I swear that I've never seen a greater variety of butterflies in my life than I have in the nearly two years we have lived in Japan. (During our trip to Hokkaido I counted at least six different varieties while sitting in one spot for two hours.) Black, black and white, yellow, orange, black with blue, and ranging from about an inch to those that look like bats or small birds fluttering by. It's been another of the unexpected pleasures of living here.
Last year these fat little fellows trimmed my fennel plants, and this year they've arrived to keep the parsley in the west wall bed in check. (Truth be told, they're straight up devouring it and we are parsley-less.) My minimal knowledge and experience with these creatures told me this could be something rather exciting. A short search for "black and yellow caterpillar" images has me believing they are swallowtails, but I'm not entirely sure if they are Old World Swallowtails or if they are Black Swallowtails. Regardless, while I await word from a professional they will keep munching and I'll be glad for the company in the garden.