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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On July 21, 2022 the migratory monarch butterfly was officially listed as "Endangered" on the Red List of Threatened Species by the International Union for Conservation of Nature (IUCN). See full details here. Although this is an important step for monarch conservation and recognition, this listing has nothing to do with listing under the Endangered Species Act (ESA), which offers federal protection. A final ruling for the listing of monarch butterflies by the US Fish and Wildlife Service is scheduled for 2024.