Not all butterflies are entrapped by the kind of simulacrum here noticed. Nature can adapt herself to every taste, and in South Africa there are spiders who make themselves attractive by appearing to be flowers. Of some of these and their modus operandi Mr. Rowland Trimen, who was curator of the Cape Town Museum, gives the following interesting account: “Many species of spiders,” he says, “are well adapted to succeed by being coloured in resemblance to the flowers in or on which they await the arrival of their victims. One that inhabits Cape Town is of the exact rose-red of the flowers of the oleander, and, to more effectually conceal it, the palpi, top of the cephalothorax, and four lateral stripes on the abdomen, are white, according remarkably with the irregular white markings so frequent on the petals of Nerium.”[[106]] These, indeed, must be beautiful spiders, and one would like to hear a little more of them, but Mr. Trimen goes at once from red to yellow. “I was led,” he continues, “to notice a yellow spider of the same group in consequence of seeing that two of a number of butterflies on the flowers of Senecio pulugera did not, on my approach, fly off with their companions. Each of these unfortunates turned out to be in the clutches of a spider, and when I released them I observed their captors very narrowly, and I found that the latter’s close resemblance to the Senecio flowers was not one of colour alone, but due also to attitude. This spider, holding on to the flower stalk by the two hinder pairs of legs, extended the two long front pairs upward and laterally. In this position it was scarcely possible to believe that it was not a flower seen in profile, the rounded abdomen representing the central mass of florets and the extended legs the ray florets, while to complete the illusion the femora of the front pair of legs, adpressed to the thorax, have each a longitudinal red stripe, which represents the ferruginous stripe on the sepals of the flower.”[[106]]
Later on, Mr. Trimen was so fortunate as actually to see a butterfly caught by another flowery impostor:—“The butterfly,” he tells us, “was engaged in honey-sucking on a white flower-head of Lantana, and explored each individual flower with its proboscis. While I was watching it, the butterfly touched and partly walked over what looked like a slightly folded or crumpled flower about the middle of the cluster. This turned out to be a spider, which instantly seized the butterfly, throwing forward its front legs, somewhat after the fashion of a mantis. In this spider the effect of the little depressions on the limb of the corolla was given by some depressed lines on the back of its smooth white abdomen.”[[106]]
Other spiders resemble snail-shells, others ants, and one, at least, is like a small scorpion, but we will return to the butterflies. As I have said, except for that wonderful copy of a leaf, already described, I cannot think of any very extraordinary resemblances amongst them, belonging to that class, but there are others which form a little class of their own. In the last chapter we have seen bees imitating bees, and in this we will make the acquaintance of certain butterflies which, as it were, pretend to be of a species which they do not really belong to. Thus in Brazil, by the great River Amazons, a number of large showy butterflies are found which belong to the family of the Heliconea, and wherever these fly they are accompanied by various other butterflies, belonging to quite different families, which are nevertheless so extremely like them that even Mr. Bates, who, for eleven years, ran up and down the Amazons with a butterfly-net in his hand, could never be quite sure which kind it was that he was going to catch. Often, when he thought he had got a Heliconea he was perfectly thunderstruck to find it was really a Papilio, Pieris, Euterpe, Leptalis, Protogonius, Ithoneis, Dioptis, Pericopis, Hyelosia, or something of that sort; or again, when it was one of these he was after, and at last he thought he had it in the net, he would be petrified, on looking more closely, to find that what he had really caught was a Heliconea.
