The Memory Made it Through the Goo

(And other incredible physiologizable facts about butterflies)

Courtney Hart

© Monarch-Butterfly.com

If you haven’t had a chance to visit the monarch butterflies clustered on the trees in Pismo Beach, now is the time. Monarchs cannot survive cold winter temperatures so they migrate to warmer areas. From October to late February, all monarchs east of the Rocky Mountains migrate to Mexico and those west of the Rockies spend the cooler months clustered by the hundreds and thousands in groves of trees. As of February 4^th, the current population count for the Pismo beach population was estimated to be around 20,000. At some point in your youth you learned about the butterfly life cycle and the amazing process of metamorphosis. But did you ever think about what actually happens during the process of metamorphosis? I bet you just think that the caterpillar wraps itself in a cocoon and wakes up as a butterfly. Although that is partially true, the actually mechanisms that take place before the butterfly emerges from its pupa is complex, mystical, and not very well understood.

© Monarch-Butterfly.com

      First, let’s take a look at the 4 basic stages of the butterfly life cycle. Adult female butterflies (in this case we’ll look at the Monarch, Danaus plexippus) lay eggs on the underside of a milkweed plant. A monarch butterfly egg is the size of a pinhead and the caterpillar that hatches from this tiny egg isn't much bigger but it will grow up to 2 inches in several weeks. They eat the leaves of the milkweed plant and split their skin, or molt, many times as they grow. Once the caterpillar is full-grown it stops eating and pupates. There is a common misconception that caterpillars crawl into or surround themselves with the chrysalis but that is not the case. As their final molting stage, caterpillars grow the chrysalis beneath their wormy skin and molt not into a bigger caterpillar, but into a chrysalis. The chrysalis that comes out from under the skin is soft and wet, but hardens over a few hours. The third stage is the transition stage and lasts for about 7-10 days (Oberhauser & Solensky, 2004). In the case of monarchs the pupa, or chrysalis, is suspended under a branch or leaf. It may look like nothing is going on but big changes are happening inside. Special cells that were present in the larva are now growing rapidly. They will become the legs, wings, eyes and other parts of the adult butterfly. Many of the original larva cells will provide energy for these growing adult cells. When the adult butterfly emerges from the pupa it enters into the fourth and final reproductive stage. The adult stage is what most people think of when they see a butterfly. They no longer have tiny eyes and stubby legs but instead have long slender legs, compound eyes and the ability to fly.

© Monarch-Butterfly.com

I want to focus on some of the amazing discoveries that occur during stage three of metamorphosis, the pupa. This is the most complicated stage of metamorphosis and little is known about the process by which a caterpillar emerges as a butterfly. The word metamorphosis is a Greek word meaning transformation or change in shape. Insects have two common types of metamorphosis. Grasshoppers, crickets, dragonflies, and cockroaches have incomplete metamorphosis. The young (called a nymph) usually look like small adults but without the wings (Baluch 2011). Butterflies, moths, beetles, flies and bees have complete metamorphosis. The young (called a larva instead of a nymph) is very different from the adults (Baluch). Massive structural development occurs during complete (Pelling, et al., 2009) metamorphosis.

http://www.biology.sbc.edu/faculty/
HomePageLPB.html

As I mentioned before, metamorphosis is a bit of a mystery but all you really need to know is that there is a lot of GOO. Dr. Lincoln Brower at Sweet Briar College studies the overwintering, migration and conservation biology of the monarch butterfly. He explains that enzymes are being released that digest all the caterpillar tissue, so that the caterpillar is being converted into a rich culture medium (i.e. goo). “During the first 3-4 days, the pupa is literally a bag of rich fluid media that cells are growing. Inside the caterpillar are several sets of little cells that are in different parts of the body and they're called imaginal disks. These are really like little groups of embryonic cells that start growing like crazy. There are imaginal disks that form the wings, legs, the antennae and all the organs of the adult butterfly” (Brower, 2000). Simply put, the caterpillar forms a chrysalis, dissolves into goo, reforms, and emerges as a butterfly. But how much of butterfly resembles its larval stages cellularly speaking?

