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Monarchs Splitting Exoskeleton For The Fifth And Final Time

In the Northeast there appears to be an amazingly large number of Monarch larvae this year, and most of these larvae will complete their metamorphosis by transforming into a beautiful green chrysalis. Once mature, the larva, or caterpillar, wanders about and finds a suitable spot (usually protected and stable) to spend the next two weeks hanging precariously in the wind.  It then spins a silk mat in this location, and puts a silk “button” in the middle of the mat.  It clasps the button with its last set of prolegs (it has three pairs of true legs, and five pairs of so-called prolegs) and spends about 18 hours hanging in a “J,” with its head down, preparing to split its exoskeleton for the last time and reveal the chrysalis within it.

Ba Rea, a Monarch specialist (and publisher of my children’s book, Milkweed Visitors), informs her “Monarchchaser’s Blog” ( readers that even though the visible changes between the larval and pupal (chrysalis) stages of a Monarch are sudden, inside the caterpillar these changes are taking place gradually and long before we can see them.  “The parts that will transform the caterpillar into a butterfly are present from the time that the egg hatches.  Inside the caterpillar are “imaginal disks.”  As wonderfully fanciful as the word imaginal sounds, it is actually referring to the adult stage of the monarch which is called the imago.  These disks are the cells that will become the butterfly’s wings, legs, proboscis and antennae, among other things.  By the time the caterpillar is half an inch long its butterfly wings are already developing inside it.

After eight to fifteen days, the adult Monarch emerges from its chrysalis and heads towards Mexico (butterflies that emerge after the middle of August migrate). It is the great grandchildren and great great grandchildren of these migrating monarchs that will return next summer.  (Photo: Monarch hanging in a “J” from Jewelweed, also known as Touch-Me-Not — not the sturdiest of plants to hang from!)

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11 responses

  1. Bill On The Hill

    …Wonderful story Mary on a most fascinating insect we are privileged to witness most years.
    It has been a good year for monarchs up here in the highlands of n. central Vt. this summer as I am not lacking in the milkweed dept!
    Both my granddaughters know as much if not more than I on the subject of monarchs going through the chrysalis stage. ” Imaginal disks, ” 1st I’ve heard of this…
    Bill… :~)

    August 23, 2019 at 8:02 am

  2. Meredeth Allen

    I currently have 20 beautiful chrysalises (?) hanging in my butterfly cage. However three other caterpillars failed to pass from the J stage. Parts of the shiny green chrysalis formed and then stopped. The caterpillar began to turn dark and some sort of liquid came from it

    August 23, 2019 at 8:11 am

  3. Libby Hillhouse

    The mystery is revealed…I had not known the exoskeleton covers the chrysalis…but I now wonder where that exoskeleton goes…..does it become part of the attachment? Does it drop away? I’ve seen other shed exoskeletons but not one near the chrysalis…and witnessed one transformation to chrysalis, but….where does the last exoskeleton go?

    August 23, 2019 at 8:14 am

    • Hi Libby! After it splits apart and the chrysalis emerges, the last exoskeleton is crinkled up at the top of the chrysalis, and when the chrysalis is first out, it wiggles and wiggles, in part to get the skin to drop off, and not interfere with the chrysalis. Sometimes it remains there (at the top of the chrysalis) for a while, but usually the chrysalis manages to move enough to get its skin/exoskeleton to drop to the ground.

      If you go to and click on “About Monarchs” at the top, and scroll down, Ba has written an excellent description and has photos of the process! Hope you’re having a great summer!

      August 23, 2019 at 12:30 pm

  4. Craig Barry

    Dear B,

    I am beginning to appreciate the incredible story of the Monarch. My 18 🐛 are a microcosm of what is going on in the Northeast. Yeah! 👍 Amy

    Sent from my iPad


    August 23, 2019 at 9:29 am

  5. Tom Farmer

    Big Monarch year apparently! —tomO

    August 23, 2019 at 9:48 am

  6. Alice

    No idea what happened to my comment this am. Here goes: it’s quite an amazing time following from ‘egg to newborn & the quick growth of the caterpillars & shedding their skin & eating it, to Js & chrysalises & eclosing & it’s a boy! or it’s a girl’! and releasing the Monarchs.’ My first year enjoying this…21st Butterfly was released this morning. About 25 to go. So glad I have the large habitat & extra happy to have a great deal of Milkweed, to help this project happen. It’s also fun to share info & have neighbor’s children help release them.

    August 23, 2019 at 2:29 pm

  7. Kathy

    I see the butterfly comes out in 8-14 days I have mine attached to a screen I’m at my summer home how strong is the attachment ? Can I take my co Rainer on a 3 hr drive ? Will it survive ?

    August 23, 2019 at 7:23 pm

    • The limited experience i’ve had with transporting chrysalises has not ended well, i’m afraid. I’m not sure whether vibrations or the temperature of the car was responsible but butterflies never emerged.

      August 23, 2019 at 10:20 pm

  8. Claire Holzner

    Fascinating post about Monarchs, thank you. It’s a good year for them here in central PA — I’ve seen many more Monarch butterflies and caterpillars this year than last. I now have 8 chrysalises and 3 caterpillars in my butterfly tent. My question is — I’ve never heard their skin referred to as an “exoskeleton.” I thought grasshoppers and crabs have exoskeletons — hard, spiky things – but soft caterpillars just have skin? When the chrysalis forms the old skin is small and wrinkled up and usually falls to the ground — but Mary, you’re saying that’s not a skin, it’s an exoskeleton? I’m curious — thanks.

    August 26, 2019 at 8:18 am

    • Hi Claire,
      It is an exoskeleton, but is also referred to as a “skin” so either term is correct, just that exoskeleton is the technical term for it. But yes, all arthropods (insects, spiders, crustaceans) have exoskeletons that do not grow with the rest of the body, and have to be molted and replaced with a bigger exoskeleton as the organism grows. To read more than you probably want to know about exoskeletons, you can go to: .

      August 28, 2019 at 12:51 pm

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