But now, as all these butterflies were alike or nearly alike, how could Mr. Bates tell—or how had anybody been able to tell before him—that they were really not all the same species—that a Heliconea was not a Papilio, or that a Papilio, Pieris, Euterpe, etc., were not all of them Heliconeas? This, at first sight, seems a difficult question to answer, but really it is not, because, in all these families of butterflies, the various species composing them bear a kind of generalised resemblance to one another: there is a family likeness, in fact, and this is not only the case in regard to their outward appearance—the shape and colour of their wings, etc.—but it applies, in a still greater degree, to their structure and internal economy. Thus, however strongly a Pieris, or one of those others, might resemble a Heliconea, the trained eye of an entomologist could easily see that it really was a member of another family, and since, in resembling the Heliconea, it departed from the general type of pattern and colouring exhibited by the family to which it belonged, whilst this one species of Heliconea it resembled was like the others, it might be inferred that the latter was the imitated and not the imitating form. Again—and this is still more decisive evidence in the cases where it applies—the resemblance is often confined to one sex of the copying species, viz. the female, so that whilst she is hardly to be distinguished from the model on which she has founded herself, the male retains the appearance, together with all the other characteristics, of the race to which both he and she belong.
But now came a further question, the most puzzling or, at any rate, the most important one of all, viz. Why should the one butterfly imitate, or rather resemble the other, in such an extraordinary degree—a degree seeming to preclude the possibility of mere chance having brought it about? This question Mr. Bates is supposed to have been the first to answer, though I cannot help thinking, myself, that he has only extended an explanation, which, in some cases, was so obvious that no one had thought of pointing it out, to these other cases where it was not nearly so easy to see. For what can be plainer, as Mr. Bates himself remarks, than that a moth, for instance, by closely resembling a hornet, would escape the attacks of birds that might otherwise have devoured it? I cannot think but that so patent an explanation had been in the minds of many long before 1862, and though no one previous to that date may have applied the principle of natural selection to such cases, it must be remembered that natural selection had been established by Darwin some ten or twelve years before.
Bates, however, besides making an ingenious application of the above principle to a special case, gave a real reason for something which was not at all obvious, viz. why one butterfly should be a gainer by closely resembling another; and this no one had hitherto been able to do. His surmise, which has since in many instances been confirmed, is as follows. Having first pointed out that the Heliconea butterflies are a numerous, flourishing race, whilst those species that imitate them are poor in numbers, he says, “What advantages the Heliconidæ possess to make them so flourishing a group, and, consequently, the object of so much mimetic resemblance, it is not easy to discover. There is nothing apparent in their structure or habits which could render them safe from persecution by the numerous insectivorous animals which are constantly on the watch in the same parts of the forest which they inhabit. It is probable they are unpalatable to insect enemies. Some of them have glands near the end of the abdomen which they protrude when roughly handled; it is well known that similar organs in other families secrete fetid liquids or gases and that these serve as a protection to the species. They have all a peculiar smell. I never saw the flocks of slow-flying Heliconidæ in the woods persecuted by birds or dragon-flies, to which they would have been easy prey; nor when at rest on leaves did they appear to be molested by lizards, or the predaceous flies which were very often seen pouncing on butterflies of other families. If they owe their flourishing existence to this cause it would be intelligible why species whose scanty number of individuals reveals a less protected condition, should be disguised in their dress, and thus share their immunity. Is it not probable, seeing the excessive abundance of the one species and the fewness of individuals of the other, that the Heliconea is free from the persecution to which the Leptalis is subjected?”[[107]]
No sooner was this suggestion made than naturalists all over the world began to test it, or rather to say that it ought to be tested. Some experiments have been made, but they have not been very numerous, and it can hardly be said that they entirely support Bates’s view. Sometimes they do and sometimes they do not, so as there is no reason to suppose that every butterfly is relished by every kind of insect-eating creature, this is not conclusive, till the same tests are employed in regard to butterflies that are not imitated in this way; for if the latter have not been imitated on that account, it need not be on that account that others have been imitated. Thus Belt says, as the result of his observations, “The Heliconidæ are distasteful to most animals; I have seen even spiders drop them out of their webs again; and small monkeys, which are extremely fond of insects, will not eat them, as I have proved over and over again.”[[108]] He also “observed a pair of birds that were bringing butterflies and dragon-flies to their young, and although the Heliconidæ swarmed in the neighbourhood, and are of weak flight so as to be easily caught, the birds never brought one to their nest.”[[109]] This seems very good evidence of the truth of Bates’s theory, but then, as against it, we learn from the same observer that “another spider that frequented flowers seemed to be fond of these very same butterflies,” and as to the spiders which were seen to drop them out of their webs, they may resolve themselves into one, since farther on Belt says, “A large species of spider also used to drop them out of its web when I put them into it.”[[109]] Then we are told that “there is, however, a yellow and black-banded wasp that catches them to store his nest with”; and which, having done so, “would quietly bite off its wings, roll it up into a ball, and fly off with it.”[[110]] Professor Poulton calls these cases “interesting exceptions,” and easily accounts for them. But might not further observation keep adding to the number of exceptions, until at last, they become so numerous that all one could say would be this: “There is a great choice of insects in tropical America, and some creatures may prefer one kind and some another, to whatever species they belong.” In India, again, where there is another family of butterflies having doubles, or understudies, only one species was refused by all the mantids which a French naturalist gave them to. Others were eaten by all of them.
