Nightly frosts brush the garden, but botanical energy continues to surge. Carrots, lettuces, and chard are all thriving in the cool sunshine. Black swallowtail butterflies have tapped this energy, laying eggs on the carrot leaves. The caterpillars fall into an inert stupor during the cold nights, but turn into animated single-minded leaf-munchers when the sun touches their skin. They are all in their last stage of molt. The next step will be transformation into overwintering pupae.
For reasons unknown to me, Linnaeus named many swallowtails for prominent figures in Homeric/Greek poems. The black swallowtail is named (polyxenes) for Polyxena, the daughter of Priam, King of Troy. She does not appear in the Iliad but other accounts tell that her death marked the end of the Trojan War. The connection between these tales and North American butterflies seems tenuous, at best. But Linnaeus invented the rules of nomenclature, so he could do as he pleased.
Our caterpillars need not fear Achilles and, with luck, the birds will not find them.
David George Haskell 
Great pictures!
Thank you !!
Fountainpen
A year ago two of them totally demolished my parsley plants. They are gorgeous and I
enjoyed watching them. Have not seen any this year.
I keep an area of fennel for my swallowtail larvae. They come back after being eaten down to nubbins, are perennial, and seem to be preferred to others in the carrot family. Not bad on the grill, either! Our biggest predator here, FL, is a wasp that eats them whole.
Thank you for the beautiful photos and the writing prompt for a new poem (or thought experiment): What link did Linnaeus see between the Swallowtail and Polyxena? Good food for thought for today’s munching.
I’ve never had black swallowtail larvae this late in the year. Like Rose, I have a clump of fennel that plays host to them year after year, but none this year. Another difference: your late instar larvae are predominantly black. Are your early season late instar larvae just as dark? The ones I see here are much greener. The black is restricted to a narrow band on each segment. It could be geographic variation, seasonal variation or polymorphism. Do you have any ideas?