So, I’m on my way home, pushing my bike up the last steep hill, when I look down and see this very, very pink caterpillar just starting to cross the road. Now, a shocking pink caterpillar is not something you see every day, so naturally I picked it up to bring it home.
I didn’t have a container or anything, so I had to carry it in my hand. It didn’t like this, and kept thrashing around pretty vigorously. It was about an inch and a half long, so it was pretty substantial and quite strong. The weird thing was, after a bit it stopped thashing and started slowly rubbing against my hand, and I could feel a vibration. And then, when I held my hand up to my ear, I could hear a faint purring. This was very disturbing, it was an odd and not particularly pleasant sensation. If I’d been a predator, I probably would have dropped it. I was a little bit afraid that it might be putting some irritating or corrosive chemical on my hand, but no, it wasn’t. But somehow, the vibration was making it feel as if it was.
So, anyway, I got it home, showed it to Sam, got some pictures, and then sat down with “Caterpillars of Eastern North America” and started browsing.
I didn’t have much luck at first, there are only a few species of pink caterpillars and most of those are kind of hairy or warty, and this one was smooth. At first I thought it was an Angulose Prominent, Peridea angulosa, based on an inset picture of one of its color phases in Caterpillars of Eastern North America – they are normally green, but in the fall they sometimes turn pink.
But, as it turns out, it probably is not a prominent at all. Down in the comments, John and Jane Balaban (who are contributing editors to BugGuide) noted that it is more likely one of the Cucullia owlet moths, which are both very colorful and highly variable in both color and pattern. For example, a few of the pictures of Cucullia asteroides are a very similar shade of pink with yellow side stripes (along with a lot of others that look completely different!)
This immediately raises a question, though: why on earth would any caterpillars sometimes be pink? Well, in the fall, it’s probably because the tree leaves that they are eating turn some shade of red or pink in the fall, and so changing color with the leaves is much better camouflage than staying green. I expect that what happens is that, when the trees start producing their reddish leaf pigments as the weather cools, the caterpillars eating the leaves take up the pigment in their skin, and so they change color too.
 The whole way home with a purring caterpillar in my hand, I kept being reminded of something that Don Marquis wrote about fishing for bullheads (a type of particularly rugged medium-sized catfish): “And then, once out of the water, they will advance on you with an odd purring noise and try to horn you with their spines, showing every indication of wanting to fight it out on land.”
 Don Marquis was mainly known for a series of poems he published in the 1920s and 1930s, written entirely in lower case, that he claimed were typed by a cockroach named Archy. Archy typed by jumping on the typewriter keys, and it was all in lower case because he couldn’t work the shift key.
 It turns out that the color change of the leaves in the fall is no accident, it’s not just a case of dying leaves exposing a base color as the chlorophyll breaks down. The color comes from pigments that the trees produce to protect them from the sunlight. I read an article about why leaves turn color (I think it was in American Scientist), and they pointed out that leaves aren’t just casually soaking up sunlight that is otherwise harmless. Rather, they are juggling nuclear fire from the heavens, trying desperately to capture the energy without getting burned. They can use pretty much all the light they catch in the warmth of the summer, but as it gets colder their ability to convert light to food drops off faster than the intensity of the light. Unless they do something, they’ll get singed. So, the leaves produce pigments (usually red or yellow) to filter out the most damaging wavelengths while allowing the leaves to keep photosynthesizing for just a few more weeks in the fall.
 The sun is a mass of incandescent gas, a gigantic nuclear furnace. Where hydrogen is built into helium, at a temperature of millions of degrees. Or, to put it another way, “There have been many arguments about how far a nuclear reactor should be from inhabited areas. Extensive historical experience has shown that 93 million miles is probably far enough.”
 OK, ok, actually the sun is a miasma of incandescent plasma, but that requires completely rewriting the song.