World's first known vegetarian spider

Henry Fountain described it (above) in his "Observatory" feature in the October 12, 2009 New York Times Science section; his piece follows.

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Jumping Spider Prefers Green Leaves to Meat

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.

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.

Some spiders occasionally eat pollen or nectar, but only as a supplement to their typical diet of insects. Through observations and isotopic analyses, the researchers found that B. kiplingi eats more Beltian bodies than ants — especially in Mexico, where about 90 percent of the diet consisted of plant tissue.

The spiders actively hunt their vegetable prey. “Every foraging event poses a novel obstacle,” Mr. Meehan said. “The spider stares at a food target, and sometimes it backs away and stares again. Then it will use a variety of ways to acrobatically move through the ants.”

Mr. Meehan said the behavior was an example of one organism exploiting a mutualism between two others. “It’s pretty well established that the interdependence between ants and acacias allowed for the emergence of the Beltian body,” he said. “And now the only known case of spider herbivory depends on this interdependence.”

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The abstract of the Current Biology report follows.

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Herbivory in a spider through exploitation of an ant-plant mutualism

Spiders are thought to be strict predators. We describe a novel exception: Bagheera kiplingi, a Neotropical jumping spider (Salticidae) that exploits a well-studied ant–plant mutualism, is predominantly herbivorous. From behavioral field observations and stable-isotope analyses, we show that the main diet of this host-specific spider comprises specialized leaf tips (Beltian food bodies) from Vachellia spp. ant-acacias (formerly Acacia spp.), structures traded for protection in the plant's coevolved mutualism with Pseudomyrmex spp. ants that inhabit its hollow thorns. This is the first report of a spider that feeds primarily and deliberately on plants.

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A very detailed and informative story on the spider appeared October 12, 2009 on ScienceDaily.com.