National Geographic : 1937 May
BUTTERFLY TRAVELERS Some Varieties Migrate Thousands of Miles BY C. B. WILLIAMS Chief Entomologist, Rothamsted Experimental Station, Harpenden, England M ANY people believe that all butter flies live but a few days, and that they keep quite close to the local ity where they hatched from a chrysalis. This is true of most species, but there are others which live for weeks, sometimes for months, and instead of fluttering around they may set off in a definite direction and fly some hundreds, or even thousands of miles from their birthplace before settling down to lay their eggs. This habit of changing location, or migra tion, has been known to occur in birds and locusts since ancient times, and has been suspected for about a century in the butter flies and moths. The Cotton Worm Moth of the southern United States (Plate VI, figure 10) was one of the first in North America to come under suspicion. Today the habit is also known among some dragon flies and beetles, particularly the ladybirds, and more rarely in other groups of insects. The butterflies may migrate singly or in large numbers. Flights estimated to con tain more than a thousand million indi viduals have been recorded. The sight of one of these butterfly movements, the in sects passing for hours and even days, steadily pressing on in one direction, is an event in the life of any naturalist.* EVIDENCE LIKE A JIGSAW PUZZLE By piecing together scattered and incom plete information, much as one might try to fit together a jigsaw puzzle of which most of the pieces have been lost, we begin in a few cases to have some idea of the ex tent of the movements; of where the butter flies start, what route they take, and where they come to rest. By far the best known of the migrants is the Monarch or Milkweed Butterfly (Danaus plexippus, Plate IV). This mag nificent insect has its headquarters in North America and has spread, chiefly in historic times, to the Cape Verde Islands * See, in THE NATIONAL GEOGRAPHIC MAGAZINE, "Butterflies-Try and Get Them," by Laurence Ilsley Hewes, and "Who's Who Among the Butter flies," by Austin H. Clark, May, 1936; also "Strange Habits of Familiar Moths and Butter flies," by William Joseph Showalter, July, 1927. and Madeira in the Atlantic, and to most of the islands of the Pacific. It is said to have reached New Zealand about 1840 and appeared in Australia about 1870. In both of those countries it is now established. In the past sixty years nearly a hundred individuals have been seen in Great Britain and a much smaller number in France and Portugal. Nearly all these were observed in the autumn. The food plant, milkweed (Asclepias sp.), does not exist wild in Europe, so the butterfly has never become established there. It is not yet known for certain whether the European specimens have flown across the Atlantic, assisted by the prevailing westerly winds, or have been carried across in ships. In North America this butterfly is found during the summer throughout the United States and Canada as far north as Hudson Bay and, in the West, occasionally as far as Alaska. In the early autumn, the butter flies congregate into bands and fly south ward, starting from Canada about the end of August and reaching the Gulf States about the beginning of November. On the west coast they do not go so far south, and may winter in the neighborhood of San Francisco. Having reached the end of their south ward flight, the butterflies settle on trees, still keeping to their large bands, and spend the winter in a state of semi-hibernation. They flutter around a little on fine warm days and in cold weather creep closer to the shelter of the trees. The same group of trees may be used year after year by hibernating Monarchs, although the same individuals never return south a second time. One of the localities, on Point Pinos on Monterey Bay, Cali fornia, is a show place for visitors. In the spring the bands begin to break up, and the butterflies fly northward indi vidually, pausing here and there to lay eggs as they go. They start about March, reach the level of West Virginia about April, and Canada at the end of May or early June. The return flight starts after about three generations in the middle States, two in the North, and after a single generation in Canada.