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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Third Instar Gallery & Molting Video
Third Instar Caterpillar
Third instar caterpillars begin resembling what we picture when we think of a monarch caterpillar. The tentacles are growing, the stripes are becoming more bold and the face capsule is more defined and may have the yellow coloration in the triangle not present in first and second instars.
Third Instar Eating Pattern
Third instars begin eating along the edges of the leaf, but may also eat in the center like first and second instar caterpillars
Third Instar Head Capsule - Yellow
Third instar caterpillars sometimes have a yellow triangle on their head capsule, sometimes it is faint and difficult to see.
Third Instar Head Capsule - White
This third instar caterpillar has a faint yellow triangle on the head capsule. Some are brighter and easier to see than this lighter triangle
Third Instar Dropped Head Capsule
This third instar is preparing to molt. Note the dropped head capsule. Prior to a molt, caterpillars become lethargic for a day or two.
(1 of 7) Third Instar Recently Molted
This third instar has recently molted. Note the pale head capsule. Once the caterpillar fills with fluid it will look like a "typical" monarch caterpillar. Go through the next several photos to see how the head and legs fill with fluid. This takes about an hour to complete.
(2 of 7) Third Instar Filling with Fluid after Molting
Note the slightly darker head capsule. You can now see the pattern beginning to form. Go to the next phots to see a darker head capsule and darker prolegs.
(3 of 7) Third Instar Filling with Fluid after Molting
(4 of 7) Third Instar Filling with Fluid after Molting
(5 of 7) Third Instar Filling with Fluid after Molting
(6 of 7) Third Instar Filling with Fluid after Molting
(7 of 7) Third Instar Filled with Fluid After Molt