Fig. 8. Caterpillar of the Buckthorn Hawk-moth, Deilephila hippophaës. A, Stage III. B, Stage V. r, ring-spots.
There could be no continuance of insect-metamorphosis if every character of the final stage had to be shunted back to the one next below, for then, for instance, the characters of the butterfly must, in the course of the phyletic evolution, be carried back to the pupa and larva. But even in the larval stage alone it can be seen that this carrying back is kept within well-defined limits. Thus, for instance, in the dimorphic caterpillars of the Sphingidæ the brown of the full-grown stage never comes so far down as the earliest stages, for the little caterpillars are all green, like the leaves and stems on which they sit. On the other hand, there are species in which the green persists, as apparently the most advantageous colour. Thus in the buckthorn hawk-moth (Deilephila hippophaës) (Fig. 8), which lives in the warm valleys of the Alps, and especially in Valais, the caterpillars are grey-green in all stages, and are exactly of the shade of the lower surface of the buckthorn leaves; they possess no oblique lines, for these would not make them more like the leaves, as the full-grown caterpillars are much bigger than an individual leaf of buckthorn, on which, moreover, the lateral veins are not very conspicuous. Nevertheless the caterpillar enjoys very fair security, as it does not feed through the day, but only in twilight and at night; it passes the daytime concealed in the dry leaves and earth about the base of the bush. Its resemblance to the leaves is very great, and is increased by the fact that it bears on the last segment a comparatively large orange-coloured spot (r), exactly the colour of the buckthorn berry, which ripens just at the time that the caterpillar attains its full growth.
But butterflies are as much persecuted, and have as much need of protection, as caterpillars, and among them, too, we find many instances of protective colouring, which are the more interesting in that they occur, as a rule, only on such parts of the body as remain visible when the insect is at rest, which is exactly what we should expect if the coloration has been wrought out in the course of natural selection. But it is well known that the resting position of diurnal Lepidoptera is quite different from that of the nocturnal forms, and is not even the same among all families, and in accordance with this we find the sympathetic colouring occurs on quite different areas in the different families.
The reason why the butterflies only require to be protected by their colour in the sleeping or resting position is that no colour whatever could make a flying butterfly invisible to its enemies, because the background against which its body shows is continually changing during its flight, and, moreover, the movement alone is enough to betray it, even if it is of a dull colour.
Thus, in general, only those parts of a butterfly's wing that are invisible at rest could safely bear bright or conspicuous colour, while the visible portions had to acquire sympathetic coloration through natural selection.
As the diurnal butterflies, when at rest, turn their wings upward and bring them together, it is only the under side which is sympathetically coloured, and that only as far as it is visible, that is, the whole of the posterior wing, and as much of the anterior one as is not covered by it. Many diurnal butterflies, when at rest, fold the anterior wing so far back that only its tip remains visible, and in such cases only this tip is protectively coloured, while in other forms, which have not this habit, almost the whole surface of the wing is sympathetically coloured.
A very simple protective colouring is exhibited by our 'lemon butterfly' (Rhodocera rhamni), in which the under surface is a whitish yellow, which protects the insect well when it settles on the dry leaves on the ground in the light woods which it is fond of frequenting.
Our gayest diurnal butterflies, the species of Vanessa, all have the under surface of a dusky colour, sometimes passing into a blackish brown, as in the peacock-butterfly, Vanessa (v. io), sometimes more into greyish brown, or brown-yellow, or reddish brown. They are never simple colours, but always consist of mixtures of different colour-tones—indeed, there is often a complex mingling of many colours, as grey, brown, black, white, green, blue, yellow, and red, made up of dots, strokes, spots, and rings, into a wonderful and very constant pattern, which, taken as a whole, has the effect of being uniform, and harmonizes with the soil, or with the highway, on which the species loves to settle, with much greater accuracy than a monochrome grey or brown would do. When the 'painted lady' (Vanessa cardui) settles on the ground it is hardly distinguishable from it, and this species in particular has a preference for settling on the ground. Other species of Vanessa, such as the peacock and the Camberwell beauty (Vanessa antiopa), are underneath of a dark blackish grey, or even black; when resting they press themselves into the darkest corners and crevices, and are thus most effectively secured from discovery.
Many diurnal Lepidoptera, on the other hand, especially the wood-butterflies of the family Satyridæ, have the habit of resting on the trunks of trees, as Satyrus proserpina does on the great beech-trunks of the forest clearings. These large butterflies, coloured conspicuously on the upper surface in deep velvety black and white, are marked on the under surface exactly to match the whitish bark of the great beech, covered over with white, grey, blackish-brown, and yellow spots, and the butterfly whose flight one has just been carefully following disappears as it suddenly alights on such a tree-trunk. As I have already stated, the protective colour only extends over as much of the insect as is seen when it is at rest. As the anterior wings are folded far back between the posterior ones, the protective colouring is limited to the whole surface of the posterior wing, and the tip of the anterior one, as far as that is visible in the resting attitude; the protectively coloured area is somewhat sharply bounded, and it is often of very different extent in quite nearly allied species, according to whether the species folds the anterior wing far back or not. Thus in our common small tortoiseshell-butterfly (Vanessa urticæ) the protective area is considerably wider than in the large tortoiseshell (Vanessa polychloros), much as the two resemble each other in other details.
This harmony between the wing tips and the posterior wings is nowhere wanting, where the under side is protectively coloured at all, but in many cases the protective colouring spreads over almost the whole of the anterior wings, and these are then not folded far back when at rest, as will be seen later in the so-called leaf-butterflies.