The Monarch (Danaus plexippus) is a common poisonous butterfly with a wingspan of 8.6 - 12.4 cm. It is bright orange with black wing veins and outer margins. The wings have white spots on outer margins, and there are three orange patches near the top of the forewings. The hindwings are rounded, and are lighter in color than the forewings. The body is black with white spots.

Male monarchs have a dark spot on the hindwing. Females don't. Males also have much thicker wing veins and small claspers at the end of the abdomen.

The monarch gets its poison (cardenolide glycosides) when it is a caterpillar, from eating the poisonous milkweed plant (genus Asclepias) while in its larval stage. The adult lays its eggs on the milkweed, and the larvae that hatch eat it.

Animals that eat a Monarch get very sick and vomit (but generally do not die). These animals remember that this brightly-colored butterfly made them very sick and will avoid all Monarchs in the future. Of course, as is nature's way, the Monarch butterfly has a mimic. The North American Viceroy butterfly (Limenitis archippus) looks almost identical to the Monarch, but is not poisonous. Its similarity to the Monarch offers it protection from predators.