Babies are cute. They’re meant to be. We are programming to delight in the differences between puppies and grown dogs, lambs and sheep, and of course our own darling miniatures. Their big eyes, over-sized heads and smooth lines are meant to trigger some parenting instinct deep inside. But of course, they’re not really all that different from the grown-up versions. Dress a little boy in grown-up clothing, and everyone will comment on how he looks like a ‘little man.’
Not so for many creatures in the invertebrate world. Put a pair of pretty wings on a chubby caterpillar, and something would still be missing. And could you possibly dress a creepy-looking cicada nymph to resemble it’s B-movie flying future? (Come on – don’t those things look like something that should be fighting Godzilla on a Sunday afternoon. Yes, I know I’m dating myself here – give a shout-out if you know what I’m talking about). That future is not very long, anyway. Nymphs of the cicada species will spend either thirteen or seventeen years underground in their nymph stage, and only a few weeks above ground as adults They don’t even eat during the last phase of their life – they just reproduce and die. (Talk about one-track minds. I really hope it’s good for them, don’t you?)
Not all metamorphosis is alike. The caterpillar above goes through complete, or holometabolic metamorphosis. This includes an egg, larvae, pupa or resting phase, and adulthood. Bees and wasps go through this progression as do ants.The cicada’s path is hemimetabolic metamorphism, which includes the egg, nymph, pupal respite, before launching into adulthood.
Metamorphosis isn’t restricted to the flying and crawling critters. It’s also common among their marine invertebrate cousins. Take the crab, for example. When it hatches from a dust-sized egg cast into the ocean – if it hatches before becoming fish food – it bears no resemblance to its future self. It’s looks more like a tiny shrimp with a lance sticking through its head.
Unlike the contented lifestyle of the caterpillar (a bit of anthropomorphising, perhaps?), the zoea, as the proto-crab is called, just floats along, another bit flotsam pushed here and there by winds and currents. It hangs out until some still unknown signal or environmental cue tells it that it’s time to move towards full crabdom. No architectural feats or deep sleep precedes this change. In fact, not only doesn’t it get a nice safe cocoon in which is transform; as any creature with an exoskeleton, it must shed its protective outer shell in order to become just a wee bit bigger and more crab-like than its former self. Again and again this shedding or molting happens, followed by incremental changes, making one wonder if all the fuss and loss of security is really worth it. Nothing dramatic here – no it just adds segments one by one, until it bears little resemblance to either its future or its former self. It’s at that point that the little half-crab must commit to its future, even before it can have any idea of what that future holds. The critter that has enjoyed – or at least survived – its early life floating aimlessly on the tide must now fight the same currents that have supported it to make its way to the shore – nursery for many marine creatures small and big – and settle down, hoping to become a crab someday soon. And not get eaten in the process.