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The Color of Animals: Euphorbia

The caterpillar of the Euphorbia Hawk-moth begins life of a clear green color, without a trace of the subsequent markings. After the first moult, however, it has a number of black patches, a white line, and a series of white dots, and has, therefore, at one bound, acquired characters which in the Elephant Hawk-moth, as we have seen, were only very gradually assumed. In the third stage, the line has disappeared, leaving the white spots. In the fourth, the caterpillars have become very variable, but are generally much darker than The Colors of Animals before, and have a number of white dots under the spots. In the fifth stage, there is a second row of white spots under the first. The caterpillars not being good to eat, there is, as has been already pointed out, no need for, or attempt at, concealment. Now if we compare the mature caterpillars of other species of the genus, we shall find that they represent phases in the development of the Euphorbia Hawk-moth. The Sea Buckthorn Hawk-moth, for instance, even when full grown, is a plain green, with only a trace of the line, and corresponds, therefore, with a very early stage of the Euphorbia Hawk-moth; there is another species found in South Russia, which has the line, and represents the second stage of the Euphorbia Hawk-moth; another has the line and the row of spots, and represents, therefore, the third stage; lastly, there are some which have progressed further, and lost the longitudinal line, but they never acquire the second row of spots which characterizes the last stage of the Euphorbia Hawk-moth.

Thus, then, the individual life of certain caterpillars gives us a clue to the history of the species in past ages.

For such inquiries as this, the larvæ of Lepidoptera are particularly suitable, because they live an exposed life; because the different species, even of the same genus, often feed on different plants, and are therefore exposed to different conditions; and last, not least, because we know more about the larvæ of the butterflies and moths than about those of any other insects. The larvæ of ants all live in the dark; they are fed by the perfect ants, and being therefore all subject to very similar conditions, are all very much alike. It would puzzle even a good naturalist to determine the species of an ant larva, while, as we all know, the caterpillars of butterflies and moths are as easy to distinguish as the perfect insects; they differ from one another as much as, sometimes more than, the butterflies and moths themselves.

There are five principal types of coloring among caterpillars. Those which live inside wood, or leaves, or underground, are generally of a uniform pale hue; the small leaf-eating caterpillars are green, like the leaves on which they feed. The other three types may, to compare small things with great, be likened to the three types of coloring among cats. There are the ground cats, such as the lion or puma, which are brownish or sand color, like the open places they frequent. So also caterpillars which conceal themselves by day at the roots of their food-plant, tend, as we have seen, even if originally green, to assume the color of earth. Nor must I omit to mention the Geometrida, to which I have already referred, and which, from their brown color, their peculiar attitudes, and the frequent presence of warts or protuberances, closely mimic bits of dry stick. That the caterpillars of these species were originally green, we may infer from the fact that some of them at least are still of that color when first born.

Then there are the spotted or eyed cats, such as the leopard, which live among trees; and their peculiar coloring renders them less conspicuous by simulating spots of light which penetrate through foliage. So also many caterpillars are marked with The Colors of Animals spots, eyes, or patches of color. Lastly, there are the jungle cats, of which the tiger is the typical species, and which have stripes, rendering them very difficult to see among the brown grass which they frequent. It may, perhaps, be said that this comparison fails, because the stripes of tigers are perpendicular, while those of caterpillars are either longitudinal or oblique. This, however, so far from constituting a real difference, confirms the explanation; because in each case the direction of the lines follows that of the foliage. The tiger, walking horizontally on the ground, has transverse bars; the caterpillar, clinging to the grass in a vertical position, has longitudinal lines; while those which live on largeveined leaves have oblique lines, like the oblique ribs of the leaves.

Red and blue are rare colors among caterpillars.

Omitting minute dots, we have six species more or less marked with red or orange. Of these, two are spiny, two hairy, and one protected by scent-emitting tentacles. The orange medio-dorsal line of the Bedford Butterfly is not very conspicuous, and has been omitted in some descriptions. Blue is even rarer than red; in fact, none of our butterfly larvæ can be said to exhibit this color.

Now let us turn to the moths. I have taken all the larger species, amounting to rather more than one hundred and twenty; out of which sixty-eight are hairy or downy; and of these forty-eight are marked with black or gray, fifteen brown or brownish, two yellowish-green, one bluish-gray, one striped with yellow and black, and one reddish-gray. There are two yellowish-green hairy species, which might be regarded as exceptions: one, that of the Five-spotted Burnetmoth, is marked with black and yellow, and the other is variable in color, some specimens of this caterpillar being orange. This last species is also marked with black, so that neither of these species can be considered of the green color which serves as a protection. Thus, among the larger caterpillars, there is not a single hairy species of the usual green color. On the other hand, there are fifty species with black or blackish caterpillars, and of these forty-eight are hairy or downy. In ten of our larger moths the caterpillars are more or less marked with red. Of these, three are hairy, one is an internal feeder, four have reddish lines, which probably serve for protection by simulating lines of shadow, and one, the Euphorbia Hawk-moth, is inedible. The last, the striped Hawk-moth, is rare, and I have never seen the caterpillar; but to judge from figures, the reddish line and spots would render it, not more, but less conspicuous amongst the low herbage which it frequents.

Seven species only of the larger moths have any blue; of these, four are hairy, the other three are Hawk-moths. In one, the Death's Head, the violet color of the side stripes certainly renders the insect less conspicuous among the flowers of the potato, on which it feeds. In the Oleander Hawk-moth there are two blue patches, which, both in color and form, curiously resemble the petals of the periwinkle, on which it feeds. In the third species, the small Elephant Hawk-moth, the bluish spots form the centres of the above-mentioned eye-like spots.

The Colors of Animals In one family, as already mentioned, the caterpillars are very often brown, and closely resemble bits of stick, the similarity being much increased by the peculiar attitudes they assume. On the other hand, the large brown caterpillars of certain Hawk-moths are night feeders, concealing themselves on the ground by day; and it is remarkable that while those species, such as the Convolvulus Hawk-moth, which feed on low plants, turn brown as they increase in age and size, others, which frequent trees, and cannot therefore descend to the ground for concealment, remain green throughout life. Omitting these, there are among the larger species, seventeen which are brown, of which twelve are hairy, and two have extensile caudal filaments.

The others closely resemble bits of stick, and place themselves in peculiar and stiff attitudes.

And thus, summing up the caterpillars, both of butterflies and moths, out of eighty-eight spiny and hairy species, only one is green, and even this may not be protectively colored, since it has conspicuous yellow warts. On the other hand, a very great majority of the black and brown caterpillars, as well as those more or less marked with blue and red, are either hairy or spiny, or have some special protection.

Here, then, I think we see reasons, for many at any rate, of the variations of color and markings in caterpillars, which at first sight seem so fantastic and inexplicable. I should, however, produce an impression very different from that which I wish to convey, were I to lead you to suppose that all these varieties have been explained, or are understood. Far from it; they still offer a large field for study; nevertheless, I venture to think the evidence now brought forward, however imperfectly, is at least sufficient to justify the conclusion that there is not a hair or a line, not a spot or a color, for which there is not a reason - which has not a purpose or a meaning in the economy of nature.