How exactly does a caterpillar morph into a butterfly?

SDStaff Doug replies:

The transformation that results in a butterfly — or fly, or beetle, or wasp, or any other insect that undergoes complete metamorphosis, from larva to pupa to adult — is one of the most remarkable stunts pulled in the animal kingdom. Saying it involves “magical happenings” isn’t so far off: it’s an astounding trick, even when you know how it’s done. And, as with most magic tricks, the secret behind it involves some specialized apparatus — a gimmick that’s hidden away from the audience, up the proverbial sleeve.

The first part of the explanation lies in understanding how insects are put together. As is the case with other arthropods, the “skin” of insects is a single layer of cells on top of which they produce a tough covering called the exoskeleton. If you read some of the more popularized stuff about insects, you’ll likely see references to the “chitinous exoskeleton,” with the implication that “chitinous” means “hard.” That’s not accurate: chitin is a polysaccharide (like cellulose) that’s the main constituent of the insect exoskeleton, but it’s not intrinsically hard. For a good idea of what chitin is like, think of the basic white mushrooms you find at the grocery store — it’s pale and soft. Some parts of the exoskeleton are more rigid than others. There are solid plates, called sclerites, but in between these the exoskeleton is soft and flexible; in some life stages — picture a caterpillar here — the soft and flexible part of the exoskeleton makes up pretty much the whole thing. The trick here is that the skin cells of insects release chemicals into the exoskeleton above them that cause modifications of the molecules and the links between them, making some parts of the exoskeleton darker and harder than others.

The second part of the explanation deals with how insects and other arthropods grow larger. The soft parts of the exoskeleton can be stretched as an insect grows, but not the hardened parts. So, what happens is that when a certain limit is reached, the skin cells essentially detach from the exoskeleton above them, and manufacture a new exoskeleton underneath the old one — but the new exoskeleton is kept in a soft, flexible condition. When the new exoskeleton is ready to go, the insect ruptures the old exoskeleton, and either crawls out of that shell, or shucks it off like a pair of pajamas, and then expands its new exoskeleton. Only then will it release the chemicals that cause parts of the new exoskeleton to get darker and harder. It’s this brief vulnerable stage which we can thank for the culinary wonder known as soft-shell crab. Mmm … soft-shell crab. But I digress.

So, far, everything I’ve said pretty much applies to all arthropods — crustaceans, arachnids, insects, etc. Now comes the final part of the explanation, and the thing that makes insects that undergo complete metamorphosis so vastly different from everything else, including other insects and arthropods. When one of these sorts of insects is an embryo, a number of tiny little buds of embryonic tissue, called imaginal discs, separate off from the rest, and just stay there inside the body cavity, doing nothing, all through the larval stages. Those discs are the gimmick hiding up the sleeve. The larva grows, sheds its skin several times, until it comes time to pupate. Underneath the last larval exoskeleton, the skin cells form the exoskeleton of the pupa, and when that last larval skin is shed, the pupa is revealed. In butterflies, the pupa is known as a “chrysalis”, though other insect pupae go by different names, and are different in appearance. Now comes the trick: inside this pupal shell, most of the innards of the insect, including the skin, basically disintegrate, except for those tiny little discs of embryonic tissue. They suddenly go into overdrive, consuming the nutrients and raw materials now floating in the body cavity and dividing, growing, and proliferating at a breakneck pace. They grow into layers of new cells, then forming new organs, finally fusing together to form a new skin. This entirely new skin manufactures an entirely new exoskeleton underneath the shell of the pupal exoskeleton. That new exoskeleton is the body of the adult insect, and — generally speaking — bears hardly any resemblance at all to the larval stages that preceded it. Now you know the secret: it’s not a case of morphing one physical structure into another, it’s a new physical structure altogether, formed by a spare set of embryonic cells that had been hidden away. In essence, insects that undergo complete metamorphosis are born twice (or, at least, reborn), which is pretty amazing and pretty novel. Pretty good trick, huh?

SDStaff Doug, Straight Dope Science Advisory Board

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