Monday, September 16, 2013

Apple #652: Pupal Soup

So I've been listening to a book on CD that I bought from the Friends of the Library for $1.  The US title is, unthrillingly, The Sister.  The UK title is The Behavior of Moths. The main character/narrator of this novel is a 60-something-year-old woman who was "quite a famous lepidopterist" and who learned the science from her father.  There is more to the novel than that, including some suspicious deaths which may or may not have been murders, but for the purposes of this entry, I'll just talk about the moths.

The Sister
I very much recommend the audio version because the woman who reads it is really a talented actor, and she makes the narrator come to life.

Many parts of the novel deal with the habits of moths or the study of moths.  While the moths are actually quite an ingenious, ongoing, and complex metaphor--one that the narrator isn't entirely aware of--the facts about the moths I found interesting in and of themselves.  The main one is that, while inside the chrysalis, the caterpillar breaks down entirely into what the narrator's father calls "pupal soup"--nothing but goo.  The caterpillar completely dissolves into this primordial goop inside the chrysalis and a few weeks later, emerges as a butterfly.

Astounding.  I knew the basic things you're taught in elementary school, caterpillar, pupa in the chrysalis, butterfly, but I never heard of this pupal soup business.  No one ever told me the caterpillar turned into a liquid.

From caterpillar to butterfly -- includes a liquid phase?
(Photo from Fairchld Tropical Botanical Garden)

Now, this is described in the novel as research the father & daughter were doing in the 1950s & 1960s.  So I wondered, is it true about pupal soup?  I mean, does the caterpillar dissolve completely into liquid, only to emerge, totally rebuilt as another creature entirely?  In other words, I found this information so stunning and wonderful, I had to know more. Especially since this is the time when Monarch butterflies will soon be emerging all over the place.

  • The short answer is, it's mostly true, but it's more complicated than that.  
  • So, you know the part about how the caterpillar eats and eats and eats non-stop.  The way caterpillars were described to me is that they are basically eating machines disguised as tube socks.  Uncomplicated, nothing much going on in there except a lot of eating and digestion.
  • Well, my friend, the truth about caterpillars is they have a lot more happening in there than just eating.

Yes, a caterpillar is an eating machine--and more.
(Photo from WDW, via Science Buzz)

  • First, everything about their development is regulated by hormones.  There is one hormone, referred to as "juvenile hormone" that keeps them in the caterpillar stage. As long as the juvenile hormone is active, the insect will stay a caterpillar.
  • There's another hormone, called ecdysone.  This one is kind of like the change signal.  A little burst of this, and the caterpillar has a growth spurt and molts.  The caterpillar goes through several rounds of ecdysone burst/molting.  But that juvenile hormone is still active, so it stays a caterpillar.
  • Finally, when the caterpillar reaches a certain size--has taken in enough food to last it through the metamorphosis--the amount of juvenile hormone drops so that at the next ecdysone burst, the caterpillar doesn't just molt, it starts becoming a chrysalis.
  • Yes, I said that correctly.  The caterpillar doesn't create a chrysalis that it climbs into, its skin becomes the chrysalis. Some caterpillars will spin a protective cocoon first, but that outer casing which is the chrysalis--that is a new skin that the caterpillar has grown.  A new molting, if you will.

You can kind of see here how the chrysalis of this Monarch caterpillar looks like a new skin.
(Photo from Shea in Michigan on Flickr)

  • Once it's safely enclosed within its new skin that is the chrysalis, the caterpillar releases a batch of enzymes.  These are digestive enzymes.  Which means that the caterpillar is effectively digesting itself. Thus turning itself into pupal soup.
  • So, yes, that novel was correct about that soup business.  But what we've learned since the 1950s and 1960s makes it more complicated.  If you were to cut open a chrysalis, caterpillar goo would spill out.  It would look like it's only liquid.  But in fact, there is actual structural stuff still lurking within the goo.
  • Some muscle tissue breaks down but the cells remain intact and persist in clumps. Organs such as the breathing tubes and the guts also stay intact; they grow larger or reconnect things in slightly different ways. How these structures remain even though all appears to be soupy, I'm not sure.  I can only tell you what the researchers have reported.

