October 12, 2009

‘Vegetarian’ Spider Discovered In Central America

A Central American species of spider, known as Bagheera kiplingi (B. kiplingi), represents the first known instance to science of an arachnid that dines primarily on plants.

About 40,000 unique spiders have been described to date, with most considered strict predators that either trap their prey in intricate webs or hunt them down directly.

The newly-discovered vegetarian spider primarily consumes something called Beltian bodies, specialized leaf-tip structures produced by acacia shrubs.

Ants living in the hollow spines of the acacia typically feed on Beltian bodies and act as the plants' "bodyguards." The ant-acacia relationship is one of the most famous and well-studied examples of natural coevolution.

"This is really the first spider known to specifically 'hunt' plants; it is also the first known to go after plants as a primary food source," said Christopher Meehan of Villanova University, who noticed the vegetarian spiders during a field course in Mexico.

Eric Olson of Brandeis University, a study coauthor, independently observed the spiders in Costa Rica, Meehan said.

B. kiplingi is the only spider known to consume solid, vegetative plant food at all, Meehan said.

And while the spiders do occasionally prey on small invertebrates, field observations and chemical analyses of the spiders' tissue confirm that these arachnids consume a primarily vegetarian diet.

"I've done the math several times, and even the most conservative estimates point to near-total vegetarianism," Meehan said.

Nearly all of the prey that the spiders do eat are acacia-defending ant larvae.

Until now, "herbivory" in spiders simply meant that a spider occasionally consumed a bit of either nectar or pollen, Meehan explained.  Pollen-feeding has only been well documented in a single orb-weaving species"”and even then only in juveniles that swallow pollen and anything else that gets entangled in their webs in the process of recycling their silk.

Nectar feeding is likely common among spiders that actively hunt instead of building webs, but this is still a distant second to carnivory in terms of their total diet.

"Spiders aren't really thought to be capable of eating solid food at all," Meehan said.

Rather, spiders digest their prey externally, and anything bigger than about one micrometer gets filtered out of the vital juices in the spider's pharynx, he explained.

However, Beltian bodies are 80 percent structural fiber, and rather large by spider standards.

"The acacia spiders consume these nutrient-enriched"”but not particularly nutrient-rich"”'vegetables' completely, often in less than five minutes."

The spiders circumvent the acacia-guarding ants by using something Meehan called their "sheer wit." 

"Jumping spiders in general possess incredibly advanced sensory-cognitive skills and eight-legged agility, and Bagheera is no exception," he said.

"Individuals employ diverse, situation-specific strategies to evade ants, and the ants simply cannot catch them."

The spiders also appear to construct their webs in less attractive plant "real estate", and to actively defend their nests against ant invaders.

Finally, they might actually imitate the ants. Indeed, young spiders in particular resemble the ants, which could possibly explain why they have escaped detection by scientists for so long despite rigorous study of the ants and acacias.

Meehan suspects the B. kiplingi spiders may also wear the ants' chemical scent, a theory he is now studying in further detail.

The discovery was published online on October 12th in Current Biology.


Image 1: Adult female Bagheera kiplingi eats Beltian body harvested from ant-acacia. Credit: R. L. Curry

Image 2: Adult female Bagheera kiplingi defends her nest against acacia-ant worker. Credit: R. L. Curry


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