Darwinism (1889) Part 18

We have hitherto considered, that mimicry could only occur when a comparatively scarce and much persecuted species obtained protection by its close external resemblance to a much more abundant uneatable species inhabiting its own district; and this rule undoubtedly prevails among the great majority of mimicking species all over the world. But Mr.

Bates also found a number of pairs of species of different genera of Heliconidae, which resembled each other quite as closely as did the other mimicking species he has described; and since all these insects appear to be equally protected by their inedibility, and to be equally free from persecution, it was not easy to see why this curious resemblance existed, or how it had been brought about. That it is not due to close affinity is shown by the fact that the resemblance occurs most frequently between the two distinct sub-families into which (as Mr.

Bates first pointed out) the Heliconidae are naturally divided on account of very important structural differences. One of these sub-families (the true Heliconinae) consists of two genera only, Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less than sixteen genera; and, in the instances of mimicry we are now discussing, one of the pairs or triplets that resemble each other is usually a species of the large and handsome genus Heliconius, the others being species of the genera Mechanitis, Melinaea, or t.i.th.o.r.ea, though several species of other Danaoid genera also imitate each other. The following lists will give some idea of the number of these curious imitative forms, and of their presence in every part of the Neotropical area. The bracketed species are those that resemble each other so closely that the difference is not perceptible when they are on the wing.

In the Lower Amazon region are found--

{ Heliconius sylvana.

{ Melinaea egina.

{ Heliconius numata.

{ Melinaea mneme.

{ t.i.th.o.r.ea harmonia.

{ Methona psidii.

{ Thyridia ino.

{ Ceratina ninonia.

{ Melinaea mnasias.

In Central America are found--

{ Heliconius zuleika.

Nicaragua { Melinaea hezia.

{ Mechanitis sp.

{ Heliconius formosus.

{ t.i.th.o.r.ea penthias.

Guatemala { Heliconius telchina.

{ Melinaea imitata.

In the Upper Amazon region--

{ Heliconius pardalinus.

{ Melinaea pardalis.

{ Heliconius aurora.

{ Melinaea lucifer.

In New Grenada--

{ Heliconius ismenius.

{ Melinaea messatis.

{ Heliconius messene.

{ Melinaea mesenina.

{ (?) Mechanitis sp.

{ Heliconius hecalesia.

{ t.i.th.o.r.ea hecalesina.

{ Heliconius hecuba.

{ t.i.th.o.r.ea bonplandi.

In Eastern Peru and Bolivia--

{ Heliconius aristona.

{ Melinaea cydippe.

{ (?) Mechanitis mothone.

In Pernambuco--

{ Heliconius ethra.

{ Mechanitis nesaea.

In Rio Janeiro--

{ Helieonius eucrate.

{ Mechanitis lysimnia.

In South Brazil--

{ Thyridia megisto.

{ Ituna ilione.

{ Acraea thalia.

{ Eueides pavana.

Besides these, a number of species of Ithomia and Napeogenes, and of Napeogenes and Mechanitis, resemble each other with equal accuracy, so that they are liable to be mistaken for each other when on the wing; and no doubt many other equally remarkable cases are yet unnoticed.

[Ill.u.s.tration: FIG. 25.--Wings of Ituna Ilione, female. Wings of Thyridia megisto, female.]

The figures above of the fore and hind wings of two of these mimicking species, from Dr. Fritz Muller's original paper in _Kosmos_, will serve to show the considerable amount of difference, in the important character of the neuration of the wings, between these b.u.t.terflies, which really belong to very distinct and not at all closely allied genera. Other important characters are--(1) The existence of a small basal cell in the hind wings of Ituna which is wanting in Thyridia; (2) the division of the cell between the veins 1_b_ and 2 of the hind wings in the former genus, while it is undivided in the latter; and (3) the existence in Thyridia of scent-producing tufts of hair on the upper edge of the hind wing, while in Ituna these are wanting; but in place of them are extensible processes at the end of the abdomen, also emitting a powerful scent. These differences characterise two marked subdivisions of the Danaoid Heliconinae, each containing several distinct genera; and these subdivisions are further distinguished by very different forms of larvae, that to which Ituna belongs having from two to four long threadlike tentacles on the back, while in that containing Thyridia these are always absent. The former usually feed on Asclepiadeae, the latter on Solanaceae or Scrophulariaceae.