Each of the world's 40,000 spider species survives by hunting and killing — except, that is, for Bagheera kiplingi, the world's first vegetarian arachnid.

Found in Central America, the order-defying jumping spider eats nutrient-rich structures called Beltian bodies, which are found on the tips of Acacia trees. Trees produce the bodies to feed ants that defend them, which is a textbook example of what's called co-evolutionary mutalism, and one that B. kiplingi has evolved to exploit.

In a paper published Monday in Current Biology, researchers describe the spider's ant-evading habits and provide a molecular analysis of its body composition, proving that B. kiplingi is indeed what it eats: plants, with a few larval ants on the side. (After all, 400 million years of evolutionary habits die hard.)

A few other spiders have been documented consuming nectar, but only as a snack. No other spider is so predominantly vegetarian. And that's not all: It looks like B. kiplingi males help care for eggs and young — something entirely unprecedented in the spider world.

The researchers are now studying whether there's a link between B. kiplingi's predilection for plants and parental concern. Maybe going veggie softened its heart.

Image: Current Biology

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Citation: "Herbivory in a spider through exploitation of an ant-plant mutualism." By Christopher J. Meehan, Eric J. Olson, Matthew W. Reudink, T. Kurt Kyser, and Robert L. Curry. Current Biology, Vol. 19, Issue 19, October 13, 2009.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.