Monarch caterpillars molt (shed their skin) five times throughout their larval stage. The "skin" that is left behind is actually the caterpillar's exoskeleton. An insect's skeleton is on the outside, as opposed to on the instead like mammals, fish, reptiles, amphibians and birds. Endoskeletons provide support inside the body in the form of bones and cartilage, whereas As caterpillars grow, their exoskeleton gets too tight, so they need to shed it in order to continue to grow. Each molt results in a new "instar" stage. Therefore, when the egg hatches, that tiny 2 mm caterpillar is in its first instar stage.
It will molt three more times and grow exponentially over a couple of weeks. Each stage lasts 3-5 days. The fifth and final molt is when the fifth instar caterpillar becomes a chrysalis.
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Life Cycle of Monarch Butterfly
Above is the life cycle of a monarch butterfly. Once the female lays an egg on a milkweed leaf, the journey begins. The egg stage lasts between 3-6 days. Once the caterpillar hatches from the egg, it is considered in its first instar, which is a stage of development for caterpillars. Once the caterpillar reaches the fifth and final instar, it sheds its skin and enters the pupal stage, and becomes a chrysalis (also called a pupa). This stage lasts between 10-14 days when an adult butterfly ecloses from the chrysalis.
At the center of the life cycle is milkweed. Without it, monarchs cannot survive.