How these Imitations have been Produced.
To many persons it will seem impossible that such beautiful and detailed resemblances as those now described—and these are only samples of thousands that occur in all parts of the world—can have been brought about by the preservation of accidental useful variations. But this will not seem so surprising if we keep in mind the facts set forth in our earlier chapters—the rapid multiplication, the severe struggle for existence, and the constant variability of these and all other organisms. And, further, we must remember that these delicate adjustments are the result of a process which has been going on for millions of years, and that we now see the small percentage of successes among the myriads of failures. From the very first appearance of insects and their various kinds of enemies the need of protection arose, and was usually most easily met by modifications of colour. Hence, we may be sure that the earliest leaf-eating insects acquired a green colour as one of the necessities of their existence; and, as the species became modified and specialised, those feeding on particular species of plants would rapidly acquire the peculiar tints and markings best adapted to conceal them upon those plants. Then, every little variation that, once in a hundred years perhaps, led to the preservation of some larva which was thereby rather better concealed than its fellows, would form the starting-point of a further development, leading ultimately to that perfection of imitation in details which now astonishes us. The researches of Dr. Weismann illustrate this progressive adaptation. The very young larvae of several species are green or yellowish without any markings; they then, in subsequent moults, obtain certain markings, some of which are often lost again before the larva is fully grown. The early stages of those species which, like elephant hawk-moths (Chaerocampa), have the anterior segments elongated and retractile, with large eye-like spots to imitate the head of a vertebrate, are at first like those of non-retractile species, the anterior segments being as large as the rest. After the first moult they become smaller, comparatively; but it is only after the second moult that the ocelli begin to appear, and these are not fully defined till after the third moult. This progressive development of the individual—the ontogeny—gives us a clue to the ancestral development of the whole race—the phylogeny; and we are enabled to picture to ourselves the very slow and gradual steps by which the existing perfect adaptation has been brought about. In many larvae great variability still exists, and in some there are two or more distinctly-coloured forms—usually a dark and a light or a brown and a green form. The larva of the humming-bird hawk-moth (Macroglossa stellatarum) varies in this manner, and Dr. Weismann raised five varieties from a batch of eggs from one moth. It feeds on species of bedstraw (Galium verum and G. mollugo), and as the green forms are less abundant than the brown, it has probably undergone some recent change of food-plant or of habits which renders brown the more protective colour.
Special Protective Colouring of Butterflies.
We will now consider a few cases of special protective colouring in the perfect butterfly or moth. Mr. Mansel Weale states that in South Africa there is a great prevalence of white and silvery foliage or bark, sometimes of dazzling brilliancy, and that many insects and their larvae have brilliant silvery tints which are protective, among them being three species of butterflies whose undersides are silvery, and which are thus effectually protected when at rest.[73] A common African butterfly (Aterica meleagris) always settles on the ground with closed wings, which so closely resemble the soil of the district that it can with difficulty be seen, and the colour varies with the soil in different localities. Thus specimens from Senegambia were dull brown, the soil being reddish sand and iron-clay; those from Calabar and Cameroons were light brown with numerous small white spots, the soil of those countries being light brown clay with small quartz pebbles; while in other localities where the colours of the soil were more varied the colours of the butterfly varied also. Here we have variation in a single species which has become specialised in certain areas to harmonise with the colour of the soil.[74]
Many butterflies, in all parts of the world, resemble dead leaves on their under side, but those in which this form of protection is carried to the greatest perfection are the species of the Eastern genus Kallima. In India K. inachis, and in the larger Malay islands K. paralekta, are very common. They are rather large and showy butterflies, orange and bluish on the upper side, with a very rapid flight, and frequenting dry forests. Their habit is to settle always where there is some dead or decaying foliage, and the shape and colour of the wings (on the under surface), together with the attitude of the insect, is such as to produce an absolutely perfect imitation of a dead leaf. This is effected by the butterfly always settling on a twig, with the short tail of the hind wings just touching it and forming the leaf-stalk. From this a dark curved line runs across to the elongated tip of the upper wings, imitating the midrib, on both sides of which are oblique lines, formed partly by the nervures and partly by markings, which give the effect of the usual veining of a leaf. The head and antennae fit exactly between the closed upper wings so as not to interfere with the outline, which has just that amount of irregular curvature that is seen in dry and withered leaves. The colour is very remarkable for its extreme amount of variability, from deep reddish-brown to olive or pale yellow, hardly two specimens being exactly alike, but all coming within the range of colour of leaves in various stages of decay. Still more curious is the fact that the paler wings, which imitate leaves most decayed, are usually covered with small black dots, often gathered into circular groups, and so exactly resembling the minute fungi on decaying leaves that it is hard at first to believe that the insects themselves are not attacked by some such fungus. The concealment produced by this wonderful imitation is most complete, and in Sumatra I have often seen one enter a bush and then disappear like magic. Once I was so fortunate as to see the exact spot on which the insect settled; but even then I lost sight of it for some time, and only after a persistent search discovered that it was close before my eyes.[75] Here we have a kind of imitation, which is very common in a less developed form, carried to extreme perfection, with the result that the species is very abundant over a considerable area of country.
Protective Resemblance among Marine Animals.
Among marine animals this form of protection is very common. Professor Moseley tells us that all the inhabitants of the Gulf-weed are most remarkably coloured, for purposes of protection and concealment, exactly like the weed itself. "The shrimps and crabs which swarm in the weed are of exactly the same shade of yellow as the weed, and have white markings upon their bodies to represent the patches of Membranipora. The small fish, Antennarius, is in the same way weed-colour with white spots. Even a Planarian worm, which lives in the weed, is similarly yellow-coloured, and also a mollusc, Scyllaea pelagica." The same writer tells us that "a number of little crabs found clinging to the floats of the blue-shelled mollusc, Ianthina, were all coloured of a corresponding blue for concealment."[76]
Professor E.S. Morse of Salem, Mass., found that most of the New England marine mollusca were protectively coloured; instancing among others a little red chiton on rocks clothed with red calcareous algae, and Crepidula plana, living within the apertures of the shells of larger species of Gasteropods and of a pure white colour corresponding to its habitat, while allied species living on seaweed or on the outside of dark shells were dark brown.[77] A still more interesting case has been recorded by Mr. George Brady. He says: "Amongst the Nullipore which matted together the laminaria roots in the Firth of Clyde were living numerous small starfishes (Ophiocoma bellis) which, except when their writhing movements betrayed them, were quite undistinguishable from the calcareous branches of the alga; their rigid angularly twisted rays had all the appearance of the coralline, and exactly assimilated to its dark purple colour, so that though I held in my hand a root in which were half a dozen of the starfishes, I was really unable to detect them until revealed by their movements."[78]
These few examples are sufficient to show that the principle of protective coloration extends to the ocean as well as over the earth; and if we consider how completely ignorant we are of the habits and surroundings of most marine animals, it may well happen that many of the colours of tropical fishes, which seem to us so strange and so conspicuous, are really protective, owing to the number of equally strange and brilliant forms of corals, sea-anemones, sponges, and seaweeds among which they live.
Protection by Terrifying Enemies.