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.
Our iOS Apps
Our iOS Apps
Monarch SOS
Monarch SOS is a butterfly, caterpillar and milkweed identification guide suitable for all ages. The app includes similar species often mistaken for monarch butterflies, chrysalises, caterpillars and eggs, as well monarch larval instar identification. Also included are milkweed species photos and profiles for users to be able to correctly identify monarchs' larval host plants. The app is used by citizen scientists who participate in the popular Xerces Society Thanksgiving count and the Western Monarch Milkweed Mapper (WMMM), as well as to record tagged monarchs, report tags, and report adult monarchs to the Southwest Monarch Study.