We must first consider the case of our lappet moth (Gastropacha quercifolia), which in its copper-red colour and in the remarkable shape and dentated edges of its wings, and finally in the quite extraordinary clucking-hen-like attitude of the wings when at rest, greatly resembles some dry oak-leaves lying one above the other.
Not unlike this is a 'shark' moth found in this country, Xylina obsoleta, which, as the name indicates, looks when at rest like a broken bit of half-rotten wood ([Fig. 10], p. 77). It 'feigns death,' as we commonly say, that is, it draws the legs and antennæ close to the body, and does not move; indeed, one may lift it up and throw it on the ground without its betraying by a single twitch that it lives. Only after it has been left undisturbed for some time does it show signs of life again, and makes off hastily, to find a better hiding-place. The colouring of this moth is so curiously mingled—brown, whitish, black, and yellow—and traced with acute-angled lines and curves, that one cannot distinguish it at sight from a bit of rotten wood. I experienced that myself once when, passing a hedge, I thought I saw a Xylina sitting on the ground, and picked it up to examine it. I thought it was a bit of wood, and, disappointed, I threw it down again on the grass, but then I felt uncertain, and picked it up once more—to find that it was a moth after all[1]!
[1] Rösel says in this connexion: 'The marvellous form of this Papilio preserves it from injuries, for, when he hangs freely on a trunk of a tree, he would be taken ten times sooner for a piece of bark than for a living creature. By day, too, he is so little sensitive, that if he be thrown down from his resting-place he falls to the ground as if lifeless, and remains lying motionless. One may also throw him into the air, or turn him about, and he will rarely give a sign of life. I have impaled many of them on needles, without seeing any sign of sensitiveness on their part. This is the more remarkable that these birds (sic), after they have submitted to all the torment and misery one can inflict on them, without showing any sign of feeling, will, whenever they are left in peace and have no further disturbances to fear, quickly creep off to a dark corner and attempt to conceal themselves from future attacks.'—Insektenbelustigungen, Nürnberg, 1746, vol. i. p. 52.
This case of Xylina is hardly less remarkable, and its likeness to the mimicked object is scarcely less wonderful than that of the often discussed mimicry of a leaf, with stalk, midrib, and lateral veins, by many of the forest butterflies of South America and India.
Fig. 13. Kallima paralecta, from India,
right under side of the butterfly at rest.
K, head. Lt, maxillary palps. B, limbs. V,
anterior wing. H, posterior wing. St, 'tail'
of the latter, corresponding to the stalk of
the leaf. gl1 and gl2, transparent spots. Aufl,
eye-spots. Sch, mould-spots.
The best known of these is the Indian Kallima paralecta, which, when it settles, is deceptively like a dead leaf, or rather like a dry or a half-withered one, on which brown alternates with red, and on which there are one or two translucent spots, without scales, presumably representing dewdrops. The upper surface of this butterfly is simply marked, but gorgeously coloured—blue-black with a reddish yellow, or bluish white band—and quite constant. The under surface, on the other hand, although it always resembles a dead leaf, shows very varied ground colours, being sometimes greyish, sometimes yellowish, or reddish yellow, or even greenish. Often it shows the lateral veining of the leaf quite as distinctly as in Fig. 13, but often quite indistinctly, and the black, mouldy spots (Sch) of our figure may be more strongly marked, or they may be absent. It would seem as if the mimicry of different kinds of leaves was here aimed at—so to speak—just as in the case of the varied and numerous species of the South American genus Anæa, which usually live in the woods, and are all more or less leaf-like, but each species is like a different leaf, or like a leaf in a different condition, dry, moist, or decomposing. It is simply astounding to see this diversity of leaf mimicry, and the extraordinary faithfulness with which the impression of the leaf is reproduced. But it is by no means always the venation which causes the resemblance, for this is often inconspicuous; the high degree of deceptiveness is due to the silvery-clear yellow, dark yellow, red-brown to dark black-brown ground-colouring, which is never quite uniform, and over which there usually spreads a whitish ripple, combined with the remarkable imitation of the sheen of many leaves. The upper side of this butterfly is almost always conspicuously decorated with violet, dark blue or red, but always without any relation to the under surface. Not in all, but in many of the species of this genus, we find the round, translucent mirrors on the wing already mentioned in the case of Kallima, and in some species quite remarkable means are made use of to make the resemblance to a leaf thoroughly deceptive. Thus Anæa polyxo, when sitting, looks like a leaf out of the edge of which a caterpillar has eaten a little piece; in reality there is nothing missing from the wing, but on the front margin of the anterior wing a semicircular spot of a bright, soft, yellow colour stands out so sharply from the rest of the chestnut-brown wing surface, that it has the effect of a hole in the leaf.
Fig. 14. Cœnophlebia archidona, from Bolivia, in its
resting attitude. mr, midrib of the apparent leaf.
st, the apparent stalk.
A modern opponent of the selection theory (Eimer) has suggested that the marking of the lateral veins, and other resemblances to a leaf in Kallima, represent nothing more than the pattern which was present in any case, inherited from ancestors, and which in the course of time arranged itself in a particular manner according to internal developmental laws. Not selection—that is, adaptation to surroundings—but the internal developmental impulse has brought about the resemblance to the leaf. It is astonishing how a preconceived idea can blind a man and weaken his judgment! It goes without saying that the adaptations do not start from a tabula rasa, but from what is already present; of course, natural selection makes use of the markings inherited from ancestors; it takes what already exists, and alters or extends it as suits best. Thus it is easy to prove that the clear mirrors ([Fig. 13], gl1 and gl2) on the wings of Kallima have arisen from a modification of the nuclei of eye-spots, just as the dark mould-spots which often occur, frequently develop in association with the inherited eye-spots; not always however, for many such accumulations of black scales occur in spots on which there has never been an eye-spot. Thus, too, the 'midribs' of the butterfly have in part arisen from a gradual displacing, extending, and altering of the direction of inherited stripes as, for instance, is clearly recognizable in the posterior wing of Fig. 13, but sometimes they are new formations. But the veining of a leaf is never found on the wing of any butterfly of a species which has not the habit of resting among leaves, or which has not had it at one time, and it never corresponds to the natural marking of any genus which does not live in forests. This impression of leaf-venation has obviously arisen from quite different patterns of markings, and it has been reached now by one way, now by another. We can see this from the fact that, in different butterflies, it lies in quite different positions on the wing. In the Kallima species the stalk of the leaf lies in the tail of the posterior wing, the tip of the midrib lies near the tip of the wing; in Cœnophlebia archidona it is exactly reversed, the tip of the anterior wing (Fig. 14) is prolonged, and forms the stalk, while a broad, dark, stripe, the midrib (mr), runs from there across the middle of both wings, and seems to give off two or three lateral ribs running outwards. If it be asked whether this butterfly always sits down so artistically that the 'upward turning leaf-stalk is in juxtaposition to a twig,' we may answer that a bird flying fast is not likely to look to see whether every leaf in the profusion of foliage in the primitive forests is properly fastened to its stalk or not, any more than we should do in the case of a painted bush, on which many a leaf has the appearance of floating in the air, just as in nature, or in its faithful copy, the photograph.