Note 22 — Chapter IV

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.