Micro-CT imaging was used to see inside a chrysalis as the caterpillar re-forms into a butterfly. This is a painted lady butterfly chrysalis.  The breathing tubes have been colored blue and the guts red.
(Image from the University of Manchester, via National Geographic)

  • But everything else about the insect--the exoskeleton, the many little feet, the head--all that gets completely reorganized and turns into wings! A head with an enormously long tongue suitable for collecting nectar!  Very long thin legs!  How does this happen?
  • Well, there's still more besides hormones and enzymes inside a caterpillar. In addition to those crucial fluids, they also have things called imaginal discs, or imaginal cells.  These exist in a caterpillar and develop to a certain point and then stop, waiting for go-time.
  • Once the caterpillar is in the chrysalis, the imaginal cells go into action. They work a lot like our stem cells do and develop into new body parts. 
  • The discs shift from being flat into a concave dome, then elongate into a sock shape.  The pointy end of the sock gets further defined as the disc eventually becomes some feature of the butterfly or moth--a wing, a leg, an antennae.
  • Four discs contain the DNA information to become 4 wings. Other discs become legs. Other discs become antennae. If one disc that was supposed to become a wing for some reason does not, the remaining 3 discs will adapt on the fly (pun) to form bigger wings to compensate.

The blue circles at the top are the imaginal cells of the Drosophila, another insect that undergoes complete metamorphosis.
(Image from Georgia Tech's Developmental Biology Initiative)

  • Meanwhile, much of the rest of the goo in the chrysalis is literally food.  It is a nutrient-rich soup that feeds the insect as it undergoes this remarkable change. If you weighed a chrysalis after it first formed and then weighed the newly-formed adult after it's emerged and its wings have dried, the weight would have dropped by about half. The majority of that missing weight is the food the moth/butterfly consumed during its transformation.
  • Here's another remarkable thing: scientists also have reason to believe that the neurons present in the caterpillar's admittedly small brain survive the self-digestion process and continue to function in the adult butterfly or moth.  Which means that they "remember" things about being caterpillars. 
  • That isn't just a nice little turn of phrase or metaphor; researchers at Georgetown University have proven that "moths retain at least some of the memories they had as caterpillars." The memories they've shown that the caterpillars-turned-moths retain are mainly scent memories. But still. That's pretty impressive.
  • OK, now, one final factoid to blow your mind.  9 out of 26 orders of insects undergo this sort of complete metamorphosis. That may not seem like very many, but those 9 orders include butterflies, moths, flies, beetles, bees, wasps, and ants.  Which is, in fact, the majority of insects.  Which is, in turn, the majority of all animals.

So simple, anyone can do this, right?  Sheesh.
(Photo via University of Miami Department of Biology)

Related entries: Monarch butterflies; White caterpillarsWoolly Bear Caterpillars

Devin Hiskey, Caterpillars "Melt" almost Completely before Growing into Butterflies in the Chrysalis, Today I Found Out, October 28, 2011
Ferris Jabr, How Does a Caterpillar Turn into a Butterfly? Scientific American, August 10, 2012
Tracy V. Wilson, How Caterpillars Work, howstuffworks
Dr. Lincoln Brower, Inside the Chrysalis, Monarch Butterfly Journey North
Richard Jones, How does a caterpillar turn into a butterfly? Discover Wildlife, September 15, 2012
Ed Yong, 3-D Scans Reveal Caterpillars Turning Into Butterflies, National Geographic Phenomena, May 14, 2013


  1. This is really a great description of metamorphosis. I'm working on a book about metamorphosis in Lepidoptera, and have always been very bothered by the popular conception that caterpillars basically turn into mush from which a butterfly or moth miraculously forms. I found your site when I was looking for examples of this, but instead found a well-researched and accurate description. Kudos to you, Apple Lady!

  2. Thanks, Karen. Let us know when your book is published!


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