Spider mimics ant to eat spiders and avoid being eaten by spiders

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It’s been just three weeks since I last wrote about the dark-footed ant-spider Myrmarachne melanotarsa, but this is one species that just keeps getting more and more interesting. To quickly recap, M.melanotarsa is a jumping spider that protects itself from predators (like other jumping spiders) by resembling an ant. Earlier this month, Ximena Nelson and Robert Jackson showed that they bolster this illusion by living in silken apartment complexes and travelling in groups, mimicking not just the bodies of ants but their social lives too.

Now Nelson and Robert are back with another side to the ant-spider’s tale – it also uses its impersonation for attack as well as defence. It also feasts on the eggs and youngsters of the very same spiders that its ant-like form protects it from. It is, essentially, a spider that looks like an ant to avoid being eaten by spiders so that it itself can eat spiders.

Its actively raids the silken nests of other spiders and snatches the eggs and hatchlings within. These youngsters would be safe from any normal ant but being a spider in ant’s clothing, M.melanotarsa has no problem with moving through silk. But they still have to get past the parents.

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Most jumping spiders avoid ants, for they are aggressive, formidably defended and will sometimes eat spiders. Nelson and Robert wanted to see how these other spiders would react to the ant-mimicking marauders. They allowed females of one species – Menemerus sp. – to build nests and lay eggs in a Petri dish. This test dish was sandwiched between two others that could be filled with various animals so that the researchers could test the reactions of the Menemerus mum without exposing her to actual danger.

When the surrounding dishes were empty, the female always stayed with her eggs. But the presence of 20 nearby ant-spiders, or indeed 20 of the ants that the spiders mimic, was enough to send Menemerus scuttling for safety. In these scenarios, around 70-80% of the usually stalwart mothers abandoned their brood. No other animals, including spiders of the same species, Pseudicius spiders (another relative) or midges (a common prey) instilled the same sense of panic, even at similar numbers.

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This puts M.melanotarsa in a rather unique position in the animal kingdom, blurring the distinction between two types of mimicry. It’s a “Batesian mimic”, an animal that deceives predators by looking like a more dangerous or unpalatable species. It’s also an “aggressive mimic”, a creature that disguises itself to lure prey into a false sense of security.

There are very few other examples of such aggressive Batesian mimics. One of them, the bluestriped fangblenny, is one I’ve blogged about before. It looks very much like a cleaner fish, an animal that eats parasites from much larger fish and gains a reprieve from snapping jaws because of the valuable service it performs. The fangblenny has a more sinister intent – it ignores parasites and uses disguise to get close enough to pinch off chunks of flesh. And so good is its resemblance that the host doesn’t fully retaliate.

Animals that are solely aggressive mimics are much more common. Even among jumping spiders, some species feed on others of their own kind by tapping webs to produce signals that mimic those of struggling prey. Some female fireflies impersonate the come-hither flashes of females from other species to lure in amorous males. Anglerfish, death adders, cantils and alligator snapping turtles attract their prey with lures that look like worms.

But the dark-footed ant-spider’s charade works in a different way. These other mimics pretend to be prey or mates to lure a target into a false sense of security. The ant-spider deceives its mark by resembling a predator, to cause unwarranted alarm. As Nelson and Robert say, it’s a “sheep in wolf’s clothing”.

Reference: Nelson, X., & Jackson, R. (2009). Aggressive use of Batesian mimicry by an ant-like jumping spider Biology Letters DOI:10.1098/rsbl.2009.0355