The Chemistry of Butterfly Metamorphosis

Introduction

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One of the first things everyone learned as a child is the natural phenomenon of caterpillars turning into butterflies. However, the way that metamorphosis occurs is never really explained. I couldn’t help but wonder, how exactly do these insects change so drastically over the course of their lifetime? What is happening inside of the chrysalis?


Though knowing the secret to butterfly metamorphosis isn’t exactly crucial to my life, I have exhausted a few ears questioning why this unbelievable process isn’t touched upon once past elementary school. Should I be left to believe it’s purely magic? No, of course it’s not magic. It’s chemistry.



Composition of ...

  • Ecdysone
    • C27H44O6
  • Juvenile Hormone
    • C16H26O3
  • PTTH
    • a homodimer of two polypeptides of 109 amino acids
  • Caspase
    • C27H35N6O9+ (Caspase 1)
  • Cytochrome C
    • C42H54FeN8O6S2
  • Phosphatidylserine
    • C13H24NO10P
Main Chemicals, Compounds, Components

Driving Hormones: PTTH (Prothoracicotropic Hormone), ecdysone, and the juvenile hormone (JH) control and regulate the stages of molting and metamorphosis. The presence of these hormones drives physiological changes of the larva. They are released once the larva has reached both minimal value weight, which is enough to ensure survival during metamorphosis, and critical weight, wherein starvation does not affect the time to pupation. The corpus allatum, located in the brain, and Prothoracic gland in the thorax produce these hormones.

 

Apoptosis: Apoptosis, or programmed cell death, essentially consists of larval cells committing suicide in order to provide nutrition for the development of adult structures. Once the larva has reached the time of pupation and has formed its chrysalis, the cells are broken down into fragments and engulfed by phagocytes.


Chemistry's Role

A larva’s transformation begins when PTTH is released by the corpus allatum. This happens once the larva has reached both minimal value and critical weight, and the time falls within an eight-hour window created by a photoperiodic gating mechanism. The release of PTTH then triggers the prothoracic gland to produce ecdysone, the molting hormone. The result of this molting is determined by the quantity of JH, also produced by the corpus allatum, present within the bloodstream. As the quantity of JH lessens, the larva progresses through five instars (stages between moltings), and once the quantity of JH has dropped below a certain amount, the larva forms a chrysalis in order to begin the pupal stage.

 

Once within the chrysalis, the larva’s cells begin the process of apoptosis. This programmed cell death is activated by the enzyme caspase, and is triggered by the ecdysone hormone. During apoptosis, cells shrink and form blebs. This is caused by the breaking of the cell’s cytoskeleton. In a process known as pyknosis, the cell’s DNA is broken down and fragmented. The enzyme cytochrome c breaks down the cell’s mitochondria. This process exposes the normally hidden phospholipid phosphatidylserine, which acts as an “eat me” signal. As a result of this signal, phagocytes engulf the cell fragments.



Background Research

During the larval stage, the larva molts several times, resulting in five distinct instars. As one would assume, the size of each instar is greater than the last. The first four last between 1 and 3 days, depending upon the temperature. The fifth and final instar lasts between 3-5 days.

 

Present within the larva are nests of cells known as imaginal discs. These are the only cells not to undergo apoptosis while inside the chrysalis. Instead, the imaginal discs become parts of the butterfly, such as eyes, wings, and thorax.

Sir Vincent Brian Wigglesworth is the actual name of an entomologist who contributed to scientists’ knowledge of insect physiology. He discovered PTTH and the juvenile hormone.



Resources

http://www.zmescience.com/ecology/animals-ecology/how-caterpillar-turn-butterfly-0534534/

General process of metamorphosis

Ecdysone hormone drives molting and, when combined with lack of juvenile hormone, creation of crystalis

Disintegrates through digestion all tissue but imaginal disc

Enzyme caspase programs cell self-destruction

http://www.zmescience.com/research/caterpillar-butterfly-metamorphosis-ct-3d-scan-65054/

Shows images of the inside of a chrysalis

http://www.biology-pages.info/A/Apoptosis.html

Describes Apoptosis--wherein cells die by “committing suicide”

The process in which they “commit suicide”

What triggers Apoptosis

http://www.biology-pages.info/I/InsectHormones.html

Chemical makeup of ecdysone

Chemical makeup of juvenile hormone

Studies of Dr. V B Wigglesworth

PTTH hormone

5 larval instars

http://www.sciencedirect.com/science/article/pii/S0960982210002915

Information on the imaginal discs

https://www.britannica.com/science/animal-development/The-larval-phase-and-metamorphosis#ref526507

Function of Prothoracic gland

Function of corpus allatum

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2359579/

Chemical makeup of PTTH

What triggers PTTH to release

https://en.wikipedia.org/wiki/Apoptosis

How enzyme caspase is involved in Apoptosis

Steps of cell death

https://pubchem.ncbi.nlm.nih.gov/compound/100953062#section=Molecular-Formula

Caspase chemical makeup

http://www.chemicalbook.com/ProductIndex_EN.aspx

Chemical makeup of Phosphatidylserine

Chemical makeup of Cytochrome C

http://monarchlab.org/biology-and-research/biology-and-natural-history/breeding-life-cycle/life-cycle#larva

Details the five instars



About the Author
Audrey Schied is a junior at Billings Senior High School. Though she mostly enjoys creative endeavors, she’s also her fair share of interests in left-brained fields. She spends her free time reading, writing, and otherwise avoiding any sort of physical activity. She also published a book, which is pretty cool.







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