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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First Instar Gallery & Molting Video
First Instar Early Stage
This first instar is only an hour or two old. Notice there are no markings on the head, the tentacles are only nubs both in the front and back, and there are not black, yellow or white stripes. There are tiny hairs called setae on the suface, which will get more dense as the caterpillar grows and molts.
First Instar Just Hatched Eating Egg
This newly hatched first instar caterpillar (larva) is having its first meal: the shell it just exited. (the shell is also called the chorion). These tiny caterpillars are very difficult to spot since they are only about 2 millimeters long when they hatch.
First Instar Just Hatched ~2 Millimeters
The millimeter ruler above this newly hatched first instar caterpillar measures just 2 millimeters, or the size of a grain of rice.
First Instar "Trenching" or Stemming the Flow of Milkweed Sap
First instar caterpillars must stem the flow of milkweed sap, or latex, or they can be overcome by it. They do this by "trenching" the leaf, or cutting off the flow of the sap by severing the veins and eating the leaf in a circular pattern. They then eat the leaf in the center where the latex is no longer flowing.
First Instar Feeding Pattern
The arc-shaped feeding pattern created by this first instar is a great way to identify a first instar's presence on a milkweed leaf.
First Instar Mid Stage
This instar is in the middle of its first instar. There are noticable bands around the body, whereas the earlier instar had no obvious bands. There is very little color at this stage.
First Instar Late Stage with Dropped Head Capsule
This instar is getting ready to molt into its second instar. Note the darker bands and light yellow beginning to show on the body. The head capsule has dropped, indicating the molt will happen fairly soon.
Video: First Instar Molting
This video shows the very first molt of a monarch caterpillar. Major changes happen between molts, but after the first molt, monarch caterpillars begin to look more like what we expect, exhibiting the black, white and yellow striped pattern and the striped head capsule.