A female Bagheera kiplingi spider eats a Beltian body harvested from an Acacia plant. (R. L. Curry)

Scientists have found a spider in Central America that eats mostly plants, the only known vegetarian in an estimated 40,000 spider species.

Bagheera kiplingi is a jumping spider that lives in Central America and southern Mexico. It feeds mostly on the nutrient-rich nectar and specialized leaf structures of acacia trees.

Acacia trees and certain ants have co-evolved into a mutually beneficial relationship. The trees gives the ants food — nectar and specialized leaf tips called Beltian bodies — and shelter in the plant's hollow spines, while the ants defend the plant against other animals.

B. kiplingi exploit this relationship by eating the acacia nectar and Beltian bodies without helping to defend the plant.

The vegetarian spider uses the sharp eyes and agility it shares with other jumping spiders not to hunt the ants that patrol the acacia bush, but to avoid them.

Eric Olson of Brandeis University first discovered B. kiplingi in 2001, and co-authored the full description appearing this week in the journal Current Biology.

"What surprised us most about discovering this spider's extraordinary ecology was to find it on the ant-acacias," said co-author Robert Curry of Villanova University, in a statement. "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."

Curry's student, Christopher Meehan, independently observed the same herbivorous behaviour in spiders in Quintana Roo, south of Cancun, Mexico.

The researchers took high-resolution video of the spiders to monitor what they were eating. In the Mexican population, the Beltian bodies of acacia trees made up 90 per cent of the 140 food items the spider ate.

The spiders also supplement their vegetarian diet with some animal products, such as ant larvae. Meat eating was more frequent in the Costa Rican spiders than in the Mexican ones.

The scientists backed up their direct observations of the spiders' eating habits with laboratory analysis of the plants, ants, Bagheera spiders and other local spiders. The molecular analysis, conducted with the help of scientists at Queen's University in Kingston, Ont., found that the diet of Bagheera spiders was more similar to the ants than to other spiders.

The researchers also found another characteristic that makes B. kiplingi unique among spiders: the males help care for eggs and young.