Vegetarianism is not exactly what springs to mind when considering spiders, which usually rely on web spinning and other finely tuned techniques to catch and eat other creatures. But one spider has now been observed to feed mostly on plants, shattering the common assumption that all spiders are strict carnivores.

Of the 40,000-plus spider species known, only a few nibble on plants—typically, as a dietary supplement of nectar or simply as an accidental ingestion of pollen. A new paper, published online today in Current Biology, details the natural history of Bagheera kiplingi, a jumping spider that has vegetarian leanings, and its interesting arrangement with a plant and the ants that protect it. 

Certain acacia plants (Vachellia collinsii) and ants (Pseudomyrmex peperi) have developed a mutually beneficial arrangement. The plant provides hollow areas for the ants to nest, along with nutritious nuggets, called Beltian bodies, for them to eat. For their part, the ants help to fend off would-be plant eaters. The crafty B. kiplingi, however, has thwarted the system—also feasting on the plant while managing to outmaneuver attacking ants.

The spider, which lives on these acacias in Latin America, was first described in 1896 by the husband-and-wife naturalist team George and Elizabeth Peckham and named for Rudyard Kipling and the black panther character, Bagheera, in the author's 1894 Jungle Book. The fierce namesake, however, may have proved to be a misnomer for this largely herbivorous arachnid, which went scurrying about its business largely unheeded for more than a century until two researchers observed its unusual feeding habits.

The spider's strange behavior was noted independently in the field by Eric Olson, a lecturer in biology at Brandeis University, in Costa Rica in 2001 and by Christopher Meehan, then a biology master's student at Villanova University, on a field exercise in Mexico in 2007. Waking early for a field research assignment, Meehan decided to observe a jumping spider he saw on an ant-covered acacia—a type of spider that had fascinated him since boyhood.

"I was waiting, expecting it to eat an ant," says Meehan, now a doctoral candidate in the Department of Ecology & Evolutionary Biology at the University of Arizona in Tucson. But rather than nabbing an ant, he says, it struck the plant and went after a Beltian body—"it grabbed the vegetable and ran away and sat on the leaf and ate [it]. I knew it was something unique, but I'd never heard of a vegetarian spider," he says, calling the whole experience "surreal." Expecting what he had seen to be an aberration, he was surprised to witness more spiders doing the same. Once Meehan and Olson heard of each other's findings, they began collaboration on this new research.

Grabbing and eating the Beltian bodies—the spider also eats nectar and ant larvae—isn't terribly different than ingesting a more traditional piece of prey, Meehan says, but it is the digestion process that has the researchers tied up in knots. Chemical analysis of the Beltian bodies shows them to be 80 percent fiber, and "they're not by any means nutrient rich in the same sense as animal tissue," Meehan says. What might allow a formerly carnivorous animal to adapt to a mostly plant diet?

Meehan notes that the peculiar food choice may have gotten started as these arachnids fed on the ant larvae, which are, themselves, fed primarily Beltian bodies. Researchers are now looking for bacteria in these ant larvae that would help the spiders digest the plant material. They also hope to establish whether such bacteria are now passed along to spider offspring or if they must be acquired individually by eating the ant larvae.

Despite their unusual meals, these spiders still exhibit some more traditional hunting behavior. "They're like miniature cats," Meehan says. "They literally stalk and hunt the plant," perhaps lending validity to B. kiplingi's feline namesake. They also use their quick movements to evade would-be ant attackers.

The plant-based diet does, however, seem to have changed this spider's interactions with its neighbors. Individuals can live in close proximity to one another like other species of social spiders, but in other species the spiders are assumed to be cooperating in the hunt (a behavior that, researchers hypothesize, can lead to more sophisticated social organizations). But in the case of B. kiplingi, in which food is simply harvested from the plant, no cooperation is needed, raising the question of why they live in such close quarters. Meehan proposes that the social behavior might be "to reduce local resource competition and reduce aggression," which would be important in instances where some plants are covered with thousands of these spiders.

As for the plants that the spiders feed on, most don't appear to be under severe stress by the arachnid attacks. Some acacias in Mexico, however, that have heavier loads of spiders might be suffering from decreased ant protection, Meehan notes. With all of those spiders harvesting the Beltian bodies for themselves, there are fewer for the ant larvae and thus fewer ants. This sort of instance raises questions about stabilization of the spider–plant–ant system. "Instead of eating the resource once-removed," like the ants, Meehan says, the spiders "go straight to the source." The eating pattern allows more spiders to subsist on a plant than otherwise could get by eating the ant larvae directly.

Regardless of how these spiders came to be primarily plant-eaters, their continued study will likely illuminate a rare species—one that Meehan calls "more extraordinary than a flying pig"—and the system the produced them. "Co-evolution can have cascading effects," Meehan says. "It can serve as evolutionary stepping-stones for third parties that intercept them." And for this oddly adapted spider, it appears to have been a boon.