Spiders gather in groups to impersonate ants

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The animal world is full of harmless liars, who mimic species more dangerous than themselves in order to avoid the attention of predators. But none do it quite like the dark-footed ant-spider Myrmarachne melanotarsa.

As its name suggests, this small species of jumping spider, discovered just nine years ago, impersonates ants. In itself, that’s nothing special – ants are so aggressive that many predators give them a wide berth and lots of species do well by imitating them. The list includes over 100 spiders but among them, M.melanotarsa‘s impression is unusually strong. It doesn’t just mimic the bodies of ants, but their large groups too.

Unlike all of its relatives, the spider lives in silken apartment complexes, consisting of many individual nests connected by silk. These blocks can house hundreds of individuals and while moving about them, the spiders usually travel in groups. Now, Ximena Nelson and Robert Jackson from the University of Canterbury have found evidence that this social streak is all part of the spiders’ deception.

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The duo exposed the mimics to predatory relatives – three species of jumping spider that will prey on other smaller spiders, but are averse to both the taste and aggression of ants. Other studies have shown that these predators are fooled by their ant-mimicking cousins, but Nelson and Jackson wanted to see if the group size of the dark-foots had anything to do with the effectiveness of their disguises.

They placed the predators in a plastic cage for an hour with an ant mimic, the ant that it impersonates, or a midge. The predators were all too happy to tackle the midges, but rarely attacked the ants or their lookalikes. On the rare occasions that the ants were attacked, they were immediately dropped. But if the mimic’s cover was blown, it was always eaten.

It therefore pays the mimic to ensure that it’s not attacked in the first place, and indeed, Nelson and Jackson found that groups of ten mimics were almost never attacked, facing significantly less danger than any lone individual. It wasn’t just group size that put off the predators, for they would readily assail groups of ten midges. It was the fact that their potential prey both looked like ants and amassed like ants that put them off.

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Of course, their aversion could be to do with something in the mimics’ behaviour that has nothing to do with its resemblance to a group of ants. For example, groups of moving animals can confuse predators by stopping them from locking onto a single target. To control for that, Nelson and Jackson exposed the spiders to dead mimics that could hardly be expected to move in an ant-like way. The spiders were placed in a transparent walkway with cylindrical chambers at either end – one was empty and the other contained four dead insects (or insect mimics), mounted in a life-like posture.

If the mounts were ants or ant-mimics, the predators’ aversion kicked in and they preferred to stay in the empty chamber; if the mounts were midges, neither option was more appealing. Clearly, the appearance of M.melanotarsa was enough to sell the illusion.

Nelson and Jackson describe the tactics of the dark-footed ant-spider as “collective mimicry” – a group of animals acting together to produce a communal masquerade that’s more convincing than any individual efforts.

There are very few other examples of this behaviour. The blister beetle is one – its larvae gather in groups of hundreds or thousands in order to crudely imitate a female bee (see this Powerpoint presentation for more). When a male visits, the larvae jump onboard and get a free ride onto an actual bee. On her back, they get a lift to the nest, where they can feed on her eggs. It’s an example of imitation through teamwork, but it’s subtly different to the antics of the ant-mimicking spider. In this case, the group gathers to impersonate an individual; for the spider, the group impersonates another group.

Reference: Animal Behaviour 10.1016/j.anbehav.2009.04.005