Co-evolution 

Mutualism - A symbiotic relationship in which both participating species benefit. 

Aphids are small insects that use their piercing mouthparts to suck fluids from the sugar conducting vessels of plants. They extract a certain amount of the sucrose and other nutrients from this fluid. However, much of the fluid -- so - called honeydew - runs out in an altered form through their anus. Certain ants have taken advantage of this fact and "milk" the aphids for the honeydew, which they use as food. The ants, in turn, protect the aphids against insect predators. 

The female yucca moth is the sole pollinator of the yucca, and the yucca is the only caterpillar host plant of the yucca moth. In fact, the yucca and yucca moth share a symbiotic relationship that is so specialized, each yucca species is pollinated by only one type of yucca moth.