Sure enough our parsley has a Black Swallowtail Caterpillar on it, Papilio polyxenes.
Younger instars lack their primary defense and instead focus on mimicking bird poop.
One size up presents a different story. Black Swallowtail Caterpillars have a horn that pops out of their head which sprays foul smelling formic acid. Upon being clutched in a bird's mouth they aim that stuff right for the eyes! In older caterpillars the horn is long enough to be pressed right into the bird's eye where they rub the acid all around. Birds quickly learn to not eat them.