Caterpillars Eating & Molting
The larval stage of a butterfly or moth is spent doing little but eating. Only as a caterpillar will these insects have chewing mouthparts, and they waste no time in using them. As they eat, caterpillars increase in size and their skin (exoskeleton) becomes tighter and tighter, as it doesn’t grow larger. The caterpillar grows a new, larger exoskeleton underneath the outer skin and then sheds, or molts, the old one. Most caterpillars molt five times. At first, the new exoskeleton is very soft and not very protective, but it soon hardens. The shed exoskeleton is often eaten before the caterpillar ingests more plant food.
There are names for the caterpillar’s stage of development in between each molt, called “instars.” When the caterpillar hatches from its egg, it is referred to as a “first instar” caterpillar. After its first molt, the caterpillar is referred to as a “second instar,” and so on up until the exoskeleton is shed for the final time, revealing the chrysalis (if it’s a butterfly).
The Monarch in the photograph is a very new 4th instar instar caterpillar (see antennae which haven’t hardened). It has shed three times. Its third exoskeleton (which it has just shed) is on the milkweed leaf behind the caterpillar. To see a real-time video of a Monarch molting go to https://www.youtube.com/watch?v=EbHyq3RwtxI.
(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin. Also to Ba Rea ((www.basrelief.org) for her instar confirmation.)
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Naturally Curious with Mary Holland 
Mary, I have long thought that the transformation stage is called “pupa” for both butterflies and moths. But the butterfly does it in a chrysalis and a moth does it in a cocoon. Am I mixed up? Do moths not all make cocoons?
July 27, 2019 at 11:32 am
You are correct, Suzanne! Both butterflies and moths undergo complete metamorphosis: egg, larva, pupa, adult. Butterfly pupae are inside a chrysalis, moths inside cocoons!
July 27, 2019 at 12:02 pm
And now I shall be searching the milkweed in the hidden field in the woods behind my house even more closely! Thanks, Mary!
July 27, 2019 at 11:45 am
Amazing video of the caterpillar shedding its outer skin. The following video of the caterpillar pupating into a crysalis is stunning.
July 27, 2019 at 12:38 pm
It’s so fascinating. My daughter sent a photo, this morning, that the Mom of a friend of hers took…she’s raising Monarchs & has 12 very large caterpillars on one stem! She has lots of milkweed. I gave my daughter a few stems last Saturday, with what we thought were 3 eggs…she has 7 caterpillars!!!!!!! & there were some eggs on the fresh stems I gave her yesterday. I brought some stems in & have some munchers, right on the kitchen table! The habitat & ‘Raising Monarchs’ book will arrive on Monday. It’s 2 by 2 by 3 feet. I took to these measures, because all but one of the many caterpillars & eggs we found last week Friday, ‘disappeared’. Alarming that so few survive. They are safer in here. Just cut 3 stems @ a neighbor’s, she said to help myself), several eggs & a baby! I watched Mommy Monarch lay an egg under a leaf, yesterday afternoon..it’s gone 😥 I suppose you must follow the Monarch & cut the Milkweed stem immediately, when you see an egg. Your photos are amazing, Mary!
July 27, 2019 at 12:52 pm
I just re-read all your info. Great for Otis for spotting the Caterpillar! Is this an ‘outdoor’ or ‘indoor?’
July 27, 2019 at 2:09 pm
Out in a very large field!
July 27, 2019 at 2:34 pm
Then that’s an ‘extra-special keen eye spot’….he is taught very well!
July 27, 2019 at 3:40 pm
I have a few tiny caterpillars and HAD a large one till I looked this morning. Might it have been eaten, or does it leave the milkweed plant to make its chrysalis? About how many days pass from the egg to the chrysalis formation?
August 3, 2019 at 9:44 pm
Hi Louise,
Yes, monarchs do usually leave the milkweed plant on which they are feeding when they molt for the final time and reveal the chrysalis. They usually don’t go very far. Roughly it’s an egg for 3-4 days, caterpillar 10-14 days, and chrysalis 10-14 days. Summer adults live 2-5 weeks, fall adults 8 or 9 months.
August 4, 2019 at 10:54 am
So, Mary, are the adults we’re seeing around now “summer adults” or “fall adults”? Most of them look very fresh/unbattered so are they “fall adults” — i.e. newly metamorphosed? I’m assuming the “fall adults” are the ones that fly up from Mexico in the spring, and they’d be dead by now, yes? We still have quite a few eggs and small larvae, so trying to track things more accurately! Thanks.
August 5, 2019 at 10:16 am
Hi Suzanne, Sorry for the delay, I had to check with Ba Rea, a monarch biologist, to be sure I gave you the right information. The monarchs that appear here in spring/early summer are the great, great, possibly great grandchildren of the monarchs that migrated to Mexico the previous fall. The monarchs that are hatching now may be mating and laying eggs or they could be migrating…certainly from mid-August on most are migrators.
Here’s what my friend Ba had to say: “What we know about when monarchs go into reproductive diapause and become migratory is that it is correlated to the angle of the sun above the horizon at noon. I haven’t heard that anyone has figured out what the mechanism for that may be. It happens earlier as you go north. I haven’t done the math lately but I used to say the change over date for Pittsburgh was around August 15th. So it would be earlier in Vermont, but a word of caution I have seen no evidence that sun angle is a sure sign of diapause…Some may be a little earlier some a little later. And diapause is flexible. With warm enough conditions and weather patterns that inhibit migration, a diapausal monarch may break diapause and become reproductive. They cannot then become diapausal again.” Hope this helps!
August 6, 2019 at 7:34 pm
Thanks, Mary! Interesting. So, the rather un-tattered, fresh looking, vigorous adults that are flying around now in my yard in southern VT are ones that have matured and emerged here this summer, the kids (or maybe grandkids) of ones who migrated north in the spring? And, from what Ba has to say above, if these individuals are laying eggs now, it means those same adults will NOT be migrating south? i.e. I think Ba is saying that the adults don’t lay and then migrate? Or can those same adults who are laying eggs right now then shift into migratory mode? I’m assuming that the eggs on my milkweed right now will also hatch, mature, and migrate? So is that two generations who will migrate? Hope that makes sense. Tell me to be quiet if this is too much detail. (I was involved as a kid in the banding research that figured out the migration route, so I’m a bit invested! :-))
August 7, 2019 at 11:17 am
Hi Suzanne,
The summer monarchs that breed and lay eggs live about 8 weeks or so. The last generation of monarchs to emerge in late summer delay reproduction until next March or April, as they migrate north (they don’t make it all the way back). They have the energy to migrate because they aren’t in reproductive mode! These migrating monarchs live 8-9 months!
August 7, 2019 at 3:03 pm
Dear Mary Holland, Thank you for all your very wonderful information! I am an artist and taught art to young children for almost 40 years. Every fall I brought in Milkweed and Monarch caterpillars to show the children the incredible miracle of transformation of a caterpillar, chrysalis, and butterfly. We painted them, made caterpillars, watched them and one summer I saw the whole thing in a glass terrarium- really just incredible!!!! I also brought Monarch caterpillars to church so everyone would know what happens in August in Vermont !! Sincerely Jeremy.S. Oberle
August 12, 2019 at 9:15 pm