MIGRATION OF LEPIDOPTERA

Abstract

LARGE-SCALE movements of butterflies have long attracted attention, as when in A. D. 1100 it was recorded, appropriately enough in a “History of Calamities”, that swarms of butterflies were seen passing from Saxony to Bavaria. o In 1508 “an innumerable swarme of whit butterflyes coming so thick as flakes of snow” was noted at Calais. It is clear that dense flocks, such as were common in the towns along the south coast of England in 1939-40, are not the result of modern conditions. The keen interest of Dr. C. B. Williams has led to much close study of the subject, on which an important paper has recently been published*. Dr. Williams claims that these movements are not a mere exodus from an area of food scarcity to a land of plenty, but a real migration in the sense that they are comparable with the familiar to and fro seasonal movements of birds. Evidence to this effect is particularly striking in the case of Danaus plexippus, the Milkweed butterfly or Monarch. This well-known species, of North, Central, and South America, occurs in different races with different, though overlapping, geographical distribution. The northern and southern races undertake large migrations, the central one does not. The northern race, p. plexippus, appears to have spread over a large part of the globe, and is the one which is occasionally captured in England. Its movements have been closely studied in the south-eastern United States, and it is clear that there is a definite northward movement in the spring, the last butterflies leaving Florida when the mean temperature rises above 75° F., and arriving in the north (reaching well into Canada) after the mean exceeds 50° F. There is definite evidence of their absence from Florida in the summer: they begin to return with corresponding temperatures.