Many examples of mutualism are known, and they provide some of the most charming details in natural history.
The photograph opposite shows an ant feeding on the caterpillar of the lycaenid butterfly. The ant is not eating the caterpillar; it is drinking 'honeydew' from a special organ, the sole purpose of which seems to be to provide food for ants.
Why do the caterpillars feed the ants?
Pierce and Mead suggest that the caterpillars feed ants in return for protection from parasites.
The caterpillars are parasitized by Braconid wasps and Tachinid flies. Alone, they are almost defenceless against these lethal parasites; but a tending ant will fight off a parasite from its caterpillar. The ants and caterpillars seem to be closely adapted to each other; the ants gain food, and the caterpillars gain protection. They form a kind of interspecific coadaptation.