Can you find the monarch caterpillar on the leaf? Typically, you'll only find one caterpillar per plant, but occasionally you may find two or three on a plant. The caterpillar goes through five stages, or instars. The hungry caterpillar becomes distinctly larger in each stage. Finally it forms a chrysalis (or cocoon) from which the butterfly will emerge.
Milkweed contains certain chemicals, called glycosides. They taste bad and can be toxic to predators. They don't affect the monarch caterpillars and butterflies. Instead, they protect them. Young birds quickly learn that monarch caterpillars and butterflies are no tasty treat.