During field studies in Mexico two years ago, Christopher J. Meehan, then a student at Villanova University, spent time watching a jumping spider on an acacia plant. The plant was swarming with ants, as ants and acacias constitute a well-known example of mutualism  the insects provide protection for the plant and in return the acacia produces nutritious leaf tips for the ants to eat.

Mr. Meehan figured that the spider was hunting dinner. “I was waiting for it to do something like prey on an ant,” he said.

Instead, to his surprise, the spider, Bagheera kiplingi, darted around the ants and plucked off a one of the leaf tips, called a Beltian body.

Image Credit... Chris Gash

Mr. Meehan, who is now at the University of Arizona, had discovered the first example of a largely vegetarian spider. He and Eric J. Olson of Brandeis University, who observed the same behavior among B. kiplingi in Costa Rica, and colleagues have published a paper on the finding in Current Biology.