Monarch butterfly caterpillars are famous for eating milkweed plants. Indeed, monarch and queen butterflies are called "milkweed butterflies" because of the food source for the caterpillars and a nectar source for the adults.
Milkweed plants are chosen because they have bad tasting toxins the caterpillars store to ward off predators. Monarch butterflies breed in the spring and summer. The females lay eggs singly on the milkweed plant. The caterpillar that emerges is black, white and yellow and has tentacles on its head. It feeds until it's about 2 inches long, then turns into a pale green chrysalis that's spotted near the top with gold. The butterfly emerges after about a fortnight.