A nearly totally vegetarian spider has been discovered in a remarkable ecological niche in Central America.

While most spiders dine on tasty insects and the like, the new species lives on acacia shrubs. These provide shelter and food for ants that in turn protect them from predators, in theory at least.

The new spider – named Bagheera kiplingi after the panther in the Jungle Book – lives on the plants, eating the nectar produced for the ants and the leaf tips that produce it.

It is not totally veggie, as it occasionally snacks on ant larvae and flies that try to dine on the nectar. However it is, say its discoverers, “the first report of a spider that feeds primarily and deliberately on plants”.

The spider was discovered independently by two research teams; Eric Olson, of Brandeis University in Massachusetts, found it in Costa Rica in 2001 and Christopher Meehan, of Villanova University, Pennsylvania, found it in Mexico in 2007.

“What surprised us most about discovering this spider’s extraordinary ecology was to find it on the ant-acacias,” says Robert Curry, also of Villanova University. (press release).

“This well-known mutualism has been studied by tropical ecologists for nearly 50 years, yet the spider’s role was not noticed until Olson’s discovery in 2001. We were lucky to find in Mexico an area where the spider is both exceptionally abundant and even more herbivorous than in Costa Rica.”

This is, as the comment piece from Duncan Jackson, of the University of Ulster in the UK, running alongside the paper in Current Biology notes remarkable for a number of reasons:

This is wholly remarkable, because all other known spiders are carnivorous predators, and unlike most other nutritional mutualism exploiters, jumping spiders are specialized hunters rather than generalized foragers.

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It is truly remarkable that a spider should thrive on a vegetarian diet, because all spiders are constrained by their narrow gut, specialized mandibles and a solids filtration system to consume a liquid diet.

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It seems that the transition from hunter to gatherer in this uniquely vegetarian spider has facilitated a suite of additional behavioural changes which might suggest an alternative route to sociality. One wonders how many more surprises await us in this remarkable system.

Image: adult female Bagheera kiplingi eats Beltian body harvested from ant-acacia / R. L. Curry