Each year, a hundred million North American monarch butterflies complete a timeless ritual: a mass migration from south to north in the spring, and from north to south in the autumn. The journey, up to 3,000 miles in either direction, is too long for any one butterfly to complete in its short lifespan. How, then, do these delicate creatures weighing less than a gram each find their way from the oyamel fir forests of mexico to as far as Canada and back?
