From The New York Times. 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. Learn more here.