I happened upon this White Admiral (Limenitis arthemis) last summer. I nearly tripped over it, at first unaware of what I had stumbled across. When I looked closely at this newly ‘hatched’ butterfly, drying its brand new wings, the whole thing took my breath away.
It’s a process we learn about as children, one of those uncontested facts that just lives in our brains: caterpillars become butterflies. However, that simple statement doesn’t even begin to do justice to what is truly an amazing process.
Insect life cycles encompass multiple stages that may involve fairly dramatic transformations from larvae to adult (like the previously celebrated fishfly). However, only a few groups of insects other than lepidopterans (butterflies and moths), such as bees, flies and beetles, undergo complete metamorphosis.
It’s a quite remarkable when you really think about it. These lumpy, worm-like creatures that lumber along, munching at leaves transform completely into delicate, colourful jewels that sip daintily at their food, the Victorian lady of the insect world. It happens at the pupal stage, when the larvae (caterpillars) form a chrysalis that then sits suspended for a few weeks up to a few years, depending on the species. From our point of view, it looks like nothing is happening, but on the inside, it’s a different story.
Like most things in animal physiology, the whole process boils down to hormones, the transformation being dictated by the relative amounts of two chemicals coursing through the critter’s hemolymph (insect blood). Just like every other insect, caterpillars moult, shedding their exoskeleton to make room from their growing bodies. Each moult is governed by a hormone called ecdysone (stemming from the word ecdysis, a fancy word for moult). Each new shed produces a larger caterpillar as long as a second hormone called juvenile hormone (thankfully, self-explanatory) is also circulating. It’s basically a chemical that tells the caterpillar to stay a caterpillar.
Then one day, often as a result of changing day lengths or temperature, the caterpillar’s body stops making juvenile hormone, so when the next moult comes around, things change, the chrysalis is formed and ultimately a butterfly emerges. But how does it go from a wiggling lump to something as complex as a butterfly? Well, that lump was carrying around little spheres of tissue called imaginal discs. These discs truly make the imagined possible, the cells differentiating into eyes, antennae, wings and legs. Each disc has it’s own part to build and if you were to move it to another place on the caterpillar you’d end up with a Picasso painting of a butterfly.
Once the process is complete, the chrysalis splits open and the new adult rests for a bit, drying its wings until it can safely take flight. The transformation doesn’t only affect what the insect looks like; it’s a complete life change. The metamorphosis from caterpillar to butterfly is a transition from the feeding stage to the reproductive stage and in many cases, that transition is absolute. Luna moths, for example, are like fishflies; once they reach the adult stage, they can no longer feed and their sole purpose is to mate in the day or so they have left before their metabolisms burn out. Of course, not all lepidopterans are as short-lived as adults. Some, like the Mourning Cloak actually hibernate at the adult stage, while Monarch adults travel thousands of kilometres.
So, why go through all that trouble? Why not stay a caterpillar? I’m not sure there is a definite answer to that and I’m sure it’s fuelled many debates among evolutionary biologists. Personally, the fanciful side of me likes the idea that a caterpillar decided one day that he wanted to fly. I think we all have days when we wish to break out of our shell and I think a little change, now and then, can be a good thing.