These butterflies have black wings with greenish-yellow spots on their borders. The butterfly larvae incorporate the plant's toxins into their bodies, where they remain even when the larvae develop into butterflies and no longer eat Aristolochia. In this way, the two species together form a larger population that birds associate with an unpleasant experience, which convinces them not to eat these butterflies.
Nature never fails to take advantage of such tricks.
This kind of mimicry is called Batesian.