Metamorphosis has been studied in Monarch butterflies as early as 1904. Two studies: Bauei, 1904 and Tiegs, 1922, suggest that all larval neurons are destroyed and that the adult brain is an entirely new structure. For those of you in Biology 502, it shouldn’t be too much of a surprise that people were already beginning to debate the destruction and regrowth of new neurons 100 years ago! But what does this mean for the butterfly? Are the larval and reproductive stages representative of the death of one species and the resurrection of another? Bernd Heinrich, Ph.D is a professor emeritus at the biology department at the university of Vermont and a prominent entomologist in his field. He suggests that the “radical change that occurs does indeed arguably involve death followed by reincarnation." For more on this debate I suggest you check out Robert Krulwich’s blog post on this exact question: http://www.npr.org/blogs/krulwich. Using radioautography, a study conducted by Ruth Nordlander and John Edwards in 1969 found that the majority of larval brain cells are incorporated into the adult brain.

(Pelling et al., 2009)

      Moving on to the heart of a metamorphosing butterfly, a study from 2009 used a novel ultra-sensitive detection method, optical beam deflection, and measured the mini motions of the pupae during transitional stage. The contractions that they were able to record were occurring at regular intervals, which they attributed to the mechanical function of the heart (Pelling et al., 2009). They concluded that the heart organ remains intact throughout metamorphosis but undergoes morphological changes. It’s hard to imagine that any animal can sequester enough time and energy in such a short life to facilitate actual changes in a necessary organ. Human hearts grow and experience some changes during embryo development but it is rare to see a natural progression as complicated as what happens during metamorphosis of a butterfly.

©Travis Witt 2010

In researching this post, I came across endless papers that use moths and butterflies as models for development, neuron generation, hormonal maturations, evolutionary arms races, and more!!! A study published in PNAS in 2013 uncovered an unknown developmental timer in the tobacco hornworm, which determines the time to metamorphosis and ensures that each individual reaches sexual maturity even in the face of adverse environmental conditions (Suzuki et al., 2013). But I saved the most radical study for the end (also the namesake of this blogpost). Martha Weiss is an associate professor of biology at Georgetown University. She conducted an experiment with her lab where she put a caterpillar in a large box and gassed it with a foul smelling odor. When they could smell the odor she gave them a 10-second zap so they would learn to hate the smell and repeated this so-called torture over and over. After pupation, when organs and muscles melt (this one lasted 5 weeks) a moth emerged and Dr. Weiss presented them with same odor. As you probably already guessed, they hated it! (Radiolab Podcast 2014, 12:4) Despite the cataclysmic changes that occur during metamorphosis, the memory makes it through the goo.

The mysteries of metamorphosis continue to perplex scientists in a variety of fields. Understanding the mechanisms behind this major transformational stage had major implications for neuron generation in the early part of last century and is now helping inform fields such as to hormone signaling. So if you can take the time to visit Pismo beach you may be dazzled by more than just pretty pictures of butterflies from your youth. 

EXTRA!!!! Stay tuned for the yet-to-be released BBC documentary on metamorphosis that will feature footage created with micro-CT scans, or x-rays that capture cross-sections of an object that can be combined into three-dimensional virtual models. (www.fastcoexist.com)

REFERENCES

Abumrad, J. and R. Krulwich. 2014Black Box. Radiolab Podcast. NPR.com. 12:4 from http://www.radiolab.org/story/black-box/

Baluch, P. 2011. Metamorphosis – Nature’s Ultimate Transformer. ASU - Ask A Biologist. from http://askabiologist.asu.edu/incomplete-metamorphosis

Bauer, V. 1904. Zur innern metamorphose des zentrinervensystems der insecten. Zool. Jb.,

Abt. Anat. u. Ontog. 20:123-152

Brower, L. 2000. Inside the Chrysalis. Journey North from http://www.learner.org/jnorth/tm/monarch/ChrysalisDevelopmentLPB.html

Nordlander, R.H. and J.S. Edwards. 1969. Postembryonic brain development in the Monarch Butterfly Danaus plexippus plexippus, L. Wilhelm Roux’ Archives 162:197-217.

Oberhauser, K.S. and M.J. Solensky. 2004. Monarch butterfly biology & conservation. Ithaca, NY: Cornell University Press.

Suzuki Y., T. Koyama, K. Hiruma, L.M. Riddiford, and J.W. Truman. 2013. A molt timer is involved in the metamorphic molt in Manduca sexta larvae. Proceedings of Natational Academy of Science. Inaugural Articles:1-8

Tiegs, O.W. 1922. Researches on the insect metamorphosis. Royal Society South Australia. 46:222-224.