Have you seen these caterpillars in your garden?
These are pretty Black Swallowtail butterfly larva that recently hatched on my dill and fennel, which are both primary food plants for this species. Without these specific plants, the caterpillars will die and no butterflies will be produced.
To preserve a colony of black swallowtails in your yard, consider planting a patch of dill or fennel away from the main garden as food for these flying flowers. That way, you can relocate destructive caterpillars found on garden crops to those in the butterfly plot. If you like butterflies, you might also like to read my article Flying Flowers: The Beauty of Butterflies.