A Monarch butterfly caterpillar (Danaus plexippus) on a milkweed (Asclepias sp.) leaf. Monarch butterflies lay their eggs primarily on milkweed plants. By eating the milkweed, the caterpillars sequester biologically active molecules, cardiac glycosides or cardenolides, which make them poisonous to vertebrates. The cardiac toxins continue in the pupa and the adult butterfly. Both the adult and the larva are aposematically colored to warn predators away from eating them.