Invasion | Within 3 to 4 days, the Monarch caterpillars will hatch. They will only eat the leaves (and sometimes flowers) of Milkweed plants, and cannot survive on anything else. | The sap of the Milkweed plant is poisonous to vertebrate animals. Monarch caterpillars build up this toxin in their bodies, which also remains when they become butterflies. | The bright colors of the caterpillars and butterflies warn predators to stay away. We don't taste good! | When the caterpillar is full grown it usually leaves the milkweed plant. It crawls (sometimes 30 or 40 feet away from the milkweed) until it finds a safe place to pupate. | After about an hour, the butterfly will extract any remaining fluid from its now hardening wings, and within a few hours will be ready to take flight for the first time in its new form. | The newly emerged Monarch butterfly will exercise and test out its new body, and may spend the remainder of the day and night resting until it is ready to take flight. | Does the butterfly remember that it once lived a life as a caterpillar?