Fig. 15. Cærois chorinæus, from the lower Amazon, in its
resting attitude. V, anterior wing. H, posterior wing. mr,
midrib of the apparent leaf. sr, lateral veins. st, hint of
a leaf-stalk.
Quite different from the leaf-marking either of Cœnophlebia or Kallima is that of one of the Satyrides of the lower Amazon valley, Cærois chorinæus (Fig. 15). If one spreads this butterfly out in the usual way it does not look in the least like a leaf, and one only sees a number of curiously placed disconnected stripes on the under surface of the wing. But if the wings be folded together to correspond with the sitting position of the butterfly, there appears the figure of a leaf, of which, however, only half is present, and whose midrib (mr) runs obliquely forward from the inner angle of the posterior wing. Here, again, it is not difficult to guess that this straight stripe has arisen, by displacement and straightening, from a curved line inherited from some remote ancestor, and it is these precise changes which are the work of the adaptive processes of natural selection. The same applies to the lateral ribs (sr), which are here four in number.
But even the division of the wing surface by a single dark line, such as that which crosses the middle of the posterior wing of Hebomoja ([Fig. 9]), an Indian butterfly, heightens not inconsiderably the resemblance of the resting butterfly to a leaf, a resemblance which has already been shown in the form and colour. Indeed, even the sharp division of the wing surface into a darker inner and a lighter outer portion, which occurs in many species of Anæa, gives a very vivid impression of a leaf crossed by a midrib.
It is not without a purpose that I have lingered so long over the leaf-butterflies. I wished to make it clear that we have by no means to do with a few exceptional cases, but with a great number, in all of which resemblance to a leaf has been aimed at, although it has been attained in varying degrees, and by very diverse ways. Whoever surveys this wealth of fact must certainly receive the impression, that, wherever it was advantageous to the existence of the species, the evolution of such a deceptive resemblance has also been possible. In any case one cannot but be convinced that it is not a case of chance resemblance, as some naturalists have recently tried to maintain.
But I have not yet quite finished my outline-survey of the facts, for I must not omit to mention that, in the evergreen tropical forests, there are also large nocturnal Lepidoptera, which mimic leaves, sometimes green ones, sometimes brown, dead ones.
Fig. 16. Phyllodes ornata, from Assam.
Upper surface with leaf-like marking only
on the anterior wing, which is the only
part visible when at rest; 2/3 nat. size.
Fig. 16 gives a good picture, reduced to two-thirds, of such a species, Phyllodes ornata, from Assam. The posterior wings are conspicuously coloured in deep black and yellow; in the resting position they are covered by the anterior wings, and these are red-brown with black markings which precisely and clearly mimic the ribs of a leaf. The midrib begins near the tip of the anterior wing, but breaks off half-way across the wing at two silvery white spots, similar to those in many of the diurnal forms, which also mimic decaying leaves. Three pairs of side veins go off backwards and forwards with remarkable regularity from the midrib, almost at the same angle, and parallel to one another, and three more are indicated by vague shading. Then the midrib begins again in the internal half of the wing, though only represented by a broad shading. The whole suggests two torn, rotten leaves, one partly covering the other; and the deception will certainly be perfect when the moth rests on the ground or among decaying leaves.
That all these extremely favourable protective colorations find their explanation in the slow and gradually cumulative effects of natural selection cannot be disputed; it is beyond doubt that they cannot be explained, so far as we know, in any other way.