The Universe of Discourse

Tue, 24 Jan 2006

Yesterday I visited the American Museum of Natural History in New York City, for the first time in many years. They have a special exhibit of butterflies. They get pupae shipped in from farms, and pin the pupae to wooden racks; when the adults emerge, they get to flutter around in a heated room that is furnished with plants, ponds of nectar, and cut fruit.

The really interesting thing I learned was that chrysalises are not featureless lumps. You can see something of the shape of the animal in them. (See, for example, this Wikipedia illustration.) The caterpillar has an exoskeleton, which it molts several times as it grows. When time comes to pupate, the chrysalis is in fact the final exoskeleton, part of the animal itself. This is in contrast to a cocoon, which is different. A cocoon is a case made of silk or leaves that is not part of the animal; the animal builds it and lives inside. When you think of a featureless round lump, you're thinking of a cocoon.

Until recently, I had the idea that the larva's legs get longer, wings sprout, and so forth, but it's not like that at all. Instead, inside the chrysalis, almost the entire animal breaks down into a liquid! The metamorphosis then reorganizes this soup into an adult. I asked the explainer at the Museum if the individual cells retained their identities, or if they were broken down into component chemicals. She didn't know, unfortunately. I hope to find this out in coming weeks.

How does the animal reorganize itself during metamorphosis? How does its body know what new shape to grow into? It's all a big mystery. It's nice that we still have big mysteries. Not all mysteries have survived the scientific revolution. What makes the rain fall and the lightning strike? Solved problems. What happens to the food we eat, and why do we breathe? Well-understood. How does the butterfly reorganize itself from caterpillar soup? It's a big puzzle.

A related puzzle is how a single cell turns into a human baby during gestation. For a while, the thing doubles, then doubles again, and again, becoming roughly spherical, as you'd expect. But then stuff starts to happen: it dimples, and folds over; three layers form, a miracle occurs, and eventually you get a small but perfectly-formed human being. How do the cells in the fingers decide to turn into fingers? How does the cells in the fourth finger know they're one finger from one side of the hand and three fingers from the other side? Maybe the formation of the adult insect inside the chrysalis uses a similar mechanism. Or maybe it's completely different. Both possibilities are mind-boggling.

This is nowhere near being the biggest pending mystery; I think we at least have some idea of where to start looking for the answer. Contrast this with the question of how it is we are conscious, where nobody even has a good idea of what the question is.

Other caterpillar news: chrysalides are so named because they often have a bright golden sheen, or golden features. (Greek "khrusos" is "gold".) The Wikipedia picture of this is excellent too. The "gold" is a yellow pigmented area covered with a shiny coating. The explainer said that some people speculate that it helps break up the outlines of the pupa and camouflage it.

I asked if the chrysalis of the viceroy butterfly, which, as an adult, resembles the poisonous monarch butterfly, also resembled the monarch's chrysalis. The answer: no, they look completely different. Isn't that interesting? You'd think that the pupa would get at least as much benefit from mimicry as the adult. One possible explanation why not: most pupae don't make it to adulthood anyway, so the marginal benefit to the species from mimicry in the pupal stage is small compared with the benefit in the adult stage. Another: the pupa's main defense, which is not available to the adult, is to be difficult to see; beyond that it doesn't matter much what happens if it is seen. Which is correct? I don't know.

For a long time folks thought that the monarch was poisonous and the viceroy was not, and that the viceroy's monarch-like coloring tricked predators into avoiding it unnecessarily. It's now believed that both speciies are poisonous and bad-tasting, and that their similar coloring therefore protects both species. A predator who eats one will avoid both in the future. The former kind of mimicry is called Batesian; the latter, Müllerian.

The monarch butterfly does not manufacture its toxic and bad-tasting chemicals itself. It is poisonous because it ingests poisonous chemicals in its food, which I think is milkweed plants. Plant chemistry is very weird. Think of all the poisonous foods you've ever heard of. Very few of them are animals. (The only poisonous meat I can think of offhand is the liver of arctic animals, which has a toxically high concentration of vitamin D.) If you're stuck on a desert island, you're a lot safer eating strange animals than you are eating strange berries.

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