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
Second Instar Gallery & Molting Video
Second Instar Recently Molted
This second instar has just molted. Once they have filled with fluid (which takes about an hour), they will often turn around and eat the skin they have just shed. The head capsule from the molt can be seen to the right of the caterpillar's head.
Second Instar Trenching
This second instar is stemming the flow or "trenching" in order to feed on the milkweed leaf. The dark spot clinging to the edge of the leaf is frass, or caterpillar droppings.
Second Instar Feeding Pattern
This second instar caterpillar has a similar feeding pattern the a first instar. They will eat layers of the leaf only occasionally eating a hole through the surface of the leaf.
Second Instar ~7 Millimeters
This early second instar caterpillar is approximately 7 millimeters. The black, yellow and white striped pattern is now obvious.
Second Instar Late Stage with Dropped Head Capsule
This second instar will soon molt and become a third instar. Look for a dropped head capsule as well as little movement for a day or two prior to a molt. Note the empty egg casing in front of the caterpillar. First instars will often only eat part of the egg when they hatch.