Introduction
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 ...
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
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.