After the caterpillar becomes a butterfly, the toxins shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste and may prevent them from ever ingesting the body of the butterfly. In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.

is there such thing as a poisonous butterfly?
yes