There were quite a few of these colorful caterpillars on the dill which was growing unexpectedly and very late in the season in our small garden -- long after the tomatoes and peppers had been harvested and enjoyed by Grenville and myself.
And, just as unexpectedly and late, there were up to a dozen of these caterpillars on nearly every available dill stalk.
When these larvae mature, they become eastern black swallowtail butterflies. The host plant is any of the carrot family: dill, parsley, fennel and others. The adult female lays fertilized yellow eggs on the host plant. The stages (instars) feed voraciously on the host plant as they mature. Many dill plants were already stripped of their leaves.
This is a younger version of this caterpillar larvae. Its "skin" is armor-like with spikes of bright orange. Several of these were munching on parsley. Two weeks earlier, I saw over a dozen of these in the dill and parsley.; all were gone by morning, perhaps as a bird treat?
Later, I read that not seeing a black swallowtail chrysalis is not unusual; the instars hide in nearby vegetation, then later string up and morph into a chrysalis and then a butterfly.