An adult butterfly seems a fragile creature to endure the long cold months of arctic regions. Yet many of our most beautiful species habitually hibernate as adults, finding shelter in such situations as hollow trees, the crevices in rocks, the openings beneath loose bark or even the outer bark on the under side of a large branch. It is significant that most of the adult-wintering Angle-wings are northern rather than southern species, some of them being found in arctic regions practically around the world. One of the few southern forms that hibernates as an adult is the Goatweed Emperor.
These examples are all cases of true hibernation in a lethargic condition. There are certain butterflies, however, which pass the winter as adults that remain active during this period. Obviously this is impossible in latitudes where the winter is severe, and it involves migration to a warmer climate. The one notable illustration of this is the Monarch butterfly which apparently flies southward to the Gulf states at least and there remains until spring, when individuals come north again. The southward migration may be begun in Canada when the butterflies gather together in enormous flocks that remind one of the gathering of the clans with the migrating birds. This is one of the least understood of insect activities but it has been observed so often and over so long a period of years that there seems to be no questioning the general habit.
Like everything else in relation to living things there are numerous variations in the prevailing modes of hibernation. In the case of many species one can find combinations of two or more stages in which the winter is passed. Probably if we could observe with sufficient care we might be able to find somewhere examples of almost any conceivable double combination—as egg with larva or chrysalis or adult—the insect hibernating in two of these stages. Many examples are known in which both chrysalis and adult of the same species pass the winter and also of those in which young and well-grown larvae pass the winter. As one would expect, the conditions as to such combinations are likely to be more variable in southern than in northern regions.
Notwithstanding all the attention which has been paid to butterfly life-histories there is still some uncertainty in regard to the hibernation of many of our species. One of the most interesting series of observations which a young naturalist could undertake would be to learn positively how each species of butterfly in his locality passes the winter.
Aestivation or Summer Lethargy
In some species of butterflies there is a special adaptation to passing through the hottest part of the summer season in a state of lethargy which is suggestive of the torpor of the hibernating period. This phase of butterfly existence has not been extensively studied and there are indications that it exists more generally than has been commonly supposed. It has been noticed even in northern New England that some of the Angle-wings seek shelter and become lethargic during August. Apparently this is an adaptation to single broodedness, helping to carry the species through the year without the exhaustion incident to the continued activity of the butterfly.
In more southern regions, especially in the hot, dry climates where vegetation withers in midsummer, it is well known that some caterpillars become lethargic, remaining inactive until the fall rains start vegetation into growth. The Orange-sulphur butterfly is a good example of this.
This summer lethargy offers excellent opportunity for careful study. Any observer who finds a butterfly hidden away in summer under boards, the bark of a tree, or in a stone pile should look carefully to see what species it is and how the butterfly behaves. Such observations should be sent to the entomological journals in order that our knowledge of the subject may be increased.
The fact has long been noticed that various butterflies have the habit at times of feigning death and dropping to the ground where they may lie motionless for a considerable period. This habit is most easily observed in some of the Angle-wings, especially those which hibernate as adults. Those species have the under surfaces of their wings colored in various bark-picturing patterns and apparently live through the winter to some extent, resting beneath the bark of large branches or upon the trunks of trees. Many of them also secrete themselves in hollow trees or beneath loose bark or in board piles or stone walls. It is probable, however, that during the long ages when these insects were adapting themselves to their life conditions, before man interfered with the natural order and furnished various more or less artificial places for hibernation, these butterflies rested more generally upon the under side of branches than they do now.