The Passing of the Butterflies

THE CONTRIBUTORS’ CLUB.

THE migratory habit of our birds has been so long a matter of observation and of precise record that it is one of the most commonly known facts in natural history ; we expect the appearance and disappearance of each kind at stated times, and in their local lists our naturalists distinguish between the “summer residents” and the “ winter residents.” It is quite otherwise with other winged creatures, such as our butterflies ; and yet they also appear and disappear at stated periods, and that, too, not simply when winter intervenes to check all their activities, but within the summer itself,—a particular kind appearing say in June, gradually diminishing in numbers, and then disappearing altogether, only to burst upon us again in August in numbers quite as large as before. Butterflies are far shorterlived creatures than birds, and this fluctuation in their numbers, this yearly double advent on the stage, is easily explained, and is known to be caused in many cases by the fact that in such cases two (or it may be more) generations of butterflies are produced the same season, the parents dying before the young are matured.

This is known to be the case in so many instances that it has been taken without question as a perfectly simple explanation of all. The reasoning is natural, but is it justified ? Undoubtedly it was the passage of birds in flocks which first drew attention to their migrations, but we now know that many birds migrate which may be said never to be seen migrating, so stealthily and in such small numbers do they appear and disappear. Now butterflies are not social, like birds, and do not company in pairs, so that, apart from their lesser conspicuity, their migrations, if any, would be far less readily detected than those of the least known birds. That their powers of flight would suffice for a very considerable migration is unquestionable ; butterflies have repeatedly been taken hundreds of miles from land. The prevailing winds of spring and autumn would aid their movements. That in many parts of the world vast swarms of single species have been seen moving steadily in a specific direction, not always with the wind, is on record ; the great swarms of the thistle butterfly which in 1879 invaded Europe, even pushing as far north as Finland, were believed to have come from Africa.

Let us stop a moment to consider to what the migration of animals is in general due. Food and temperature are without doubt the direct agencies. Just as the secular ebb and flow of the ice of glacial times caused the winged inhabitants of our Northern States to move slowly northward and southward, driven from the fields the struggle for existence had compelled them to venture upon, only again and again to occupy and lose them, so now the advancing and retreating winter’s cold, on a lesser scale, plays annually the same part. Be fruitful and multiply and occupy the land is the law to each species.

The physical conditions being nearly the same for both birds and butterflies, why may we not look to find among the latter some instances, at least, of that same instinct for migration so pronounced in the former ? Now we have among our butterflies one kind in which, from certain features in its history and structure, we might more reasonably look for such a phenomenon than in most others ; and could it be proved of this, the way would be opened for future inquiries concerning many others. It is one of the commonest and showiest butterflies of the United States, — our milkweed butterfly, well known to every boy collector. Not one of our butterflies exceeds it in powers oE flight. Within our knowledge, it has spread from this continent over the Pacific Ocean to Australia and Java, and eastwardly to the Atlantic borders of Europe, from England to Portugal and the Azores. This may be, probably has been, due in part to accidental transportation by vessel, but its spread among the Pacific islands shows that a flight of five hundred miles is easily accomplished. Again, it belongs to a distinctively tropical group of butterflies, and is itself par excellence a tropical insect, and its appearance at all in our Northern States is almost an anomaly yet every season it is found in Canada, and has even been credibly reported from Hudson Bay and the Athabasca region beyond. Further, its history is known sufficiently to leave no possible doubt that in our winter season it exists only in the perfect stage, or butterfly. Now, nearly if not quite every one of our hibernating butterflies has been found at one time or another in its winter quarters in crevices, under eaves, in sheds and garrets, in old walls and hollow trees ; yet this butterfly is twice as large as, and far more conspicuous in color than, any one of them, and has never been detected in hibernation. Butterflies of this species which have passed the winter in the extreme South, which they do, for aught we know to the contrary, upon the wing, are invariably dull colored and battered (as are hibernating butterflies of all kinds), and even in flight can readily be distinguished from the butterfly from the same season’s chrysalis. If the butterfly lived through the winter in our northernmost States, we ought to see such individuals in the spring, but no instance of their occurrence in Canada has ever been recorded, and two or three individuals at most have ever been heard of in New England. In New England, our ordinary hibernating butterflies come out in March, April, and early May, but the milkweed butterfly does not appear until June, and then in fine livery.

Where, then, do these June butterflies come from ? Why may they not have flown hither from the South ? We know that the opposite movement takes place. Enormous flocks of the milkweed butterfly have repeatedly been recorded, containing myriads upon myriads of individuals, clustering at nightfall upon trees to such an extent as to change their color and to bend the weaker twigs. These clusters invariably occur in the autumn, and are accompanied by movements en masse in the daytime, which have also been many times recorded, and are invariably found to take a southern direction. The first of our naturalists to collect the evidence for this migration remarks, “ There is a southward migration late in the . . . season in congregated masses, and a northward dispersion early in the season through isolated individuals.” This, I believe, expresses precisely the state of the case, though the northward movement has not been so definitely determined as is necessary to conviction, nor have the limits within which the butterfly continues on the wing throughout the winter (as it is reported to do in Florida) ever been at all determined ; but the facts of its history, too numerous even to summarize here, render it highly probable that the hibernating butterflies fly northward in the spring, depositing their eggs upon the way, some here, some there, up to about the latitude of New York. To accomplish this extreme distance from Florida, the butterfly would have to fly no faster than a man walks, counting only the daytime, and deducting half of that for bad weather. The progeny of these, when matured, not only raise another brood in the same territory, but fly still further north to New England and Canada and lay their eggs. This second brood of the season’s butterflies are the only ones that are ever reared in the northernmost regions ; and in the latter part of the season they congregate and migrate to or beyond the region where their parents were born, as do the young birds. Taking this view, we may regard northern examples of this butterfly, indeed probably those of most of the country, as mere interlopers, vagrant tropical butterflies feeling their way northward for wider pastures, immigrants seeking new homes only to retrace their steps with the close of the year.