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.
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.
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
A gallery of incredible spiders
June 3rd, 2009 at 10:05 am
I have always loved jumping spiders. There is just something about how they look up at you… And you don’t have to spend much time with them to realize that they’re very intelligent.
June 3rd, 2009 at 10:11 am
When I first read this title on twitter, I thought it said “Scientists gather in groups to impersonate ants”. I don’t know which is funnier – the fact I mis-read it, or the fact that I still clicked through. Dear me, I think it’s time to go home…
June 3rd, 2009 at 10:16 am
I bet you that if you had a group of 10 scientists, they wouldn’t be attacked by predatory jumping spiders either. Therefore scientists mimic ants. TRUFAX.
June 3rd, 2009 at 11:19 am
I love waking up to your blog posts!
June 3rd, 2009 at 11:32 am
Fascinating stuff – I had no idea such things existed.
June 3rd, 2009 at 11:48 am
That is soooo cool!
June 3rd, 2009 at 12:20 pm
Fascinating information! Based on this post I think their experimental designs are rather elegant too. Nice job by the researchers.
“Scientists gather in groups to impersonate ants…” heh-heh. I like that. It sounds like the headline for an Onion article.
June 3rd, 2009 at 12:57 pm
I remember when I was young and living in Papua New Guinea, observing an ant-mimicking spider. It looked in every way like an ant, except for one thing – I was scared of it in exactly the same way I am of all spiders, which gave it away under only slightly closer observation (visual clues of its modified arachnid origins). However, insects themselves don’t elicit this response in me at all – indeed, even from those early years I was very deeply and scientifically into entomology, and was always out photographing and handling insects of all varieties, furthering my knowledge of the field.
My personal theory is that there’s something subliminally acoustic that spiders do that insects don’t, that elicits fear in me in a way that an insect simply cannot. It’s not really a visual thing – although that submodality or sensory input is obviously tightly coupled to the sensation.
June 3rd, 2009 at 2:41 pm
That’s fascinating. Is it wrong that I now want to systematically expose you to a wide range of spiders that do or do not mimic other arthropods to test the sensitivity and specificity of his arachnophobia?
June 3rd, 2009 at 6:46 pm
That is fascinating! I showed the photo to h & kids, and at supper I am going to tell them about Ian’s comment re his fear of spiders uncovering the mimic’s cover!
June 3rd, 2009 at 11:25 pm
So spiders have cosplay conventions?
June 4th, 2009 at 10:20 am
This a beautiful picture. I never would have guessed that “one does not belong”. My little one plays those games all the time and here it is happening in the natural world.
June 5th, 2009 at 3:56 pm
#8
I’m pretty much the same way. I identified the spider in the picture immideately, and I will recognize a spider on the wall from across the room. No matter the size, I always recognize spiders immideatly.
I do not react to insects, or other arthropodes, only spiders. I didn’t even flinch when a large housecentipede ran across the wall right in front of my face, but came close to hyperventilating and exiting the car at speed when I discovered a spider on the inside of the windshield… We only have to fear fear itself.. and spiders
June 6th, 2009 at 11:18 am
Totally fucking coolio!
March 13th, 2011 at 1:17 pm
Those spiders are living a lie. I wonder if it haunts them at night?