Becoming a Butterfly

by Barbara Walvoord

Adele Dowell recently found this beautiful caterpillar in her garden on the east campus. It’s going to become an Eastern black swallowtail butterfly (Papilio polyxenes).


That is, if no bird eats it. Caterpillars make up a huge portion of the diet that birds feed their nestlings. They’re full of protein, fat, and other nutrients.

And, that is, if it can pass through several wonderfully complicated stages. This caterpillar has already gone through 4 of its 5 “instars” or stages. At each stage, it outgrows its skeleton or “cuticle,” which is on the outside of its body, protecting the soft organs underneath. As the caterpillar gets too big, cells just under the cuticle release enzymes that absorb most of the cuticle. The remainder slips off.

When it’s ready to form its cocoon, the caterpillar chooses an object such as a stick, and spins a fine thread to attach itself. Then it molts one last time, after which it’s no longer a caterpillar but a pupa that has formed a chrysalis that is usually brown, to make it look like the stick.

And now it stays there for 9-11 days, or perhaps over the winter–that is, if no one destroys or cuts down its stick. One reason we at Lathrop now mow only 1/3 of our fields each year in October is to let lots of cocoons mature.

And then it will emerge as a butterfly.

And the butterfly will, in its turn, find a mate and then lay eggs that will hatch into more caterpillars–that is, if mom can find a host plant that its larvae can eat. Like most insects, butterfly larvae are very picky eaters. Their body chemistry has developed to digest a certain plant and to counteract the poisons that the plant manufactures to fend off predators. We’ve heard a lot recently about the decline in monarch butterflies because of the disappearance of milkweed–the only food that monarch larvae will eat. For black swallowtail larvae, it’s members of the carrot family such as fennel, dill, and Queen Anne’s lace.

And the eggs will hatch into caterpillars–that is, if no wasp or fly eats them.

Nature’s web–complex, beautiful, and full of “if’s.”


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