Has any man tried eating one of these butterflies? That was what Professor Wheeler did to test another supposed case of the same sort. Here the insect was a large and very conspicuously coloured day-flying moth. This moth has not an understudy, as far as is known, but it was supposed, then, to be an example of what is called “warning coloration,” that is to say, its bright colours were believed to be flourished in the face of any and every animal it might meet with, in order to warn them that it was not good to eat. Otherwise some bird, or lizard, or other creature might kill it before it had time to find out that it wasn’t. For instance, had it been just a brown moth—there are so many of these and most of them good to eat—how was it to be distinguished from others? But such a get-up as that—black and white wings and a black and orange body—once seen it was not to be forgotten. It was like the red flag at a rifle range, warning one off, and this is the theory of warning coloration. So Professor Wheeler, as he rode through the deserts of Wyoming, with the moths all about him, resolved to test this theory which had lived for a long time, and still goes on living a good deal on trust. “He dismounted from his horse and proceeded to masticate the body of one of the moths. To his astonishment the little flavour that it contained was mild and pleasant, one may almost say nut-like.”[[111]] Perhaps it may be thought that, on the “de gustibus” principle, what is pleasant to a human being might be disagreeable to a bird or a lizard; but Professor Wheeler tried another experiment. “Another day-flying moth, common in our eastern States, has deep black wings, each adorned with a pair of large yellow spots, and there is a dash of orange on its legs. It certainly cannot be a mimetic species (if it were, of course, one would not expect it to be nasty) as there is no other day-flying moth which could serve as its model. Several of these moths were given to some lizards that had previously been well fed on house-flies and could not, therefore, be very hungry. The moths were seized at once, and devoured, with evident signs of relish.”[[111]]
As a result of these experiments Professor Wheeler concludes that “naturalists should be more careful in imputing nauseous or disagreeable qualities to some conspicuously coloured animals,” and he suggests that “if every field entomologist could only bring himself to repeat the writer’s experiment on one of many cases of ‘flaunted nauseousness’ and place his taste impressions on record, we should in the course of time have a really valuable body of evidence, for we can hardly assume that beasts, birds, and reptiles can find things ‘nauseous’ which are quite tasteless, or even pleasant, to the human palate.”[[111]] “Il n’y a pas de réplique à cela,” and how it is that so simple a plan did not occur to Mr. Bates during all the eleven years he was on the Amazon it is not very easy to imagine. On the whole, perhaps, it may be said that the reason why certain butterflies are imitated by other butterflies has not been so satisfactorily settled as the fact that they are so imitated. But, on the other hand, there is some—perhaps much—evidence of the truth of Bates’s theory, and, moreover, that theory is in itself so plausible that it seems to require a good deal of evidence to overthrow it.
It is not only in South America that butterflies dress up like one another. Instances similar to those here given occur also in Africa and the Malay Archipelago, as well as in other parts of the world. There is even one doubtful case in England, both the copy and original being moths. Moths, especially the day-flying ones, are represented in these phenomena as well as butterflies, which are sometimes imitated by them.