The eggs, layed on milkweed leaves, hatch into caterpillars. The baby caterpillars eat the milkweed, and grow very quickly. The milkweed contains a poison that the monarchs use as a defense. While the poison doesn't hurt the monarchs, it makes them taste bad to birds and other predators. Predators soon learn to avoid the bright colors of the monarch caterpillars and butterflies.
Monarchs are caterpillars for a couple of weeks.
They spend that time eating milkweed leaves. After eating their fill of milkweed, the caterpillar forms a shiny green and gold speckled crysalis. This is the pupal stage. After about 14 days, the adult monarch butterfly emerges from its crysalis. The Aztec believed the adult Monarch butterflies to be the incarnation of their fallen warriors, wearing the colors of battle.