Underside of a leaf (family Melastomataceae), a shrub growing in the tropical rainforest in southern Venezuela. The petiole (leaf stem) is enlarged and hollow and occupied by a nest of a tiny species of ant. The ants enter the hollow stem through an opening on the underside of the leaf at the base of the petiole, seen here between the veins of the leaf. The ants receive protection and a source of food from extra-floral nectaries on the stem. In return, the ants' presence deters other insects from grazing on the leaves of the plant. The association is of mutual benefit to the plant and ant.