takes place inside the chrysalis, after it has formed?
What is happening is a biological miracle going on inside that caterpillar. Enzymes are being released that digest all the caterpillar tissue, so that the caterpillar is being converted into a rich culture medium.
Inside the caterpillar are several sets of little cells that are in different parts of the body and they're called "imaginal disks." These are really like little groups of embryonic cells. And as soon as the metamorphosis gets going and as that chrysalid forms the skin is shed off the larva, and now the larva has turned into a chrysalid. These little cells start growing like crazy. And one imaginal disk will become a wing (so there are at least 4 imaginal disks because there are 4 wings in the butterfly). There are imaginal disks that form the legs, the antennae and all the organs of the adult butterfly.
And so inside that chrysalis, during the first 3-4 days is literally a bag of rich fluid media that these cells are growing on. And so the transformation of metamorphosis goes. Nothing likes this happens in vertebrates — ever. It's a phenomenon of insects and it truly is a miraculous biological process of transformation.
These little groups of cells that start developing very early in the caterpillar's life but then they stall, and so they're just in there waiting, and they don't start growing until the very end of the 5th instar (the last caterpillar stage). Then they start growing really rapidly and differentiating into the different tissues, so that literally the entire internal contents of the caterpillar — the muscles, the entire digestive system, even the heart, even the nervous system — is totally rebuilt. It's like you took your car, you took a Ford into the shop and left it there for a week and it came out as a Cadillac.
During the development of the adult, the chrysalsid loses nearly half of its weight. If you were to weigh a chrysalid 3 days after it formed, and then weigh the adult about 24 hours after it emerges, it would have lost nearly half its weight. This shows that the process of metamorphosis consumes a tremendous amount of energy. (Some of the weight would be water, of course.) During whole time it's a chrysalis it can't excrete or defecate, so all of the waste products accumulate. You may have noticed a reddish-colored liquid under the adult after it emerges. This is the nitrogenous waste that has accumulated the whole time during metamorphosis.
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The monarch "changes its ecological niche entirely when it transforms from a caterpillar to an adult butterfly," says Dr. Brower. "They are two ecologically different organisms, as distinct as a field mouse and a hummingbird."