Metamorphosis: Chunky Caterpillar Soup Part I
A caterpillar’s transformation into a moth is one of the many radical changes we can see in nature. While I have always appreciated moths (as far as my fear would allow me!) it was the discovery of a book chapter on complete metamorphosis that tipped this appreciation to slight obsession.
When I first came to grips with this magical process I would ecstatically share it with anyone who gave me a moment. I even wrote about it in my Masters thesis! But I wanted to share the mystery of metamorphosis and why it is so darn incredible with more people. So, welcome, new victims, to my explanation of metamorphosis, or “chunky caterpillar soup”.
[Disclaimer: I am by no means a professional or entomologist, or professional entomologist. I am just an artist who loves embroidering moths and has an amateur interest in entomology.]
Complete metamorphosis, or holometabolism, is the process that transforms all holometabolous insects from wee immature larvae (like caterpillars) into imagos (or adults, like moths). Complete metamorphosis includes four stages: egg, larva, pupa and imago. If you kept silkworms as a child you would have been able to watch this process. From the outside it’s a pretty straight forward cycle. Very tiny and very hungry larvae hatch from bead-like eggs and eat their way through loads of plants until they become beautiful, hairy noodles. As they grow caterpillars usually moult, or shed their skin, about five times; the stages between moulting are known as instars. Two hormones are responsible for telling the caterpillar to shed its skin for the next instar. Ecdysone starts the moulting process while juvenile hormone makes sure that the caterpillar remains a caterpillar.
Are you still with me? Good, because here is where it starts to get really incredible! Once the caterpillar is “fully grown” the ecdysone once again sends a message to the caterpillar telling it to moult. However, this time around juvenile hormone is produced at a very low level or not at all. This means that there is nothing keeping the caterpillar a caterpillar anymore, but it will still move into a moulting process- and metamorphosis begins!
A caterpillar destined to become a butterfly sheds the skin of its last instar and forms a chrysalis to protect the pupa within. Some moth caterpillars build cocoons with silk, sometimes with dead leaves and twigs or even their own hair. Others will burrow into the ground to safely metamorphose. The pupa inside is vulnerable but not entirely inactive. Most can wriggle or make a vibrating sound to scare away predators.
Now that the caterpillar is all safe and sound in a chrysalis, cocoon or underground it is time for metamorphosis to begin! Histolysis (“histo” meaning cell, and “lysis” meaning disintegration) is step one of metamorphosis. Hormones in the pupa signal for enzymes to be released. These enzymes begin to break down the larval tissues- that’s right, the caterpillar digests its body! People used to believe that at this stage the pupa was completely liquid and soup-like. It has since been proven that many cells actually do remain intact after the self-digestion, so it is more of a “chunky soup”.
Because this post got really freaking long I decided to break it into two parts. You can read everything that happens after histolysis here.