In Latin America, there lives a unique spider called Bagheera kiplingi. It's a jumping spider and it shares the group's large, acute eyes and prodigious leaping ability. But it also has a trait that singles it out among all 40,000 species of spider - it's mostly vegetarian.

Virtually all spiders are predators. They may hunt using different methods but they all end up sucking the liquidised innards of their prey. If they consume plants, they do so rarely, even accidentally. Some take the odd sip of nectar to supplement their diet of flesh. Others accidentally swallow pollen while recycling the silk of their webs.

But B.kiplingi is an exception. Christopher Meehan from Villanova University has found that this spider exploits a partnership forged between ants and acacia trees. The trees employ ants as bodyguards and it pays them with shelter inside hollow thorns, and nutritious nodules called "Beltian bodies" that grow from its leaves. B.kiplingi has learned to steal these delicacies from the ants, and in doing so, it has become the world's only (mostly) vegetarian spider.

Meehan spent seven years observing the spider and filming its foraging trips. He showed that the spiders are almost always found on acacia trees that are occupied by ants, for the trees only grow the tasty Beltian bodies when ants are around. In Mexico, Beltian bodies make up 91% of the spider's diet, while in Costa Rica, they make up 60% of it. More rarely, they will also drink nectar and evern more rarely, they will have a meat treat by taking ant larvae, flies and even others of their own kind.

Meehan confirmed his results by analysing the chemical make-up of the spiders' bodies. He looked at the ratio of two types of nitrogen: N-15 and N-14. Plant-eaters tend to have relatively less N-15 than meat-eaters do, and sure enough, B.kiplingi's body had 5% less of this isotope than other species of jumping spiders. Meehan also considered the ratio of two carbon isotopes, C-13 and C-12. Meehan found that the vegetarian spider and the Beltian bodies had virtually identical ratios, as is usually the case between an animal and its food.

Feeding on Beltian bodies is worthwhile but far from straightforward. First, there's the problem of the bodyguarding ants. B.kiplingi's strategy is stealth and evasion. It builds its nests at the tips of the oldest leaves, where ants rarely patrol. They will actively avoid ant guards if they see them approaching. If cornered, they will use their powerful legs to leap away. Sometimes, they even drop to safety using a line of silk, hanging in midair until the danger passes. Meehan documented several different strategies, all evidence of the impressive mental skills that jumping spiders are known for.

Even if it avoids the sentries, B.kiplingi has another problem. Beltian bodies are extremely high in fibre and spiders really shouldn't be able to handle that. Spiders can't chew their food; they rely on digesting their prey outside their own bodies using venom and digestive juices, and 'drinking' the liquefied remains. Plant fibre is a much tougher mouthful and we still don't know how B.kiplingi copes with it.

Even so, it's clear that the rewards are worth it. Beltian bodies are a ready-made source of food that's available all year round. By exploiting this feast that's produced for others, B.kiplingi has become very successful. Today, it's found throughout Latin American, wherever ants form partnerships with acacias.

Reference: Current Biology in press

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