by Joseph DeSisto
The idea that an animal that looks like this:
can transform into something like this:
has captivated us for centuries. The caterpillar and moth above belong to the species Callosamia promethea, commonly called the promethea moth. Prometheas can have wingspans approaching 4 inches across, and throughout their eastern North American range they are some of the biggest moths around. The moth is truly a spectacular beast. What often goes unappreciated, however, are the changes that go on before the caterpillar even forms its cocoon.
All caterpillars have to molt several times before they are large enough to go through metamorphosis. The stages between molts are called instars, and sometimes successive instars can look very different from one another.
The first photo was of a caterpillar in its second instar, meaning it has molted once since it hatched out of an egg. At this point the caterpillar has grown from nearly microscopic to a respectable centimeter or so — now it’s ready to molt again. It does so by splitting the front of its exoskeleton and, slowly and patiently, pulling itself out:
The old, empty skin is left behind:
When we started rearing these caterpillars in the lab, I wasn’t familiar with their life history. You can imagine my surprise when a container full of black-and-yellow-striped caterpillars, overnight, became a container full of these charming little creatures:
Promethea caterpillars are generalists and eat leaves off a variety of woody plants — these ones are munching on black cherry (Prunus serotina).
Why do the caterpillars change so radically and suddenly? That question remains very much unanswered. Perhaps the two instars simply have slightly different lifestyles, and different lifestyles require different adaptations. Or maybe having two different-looking life stages keeps predators from developing an accurate “search image.” In other words, by the time a bird learns to recognize the second instar as prey, it changes into a new, unfamiliar caterpillar.
Tomorrow I leave to spend a week in Arizona, New Mexico, and Texas. I’m supposed to spend that time looking for caterpillars and moths although I hope to see many other interesting animals — scorpions, rattlesnakes, and giant centipedes are at the top of my list. No writing while I’m gone, sadly, but plenty of picture-taking, so brace yourselves.