Chapter IV

NEW-WORLD MIMICS

Of all the continents South America affords the greatest wealth of butterfly life, and it is in the tropical part of this region that many of the most beautiful and striking cases of mimicry are to be found. Viewed as a whole the butterfly population presents several features which serve to mark it off from that of the other two great tropical areas. In the first place the proportion of gaily coloured forms is higher. Bright red, yellow or fulvous brown contrasted with some deep shade approaching black form the dominant notes. Sombre coloured species are relatively scarcer than in the Oriental and African regions. In the second place when looking over collections from this part of the world one cannot help being struck by the frequency with which similar colour combinations occur over and over again in different as well as in the same groups. Now it is a simple scheme of black with an oblique scarlet band upon the fore wings—now an arrangement with alternating stripes of bright brown and black relieved with patches of clear yellow—now again a scheme of pure transparency and black. Gay and pleasing as are the designs turned out the palette is a small one and invention is circumscribed. Under such conditions it might well be supposed that instances of close resemblance between different species would be numerous, and this in effect is what we find.

As in Asia with its Euploeines and Danaines, and in Africa with its Danaines and Acraeines, so in S. America are the fashions set by two dominant groups of models. These are the Heliconinae and the Ithomiinae, both peculiar to this region and both characterised, like the Old-world Danaids, by slow flight and great tenacity of life. Both live on poisonous plants—the Heliconines on Passifloras and the Ithomiines on Solanaceae. In both groups, but more especially in the Ithomiinae, the species are numerous, and the number of individuals in a species often beyond computation. From the point of view of mimicry these two groups have so much in common that they may conveniently be considered together.

It was from among the Ithomiines, as already pointed out, that the models came for the Pierine mimics of the genus Dismorphia upon which Bates founded the theory of mimicry. Though the Pierine mimics are the most striking the Heliconines and Ithomiines are mimicked by members of other groups. A few Papilios (Pl. X, fig. 8), certain Nymphalines such as Protogonius (Pl. X, fig. 9), Eresia, Phyciodes and Colaenis (Pl. XI, fig. 4), together with various day-flying moths, more particularly of the genera Castnia and Pericopis, are among the well-known mimics of this group of models. The models themselves are very variable in appearance. In one locality the predominant pattern is black with a warm red-brown diagonal bar occupying rather more than a third of the fore wing (Pl. XV, fig. 5), in another it consists of parallel bands of black and fulvous brown with clear yellow patches at the tips of the fore wings (cf. Pl. X, fig. 7), while in yet another locality it is different again. Different localities often have their own peculiar pattern and this affects the various mimics as well as the Ithomiine and Heliconine models.

It is somewhat remarkable that no Satyrine exhibits mimicry in S. America, in spite of the fact that transparency of the wings, as in so many of the butterflies of this region, is quite common in the group. On the other hand the relatively large number of more or less mimetic Pierines is a striking feature of S. America. For the most part they belong to the genera Dismorphia and Perrhybris, and resemble the yellow, black, and brown Heliconines and Ithomiines, though some of the former genus are mimics of the small transparent Ithomiines. Some of the species of Pereute with their dark ground colour and the bright red bar across the fore wing (Pl. XI, fig. 6) resemble Heliconius melpomene, as also does Papilio euterpinus. But some of the most interesting Pierine mimics are several forms belonging to the genus Archonias (Pl. XI, fig. 10) which exhibit the simple and striking arrangement of black, red and white so characteristic of the Swallow-tail Poison-eaters of S. America. They form one of the rare instances of a Pharmacophagus Papilio being mimicked by a butterfly which does not belong to the Swallow-tail group.

As everywhere in the tropics the Papilios of S. America supply a goodly proportion of the mimicry cases. A few, such as P. zagreus (Pl. X, fig. 8), enter into the black-brown and yellow Ithomiine-Heliconine combination; P. euterpinus resembles Heliconius melpomene (Pl. XI, fig. 5); P. pausanias is like Heliconius sulphurea (Pl. XI, figs. 1 and 2). But this practically exhausts the list of Papilios which mimic Heliconines and Ithomiines. The great majority of mimicking Swallow-tails in S. America find their models among the Poison-eaters of their own family, offering in this respect a contrast to those of Asia where the majority of models are among the Danaines and Euploeines, and of Africa where they are exclusively Acraeines or Danaines.

The Poison-eaters of S. America fall into two well-marked groups which we may call the red-spotted and the dark green groups respectively. The red spotted group form a remarkably compact and uniform assemblage. The general ground colour is a deep black-brown (Pl. XI, figs. 8 and 9), the hind wings are almost invariably marked with red near the centre or towards the outer margin, and the fore wing may or may not bear a patch which is generally whitish in the female, though often of a brilliant blue or green in the male. This simple colour scheme with variations runs throughout about three-quarters (some 40 species) of the Poison-eaters. The same general colour scheme is also found in about two dozen species of the unprotected Swallow-tails. As the total number of the unprotected species is placed by Seitz at less than 100 this means that fully one-quarter of them fall into the general colour scheme adopted by the majority of the Poison-eaters. In many cases the resemblance between mimic and model is so close as to have deceived the most expert entomologists before the structural differences between the groups had been appreciated (cf. Appendix II). The matter is further complicated by the fact that polymorphism is not uncommon, especially among the females of the mimetic forms. Papilio lysithous for instance has no less than six distinct forms of female, which differ chiefly in the extent and arrangement of the white markings on the wings, one form lacking them entirely. Several of these forms may occur together in a given locality, and may resemble as many distinct species of Poison-eaters. Thus the three forms lysithous, with white on both wings, rurik, with white on the fore wing only, and pomponius without any white, all fly together in Rio Grande do Sul and respectively mimic the three distinct Pharmacophagus species nephalion, chamissonia, and perrhebus (Pl. XIII). It is worthy of note that mimics are provided by both unprotected groups of Swallow-tails in S. America, whereas in Asia the Cosmodesmus division never provides mimics for Pharmacophagus models (cf. Appendix II).

In the second and smaller group of the Pharmacophagus Swallow-tails the general colour scheme is a more or less dark metallic blue-green with a tendency towards the obliteration of light markings. Some idea of their appearance may be got from the figure of the Central and N. American P. philenor on Pl. XVI, fig. 1. Though one or two unprotected Papilios in S. America fall more or less into this colour scheme, the group, from the point of view of mimicry, is not nearly so important as the red-spotted one.

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.