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Caterpillars Molting

6-25-14 caterpillar molting 044A caterpillar is the larval stage of a moth or butterfly. It is the only stage that has chewing mouthparts, and therefore a caterpillar spends most of its waking hours eating. This consumption of food results in massive growth, making its skin/exoskeleton very tight. When this happens, a hormone called ecdysone is produced, prompting the caterpillar to molt, losing its old exoskeleton (to left of caterpillar in photo) under which is a new and larger exoskeleton. After the molt, while the new exoskeleton is still soft, the caterpillar swallows a lot of air, which expands its body. Then, when the exoskeleton hardens, it lets the air out and has room for growth. Caterpillars molt four or five times as they grow. Each different caterpillar stage is called an instar. (Photo: Forest Tent Caterpillar)

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12 responses

  1. Another fascinating entry, Mary! Do you know what the caterpillar in the photo will become? I’ve had several in my yard in Rhode Island, and I’ve been meaning to track that info down. Thanks!

    June 25, 2014 at 12:11 pm

    • Hi,
      Thank you for your kind words. Yes, they turn into small brown moths, pretty nondescript — Malacosoma disstria.

      June 25, 2014 at 12:37 pm

      • A bit ironic that such interesting caterpillars should turn so… blah. Of course, even drab creatures have their place. Thanks for the reply, and keep doing what you do!

        – Hugh

        p.s. I have a blog called “Science and Nature for a Pie” (old Trivial Pursuit reference) that focuses on Rhode Island flora and fauna: .

        June 25, 2014 at 1:17 pm

  2. Beautiful. Truly lovely. I want to pet him. Colors are amazing.

    June 25, 2014 at 1:16 pm

  3. Totally fascinating post Mary! I was intrigued by the pattern on its back. It looks like a chain of little spiders. Do you suppose that fends off predators? Does anything eat these?

    June 26, 2014 at 12:43 am

    • Hi Eliza,
      Yes! The hairs probably do fend off predators, but not all! Cuckoos (both Black-billed and Yellow-billed) and Baltimore Orioles eat Forest Tent Caterpillars, as well as other hairy larvae. The Black-billed Cuckoo sometimes will squeeze out the innards and just eat them, but usually downs the whole caterpillar. The hairs pierce the inner lining of the cuckoo’s stomach and remain there. When the stomach is opened, it appears to be lined with a coating of fur…periodically the cuckoo sloughs off its stomach lining and regurgitates it as a pellet. Isn’t nature amazing? According to Birds of North America Online, “1 cuckoo ate 36 forest tent caterpillars in 5 min, continued to consume 29 more in next several minutes, took brief rest, then ate 14 additional larvae.” In Manitoba, the nesting density of Baltimore Orioles nearly doubled in the second year of a 2-year outbreak of Forest Tent Caterpillars!

      June 26, 2014 at 12:43 pm

      • That is amazing. Talk about gorging! It’s good to know something eats them as their cycles can be overwhelming some years.

        June 26, 2014 at 2:12 pm

      • jamie honey

        Hi Mary,

        If the Mullein caterpillar pupates in late spring when does the Mullein moth hatch??


        June 13, 2017 at 10:49 am

      • They pupate underground and it can take up to five years to metamorphose into an adult moth.

        June 13, 2017 at 1:19 pm

  4. Michele Girard

    Very fascinating! I didn’t realize that caterpillars molted their exoskeletons, but it makes sense. And breathing in air to expand the exoskeleton–so simple and yet so effective. Thank you.

    June 26, 2014 at 10:37 am

  5. Jenny Sawyer

    Good morning to you Mary Holland. What a huge fan club you have. This is only my third year in New Hampshire. A friend gave me your book and connected me to your emails. Lucky me. I forward pertinent blogs to grandchildren etc all over the country. You are greatly appreciated.

    I would like to send you a check but need a snail address. I think I did it last year but can’t remember how. Thank you. Jenny Sawyer


    June 26, 2014 at 11:16 am

  6. viola

    excellent explanation! How interesting!

    June 26, 2014 at 4:52 pm

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