Millions of monarch butterflies arrive each year in Mexico after travelling, in some cases, thousands of miles from the United States and Canada.

A Monarch butterfly (Danaus plexippus) is pictured at a butterfly farm in the Chapultepec Zoo in Mexico City on April 7, 2017.
Millions of monarch butterflies arrive each year to Mexico after travelling more than 4,500 kilometres from the United States and Canada. / AFP PHOTO / Pedro Pardo (Photo credit should read PEDRO PARDO/AFP via Getty Images)