DANAINE BUTTERFLIES The figures on Plate I. illustrate facts consistent with the belief that mimicry is an advantageous resemblance and has been developed for its own sake—facts which are meaningless on any suggested interpretation except one based on selection. The but terflies most generally mimicked by species of other groups and by day-flying moths belong to a few dominant tropical sub-families, the Danainae and Acraeinae of the Old World, and, in the New, the Ithomiinae, allied to the first, and the Heliconinae allied to the second, together with very few Danainae and many Acraeinae. Now, however widely the types of pattern vary within each of these sub-families, they are still mimicked by butterflies of other groups and often by moths. The South American Acraeas have very different patterns from those of the African Acraeas, yet both are extensively mimicked. The most striking evidence is, how ever, supplied by the divergent colours and patterns of the Danaine models, of which a few examples are figured. The Oriental Hestia and its allies are large, black-and-white butterflies with thin papery wings. A characteristic example (Hestia leuconoe) from the Phil ippine islands and its swallowtail mimic (Papilio idaeoides) from the same locality are shown in the fourth row. In this and the fol lowing pair it must be borne in mind that the mimic is but a single example selected to represent many butterflies of different groups, and also day-flying moths, which mimic the same type of pattern in various parts of the Oriental region. In top row is the male of Euploea midarnus, an example of a far more dominant and widespread Danaine type, that of the blue Euploeas, at its right is its swallowtail mimic (Papilio paradoxes). The originals of these two figures were both taken in the same part of the Malay peninsula. In Africa the Danaines are much less numerous but still supply the chief models for mimicry. The majority of the species belong to the genus Amauris, black butterflies with white or yellow ish markings, very unlike the Oriental Danainae.
An entirely different type of Danaine pattern (third row, left) is borne by a series of nearly related species in the Oriental region, which must be regarded as the original home. This pattern is of especial interest for the present argument, because one species bearing it has migrated westward and become the chief model for mimicry in Africa, while two others have travelled eastward and become the ancestors of American species mimicked by indigenous American butterflies. In the third row is the best-known invader,
Danaus plexippus, the monarch, the figured specimen, together with its mimic, the viceroy, having been caught near Chicago by the writer on Aug. 5, 1897. The mimic shown is a Nymphaline butterfly, Basilarchia archippus, closely related to the British white admiral and itself so recently descended from a North American white admiral that the two will interbreed and produce hybrid offspring. In all the earlier stages of its life-history the mimic remains a white admiral, the mimetic resemblance being restricted to the final colour and pattern. If, as some have supposed, mimetic likeness is the result of local influences, we should expect that the invader would have come to resemble the native rather than that the native should mimic the invader.
The above facts point directly to the conclusion that there is some advantage in mimicking Danaine butterflies, and, if space permitted, similar evidence could be brought forward to show that the widespread mimicry of the other great tropical groups is also advantageous. The proof becomes especially convincing in certain butterflies with many forms of female. Thus, three Uganda females of the African swallowtail Papilio dardanus mimic respec tively three very different Danaine patterns, and the fourth an Acraeine ; and in other parts of Africa where the Danaine patterns alter, and one of them or the Acraeine model is absent, we find that the corresponding female is similarly modified or wanting. In another African swallowtail, Papilio cynorta, the females mimic an Acraeine, changing geographically with the changes of the model. But in Abyssinia, where there is no appropriate model of this group, the female mimics a Danaine. All the local females of P. dardanus have often been bred, together with the non-mimetic males, from the eggs laid by a female of one form. From this it has been argued that they must have arisen suddenly from a female like the one found in Madagascar and Abyssinia and re sembling the male. In recent years, however, numbers of inter mediate females have been discovered and also bred in the Nairobi district. Furthermore the mimetic females which exist with the male-like females in Abyssinia, retain the "tails" on their hind wings which elsewhere have been lost by the fully formed mimics of the tailless Danaine and Acraeine models.