I started researching this at the request of a good friend, who was amazed by some reading they had done around it. When they asked what I knew about metamorphosis I had to admit fairly little – my knowledge faltered after “there was a very hungry caterpillar who stuffed himself with leaves”. We’re told as children that the caterpillar eats until it’s full, spins itself a cocoon, has a sleep, then emerges as a beautiful butterfly. To be honest, the truth is a little more gruesome so maybe best to leave it at that for children.
Let’s start with eating until “they are full”
The caterpillars (more accurately known as larva) have one main objective at this point in their lives – eat. They need to eat enough to sustain them as a larva, but also to sustain them through metamorphosis into adult life. Malnutrition can mean not making it through metamorphosis, or not being able to reproduce when they are adults, so gorging is very important.
The point at which they stop eating and start spinning is driven by a number of factors such as temperature, stress factors, availability of food, and a set of chemicals including the juvenile hormone (JH) and ecdysone. Throughout the larval stage JH is released, this suppresses ecdysone. When the larva nearly outgrows its first layer of skin the level of JH starts to fall, allowing ecdysone to increase. Ecdysone triggers the moulting process and the larva sheds an average of 5 times before it reaches the chrysalis/cocoon stage.
Usually the number of skin sheds isn’t overtly clear, but one caterpillar actively broadcasts its stages of moulting, and this caterpillar is the gum-leaf skeletoniser caterpillar. This caterpillar actually stacks it’s old heads it has shed from each moult and wears them as a hat. These caterpillars can moult up to 12/13 times before it undergoes metamorphosis, so it can end up having quite an impressive tower of heads.
Next, spinning their protective layer
There are two options of what form of protection they make for themselves depending on the species: the first option is spinning a cocoon. Cocoons are spun from strands of silk that they produce from glands near their mouths. The silk has a glue-like consistency, and can form cocoons that vary in being soft, hard, translucent, or even colourful. Different caterpillars choose different places to spin their protective cocoon, some, like the monarch larva below, choose hanging from tree branches, others nestled on the underside of a leaf.
Some species don’t spin cocoons at all but form a chrysalis. I thought that the chrysalis was made in a similar fashion, but a chrysalis is actually the last shed of the skin. When the caterpillar moults for the final time the skin underneath splits out of the skin and forms the outer shell of the chrysalis. Again, each species choose a different place to undergo metamorphosis, some, like the tomato moth, bury themselves underground to pupate.
Now, the “sleep” phase.
A common misconception of this phase is that it is a resting phase for the caterpillar, but it is far more fantastic and complex than that, and where it gets even more gruesome than some skin splitting and wearing your own severed head(s).
The fall in JH also causes genes in the imaginal discs to no longer be blocked. Imaginal discs are bundles of cells that will turn into each butterfly body part, but to do this they need fuel, and this fuel comes from recycling their current body. Enzymes called caspases break down a large amount of the caterpillars body (except the imaginal discs) into a chunky soup – they actually dissolve themselves! The imaginal discs can now use the proteins in the soup around them to undergo rapid cell division to form antennae, wings, legs, eyes etc.
When the transformation is complete they can release a liquid that softens the protective shell enough to push through it, or secrete enzymes that digest the layers making it easier to push through. You can see from the images above that when they emerge their wings are furled up, wrinkled, and definitely not capable of getting airborne. To get flight ready the butterfly pumps a fluid called meconium (a red metabolic waste liquid) from its abdomen into its wings to inflate them. When they are fully inflated the meconium is pumped back into the abdomen leaving a small amount in the wings veins, this hardens to give them a more sturdy structure. Any excess meconium is then expelled from the body when the wings are fully dry.
The weirdness isn’t even over yet – some researchers have found that some species of moth retain memories from when they were larva. Tobacco hornworm caterpillars were placed in the end of a Y-shaped tube with the two tube options ahead of them. One of tubes led to an area treated with ethyl acetate to give of a chemical smell, while the other led to fresh air. If the caterpillar chose the chemical smell they would be given a small electric shock. Unsurprisingly, after this 78% of them avoided the chemical smell. Post metamorphosis the adult moths were placed in the same experiment – 77% of the moths still avoided the ethyl acetate smell. The lead researcher Martha Weiss suggests means that parts of the brain initially thought to be dissolved are retained despite the transition.
I hope you have enjoyed this dive into the unusual and have some newfound admiration for the furry little creatures before they undergo quite a horrific transformation!