One of the most impressive sights in the insect world must be the mass migration of monarch butterflies (Danaus plexippus) each autumn. In flight they appear as a flickering orange cloud and when they land on a tree they can completely obliterate its shape. Hundreds of millions of monarchs make an incredible journey of up to 4800km from eastern United States and Canada to the mountain fir forests of Mexico. How they find the overwintering site is one of nature's unsolved mysteries. The great-grandchildren of butterflies which flew north the previous year return to the same site and sometimes to the same tree as their great-grandparents.
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