Nevertheless the blue-green Pharmacophagus group as represented by P. philenor is supposed to play a considerable part in mimicry in N. America. P. philenor is found throughout the greater part of the Eastern United States, straggling up as far as the Canadian border. On the west it is also found reaching up to North California. Over considerable parts of its range are three other Swallow-tails, belonging to the unprotected Papilios, which are regarded by Professor Poulton and others as mimics of philenor[[22]]. One of these, P. troilus, is dark brown with a dusting of blue scales over the hind wing ([Pl. XVI], fig. 2). The sexes here are more or less alike. Troilus stretches up into North-west Canada some way beyond the limits reached by its model. P. glaucus is a black and yellow Swallow-tail with two forms of female.
One of these resembles the male while the other is darker and is said to mimic philenor. It is known as the turnus form and is found more commonly in the southern part of the range of the species, i.e. in the country where philenor is more plentiful. The third species, P. asterius, has a more southerly distribution. Its female is darker and nearer to philenor than the male. It must, however, be admitted that none of the three species bears a very close resemblance to philenor. It is suggested that this is because P. philenor is a tropical form which has only recently invaded N. America. The crossing of philenor has, as it were, induced the three mimicking Papilios to turn dark, but the model has not been long enough in contact with them for the likeness to become a close one. The explanation, however, hardly accounts for the fact that the best mimic of the three, P. troilus, in which both sexes are dark, is found far north of philenor. Either the dark colour was established without the influence of the Pharmacophagus model, or else the species rapidly extended its range northwards after having been modified under the influence of philenor in the south. But in that case the critic may ask why it does not revert to the original pattern now that it has got beyond the model's sphere of influence. On the whole it seems at present quite doubtful whether any relation of a mimetic nature exists between P. philenor and these three species of Papilio.
P. philenor is also regarded as serving as a model
for two Nymphaline butterflies in the United States. One of these is the large Fritillary Argynnis diana of which the dark female has a markedly blue tint on the upper surface ([Pl. XVI], fig. 3). The other is a Limenitis[[23]] related to our own White Admiral. This form, L. astyanax ([Pl. XVI], fig. 5), is a dark form with a bluish iridescence on the upper surface. It is found, like P. philenor, over the greater part of the Eastern States, while to the north, near the Canadian boundary, its place is taken by L. arthemis with prominent white bar across both wings ([Pl. XVI], fig. 4). There is reason for believing that where the two overlap there is occasional inbreeding, and that the hybrid is the form known as proserpina, resembling astyanax more than arthemis. It must be admitted that in general appearance L. astyanax and Argynnis diana are more like Papilio troilus than P. philenor. In explanation it has been suggested that all the mimics are on the way to resembling P. philenor, and consequently we should expect them at certain stages to shew more resemblance to one another than to the form they have all as it were set out to mimic. On this view they will all arrive at a close resemblance to philenor in time. Another explanation is that favoured by Professor Poulton on which it is assumed that we are here dealing with a case of Müllerian Mimicry, all of the species in question being distasteful with the exception perhaps of A. diana. Thus troilus and astyanax though distasteful are less so than
philenor. Hence it is of advantage to them to have even a chance of being mistaken for the more obnoxious philenor, and so the one has come from the black and yellow Swallow-tail pattern and the other from the white-banded arthemis form to what they are, i.e. more alike to one another than to philenor. They now form a Müllerian combination for mutual protection along with the dark females of glaucus and asterius. But they are themselves still moderately distasteful so that it is to the advantage of the female of Argynnis diana to mimic them. Whether they are all on the way to resembling philenor more closely, or whether they have sufficiently vindicated their inedible properties and are now stationary, it is for the future to reveal to posterity. Lastly we have the view that these different species have attained their present coloration entirely independently of one another, and that we are not here concerned with mimicry at all. Since the sole evidence available at present is that based on general appearance and geographical distribution, the view taken of this case must rest largely upon personal inclination.
Though the cases just quoted are only very problematically mimetic, N. America has yet several examples of resemblance between distantly related forms as close as any that occur in the tropics. In this region are found two species of the genus Danais—D. archippus occurring all over the United States and reaching up northwards into Canada, D. berenice found in the South-eastern States, e.g. in Florida, where it is said to be more abundant than archippus.
D. archippus ([Pl. XVI], fig. 8) is very similar to the oriental D. plexippus ([Pl. IV], fig. 2), from which perhaps its most notable difference lies in the extent and arrangement of the white spots near the tip of the fore wing. D. berenice is not unlike archippus in its general colour scheme but is smaller and darker ([Pl. XVI], fig. 9).
We have already had occasion to mention the common Nymphaline, Limenitis arthemis ([Pl. XVI], fig. 4) which is found in Canada and the Northeastern States. Widely spread over N. America is a close ally of this species, L. archippus, which, though so similar in structure and habits, is very different in external appearance. As appears from [Pl. XVI], fig. 6, L. archippus is remarkably like the Danaid which bears the same specific name. In the Southern States L. archippus is replaced by a form slightly different in details of pattern and distinctly darker, L. floridensis (= eros) ([Pl. XVI], fig. 7). In Florida occurs also the darker N. American Danaid, D. berenice, to which the colour of L. floridensis approximates more than to D. archippus, and it is of interest that although the last named is also found in this locality it is said to be much less abundant than D. berenice. Nevertheless it appears to be true that the range of L. floridensis is much more extensive than that of its model; in other words, that there are considerable regions where L. floridensis and D. archippus coexist, and from which L. archippus and D. berenice are wanting.