Today I will show you my caterpillars during the 3rd and 4th Instar stage. The time of an instar is quite short between moltings as they grow very quickly. This is usually between 2 - 3 days. As you can see in the first two photographs the babies have grown considerably. I will now separate them and put only two into each cage. I do not want them to become territorial over the leaves.
Below are a few details given by the Monarch Lab at the University of Minnesota SOURCE
3rd Instar
Specifications
Body Length 10 to 14 mm
Body Width 2 to 3.5 mm
Front Tentacles 1.7 mm
Back Tentacles 0.9 mm
Head Capsule 1.5 mm in diameter
Appearance
The black and yellow bands on the abdomen of a third instar larva are darker and more distinct than those of the second instar, but the bands on the thorax are still indistinct. The triangular patches behind the head are gone and have become thin lines that extend below the spiracle. The yellow triangle on the head is larger and the yellow stripes are more visible. The first set of thoracic legs are smaller than the other two and is closer to the head. Time in this larval stage is usually 1-3 days, temperature-dependent.
Third instar larvae usually feed using a distinct cutting motion on leaf edges. Unlike first and second instar larvae, third (and later) instars respond to disturbance by dropping off the leaf and curling into a tight ball. Monarch biologist Fred Urquhart called this behavior "playing possum."
Below are a few details given by the Monarch Lab at the University of Minnesota
SOURCE
4th Instar
Specifications
Body Length 13 to 25 mm
Body Width 2.5 to 5 mm
Front Tentacles 5 mm
Back Tentacles 2 mm
Head Capsule 2.2 mm in diameter
Appearance
Fourth instar larvae have a distinct banding pattern on the thorax which is not present in third instars. The first pair of legs is even closer to the head, and there are white spots on the prolegs that were less conspicuous in the third instar.
Male and female larvae can't be distinguished by the naked eye until the pupal stage. However, male and female respective reproductive organs are visible in dissected, third, fourth, and fifth instars. Time in this larval stage is usually 1-3 days, temperature-dependent.
Below are a few details given by the Monarch Lab at the University of Minnesota SOURCE
5th Instar
Body Length 25 to 45 mm
Body Width 5 to 8 mm
Front Tentacles 11 mm
Back Tentacles 4 mm
Head Capsule 3.5 mm in diameter
Appearance
The body pattern and colors of fifth instar larvae are even more vivid than they were in the fourth instar, and the black bands look wider and almost velvety. The front legs look much smaller than the other two pairs and are even closer to the head. There are distinct white dots on the prolegs, and the body looks quite plump, especially just prior to pupating.
Fifth instar monarch larvae often chew a shallow notch in the petiole of the leaf they are eating, which causes the leaf to fall into a vertical position. They move much farther and faster than other instars and are often found far from milkweed plants as they seek a site for pupating. Time in this larval stage is usually 3-5 days, temperature-dependent.
Cleaning Cages
It amazes me that when I take them out of their cages to clean them, the caterpillars continue to munch away on the leaves. It is extremely important to clean the bottom of the cages 1 - 2 times per day as needed. Clean cages = happy and healthy caterpillars. This big guy is about ready to move into the next stage. Stay tuned for our upcoming post.
In case you missed my first post on rearing Monarch caterpillars you can see it here SOURCE
Thank you 
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