Danaus plexippus - Monarch Larva
From the Massachusetts Butterfly Atlas:
Monarchs are perhaps best known for the remarkable migrations. "Several generations of Monarchs occur in Massachusetts each season; adults of the summer broods are on the wing for approximately three weeks, while the final, late summer flight of migratory individuals may survive for up to nine months during which time they migrate to the central Mexican highlands and overwinter at several sites in the Transvolcanic Mountains. In March these same individuals begin the northward migration, normally reaching only the southern regions of North America, where they produce the next generation of Monarchs that will continue the migration northward.
As caterpillars, Monarch‘s feed on milkweed plants (Asclepias spp.). Some milkweed species contain heart poisons (cardiac glycosides). This toxic chemical, when ingested, is sequestered by the caterpillar and passed on to the adult butterfly. Experiments have shown that birds attempting to feed on these toxic butterflies react by vomiting and soon learn to avoid this prey. Different populations of Monarchs have varying amounts of poison due to the specific larval hostplant on which they feed. The story of the Monarch also includes its relationship with the Viceroy (Limenitis archippus). Traditionally the Monarch was seen as the model and the Viceroy as the mimic in a classic Batesian relationship in which a non-toxic species gained protection by its look alike adaptations. Recent research, however, suggests that both the Monarch and the Viceroy may be toxic."