50,000-plus species of spider live and work alone. As undiscriminating predators with cannibalistic tendencies, spiders don’t generally have the temperament for communal living. But there are some spectacular exceptions.
For most of the 80 or so species that show some degree of social behaviour, company is something that is tolerated rather than encouraged.
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Take the dewdrop spiders, a group of tiny tropical species that specialise in pilfering prey caught by larger orb-web spiders, such as the giant golden orb-weaver, which boasts a leg-span as big as a human’s hand and is hundreds of times the weight of its light-fingered guests.
A single host web may support many dewdrops. Most of the time, they are aggressive and territorial, defending their small patch of the web from competitors. Hostilities are suspended, though, when they come together to feed communally on larger prey items.
Australian huntsman spiders are more cooperative than dewdrops. With leg-spans up to 20cm, they are also much bigger. They live in stable family groups of up to 300 individuals under the bark of dead trees.
When the makers of the 1990 movie Arachnophobia were looking for a social species that would look sufficiently terrifying on camera, this is the one they chose.
Each spider hunts alone, but they don’t consume their prey where they catch it. Instead, they bring it back to the nest to share with younger colony members. It’s one of only about 25 species that raise their broods communally.
Another is an Ecuadorian species named Anelosimus eximius, which makes up for its small size (less than 1cm long) through sheer weight of numbers. A single colony may contain thousands of individuals occupying a three-dimensional web that envelops several cubic metres of shrubbery. Working together, they can subdue prey 700 times the weight of a single spider - even small birds and other vertebrates are not safe.
But Anelosimus must overcome a challenge not faced by solitary web-builders. With the pitter-patter of so many tiny feet on the web, it can be hard for the occupants to distinguish the vibrations created by struggling prey. They get round the problem by synchronising their movements, alternating between taking a few steps and then pausing in unison in order to get a bearing on the prey’s location.
An intriguing feature of spider colonies - one that makes them strikingly different from the societies of ants, bees, wasps and other social insects - is the absence of a social hierarchy.
Queen spiders exist only in fictional spider societies. The Acromantulas stretch the comparison even further: their society is ruled by a king named Aragog. Neither do spider colonies contain the distinct castes found in social insects - workers, soldiers, nursemaids, etc.
However, there is some evidence for personality differences between members of a spider colony. Bolder, more aggressive individuals, for example, tend to specialise in prey capture. Some biologists have suggested that such differences represent an early stage in the evolution of a system of division of labour.
Given spiders’ solitary, predatory tendencies, it’s perhaps surprising that any have managed to get along well enough to live communally. It’s thought that spider sociality has its roots in the egg-guarding behaviour of many solitary female spiders. In most cases, the spiderlings disperse as soon as they hatch, but in some species, they hang around for longer, and may even eat their own mother before they depart.
From here, it’s not such a great leap to the evolution of a colonial system. The crucial step is simply that the spiderlings never leave home.
However, this route to sociality comes at a cost. If colonies are little more than sprawling extended families, high levels of inbreeding are almost inevitable, making them vulnerable to disease and environmental change - another reason, perhaps, for the rarity of spider societies.
Reassuringly, perhaps, with the notable exception of the Australian huntsman, the few species that have developed complex societies tend to be at the smaller end of the size spectrum. Even the huntsman is no match for the tarantulas and larger orb-spiders, which all remain stubbornly solitary. That said, the giant golden orb-weaver is one of those species whose spiderlings live together for a while in a communal nursery web before dispersing. Should they ever make the evolutionary leap to never leaving home, the giant spider colonies of fantasy and fiction might not seem quite so far-fetched.
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