What is the relationship between the viceroy and monarch butterfly?

The relationship is called Batesian comimicry. Both butterflies are distasteful,so they mimic each others' patterns for mutual protection. A common misconception is that only the monarch is poisonous and the viceroy mimics it to gain protection even though it is non-poisonous. This is not true though, as studies in the 1980s proved that the viceroy is poisonous as well.