Butterflies in Disguise

EVERY observer, even the most casual, has at some time had his attention arrested by the strange resemblance of some creature to the object upon which it rested ; to this form of imitation the term “ mimicry" was applied as long ago as 1815 by Kirby and Spence, in the introductory letter to their treatise on entomology. “ You would declare,” say they, “upon beholding some insects, that they had robbed the trees of their leaves to form for themselves artificial wings, so exactly do they resemble them in their form, substance, and vascular structure ; some representing green leaves, and others those that are dry and withered. Nay, sometimes this mimicry is so exquisite that you would mistake the whole insect for a portion of the branching spray of a tree.”

It is not a little curious that it was on the very eve of the publication of the Origin of Species, at the meeting of the British Association for the Advancement of Science, in 1859, that the first attempt was made to collect facts of this nature, and to inquire into the laws which regulate them. At this meeting the late Mr. Andrew Murray read a paper upon the Disguises of Nature, in which he showed that the most perfect imitation of inanimate objects occurs, not rarely or exceptionally, but in some groups so commonly that the want of it might be regarded as the exception, and that the concealment of the animal was the plain purpose of the disguise. He confesses, however, that be cannot tell what law has set in motion such endless provision of protection, and can only suggest that it may he found in some force analogous to the great law of attraction ; that “ like draws to like, or like begets like.”

The theory of natural selection, immediately afterward proposed by Darwin, was the key to this puzzle. Its use for this purpose by Bates, in 1862. was one of the earliest independent contributions to the theory from new observations. Buried in the depths of a special systematic paper, there were presented by Bates some of the most striking instances that are known of such protective resemblance, in which the animals imitate, not the objects on or near which they live, nor such other creatures as are in themselves frightful or predaceous, but butterflies quite like themselves, to all external appearance as harmless and as much in need of protection as they. He pointed out, moreover, that there is a special group of butterflies (Heliconinæ), of vivid coloring and slow and easy flight, which are the constant subjects of mimicry, while the greater portion of the mimicking butterflies he observed belonged to a very different group (Pierinæ), normally white and tolerably uniform in color, but which had so changed their livery and even the form of their wings as closely to resemble the objects they mimicked in brilliancy of color and variegation, and even in mode of flight. Some, says he, “ show a minute and palpably intentional likeness which is perfectly staggering.” Indeed, the likeness proved so close that even after he became aware of the mimicry his practiced eye was often deceived. Or if he wandered to a new locality, where occurred a new set of Ithornyiæ (the most numerously represented among the mimicked genera), the Leptalides (the mimickers) would vary with them so as to preserve the mockery band for band and spot for spot. Now his field observations showed him that the mimicking species belonged to a group of butterflies very subject to attack by birds and other foes, while the group which they mimicked had an offensive odor and apparently a taste obnoxious to insectivorous animals, so as to be exempt practically from their attacks. This was shown partly by their exceptional abundance, which did not seem to accord with slow and easy flight and conspicuous coloring, features that naturally would render them an easy prey to their enemies. That these butterflies were truly distasteful to birds has been shown again and again. Thus Belt says, in his Naturalist in Nicaragua : “ I had an opportunity of proving in Brazil that some birds, if not all, reject the Heliconii butterflies, which are closely resembled by butterflies of other families and by moths. I observed a pair of birds that were bringing butterflies and dragonflies to their young, and although the Heliconii swarmed in the neighborhood, and are of weak flight, so as to be easily caught, the birds never brought one to their nest. I had a still better means of testing both these and other insects that are mimicked in Nicaragua. The tame, white-faced monkey I have already mentioned was extremely fond of insects, and would greedily munch up any beetle or butterfly given to him, and I used to bring him any insects that I found imitated by others, to see whether they were distasteful or not. I found he would never eat the Heliconii. He was too polite not to take them when they were offered to him, and would sometimes smell them, but invariably rolled them up in his hand, and dropped them quietly again after a few minutes. A large species of spider (Nephila) also used to drop them out of its web when I put them into it. Another spider that frequented flowers seemed to be fond of them, and I have already mentioned a wasp that caught them to store its nest with. There could be no doubt, however, from the monkey’s actions, that they were distasteful to him.”

Bates very naturally argued that if these offensive properties gave the I thomyiæi® such exemption from attack as enabled them to swarm in spite of lazy habits and brilliant coloring, then other butterflies living in the same places would gain a certain amount of freedom from attack if their flight and coloring so nearly resembled those of the offensive species as actually to deceive insecteating animals, even though they were themselves in no way distasteful.

The fact of a resemblance so close that it is to all appearances a “ palpably intentional likeness ” is impossible to question. But how explain it ? How could a butterfly change its appearance to such a degree, its wings from a uniform color to a banded, streaked, and spotted pattern, and at the same time lengthen their form and extend the antennæ ? “Can the Ethiopian change his skin or the leopard his spots ? ”

The answer, as Bates clearly saw, was to be looked for in the same direction as when accounting for the assumption by animals of the color of their surroundings. Both are produced in the same way, and have the same cause and end. It is only by keeping in view this tolerably obvious truth that we can explain all the freaks of mimicry. “ The specific, mimetic analogies,” says Bates, “ are adaptations, — phenomena of precisely the same nature as those in which insects . . . are assimilated in superficial appearance to the vegetable or inorganic substance on which or amongst which they live.”

To gain an idea, then, of the processes by which the “staggering” examples of mimicry are produced, we must look first at the simplest forms of protective resemblance. Go to the sea-shore and observe the grasshoppers among the beach grass. They fly up at your approach, whiz off a rod or so, and alight. Can you see them? They are colored so nearly like the sands they live upon that detection of one at rest is almost impossible. On yonder grassy bluff, a stone’s-throw away, you will find none of them, but other kinds equally, or almost equally, lost to sight by their harmony with their surroundings. What chance of life for either if they suddenly changed places ? They would be so conspicuous that every passing bird or other insectivorous creature would sight them. Of course these protective colors have been gained by slow steps. Every grasshopper that found its preferred food among the sands was liable to be eaten. In the long run just those would be eaten which were most easily seen. One which varied in coloring in never so small a degree, so as to be less easily seen than his brother, would live to perpetuate his kind, and his brother come to an untimely end ; the progeny would show the fortunate variation, and be more likely to be spared to transmit in increased volume the probability of the happy coloring. Given, then, a brood of grasshoppers that find their preferred food in sandy spots, and unless other and more powerful forces act upon them it must result, from their liability to be eaten by creatures fond of grasshoppers, that in time they will resemble in coloring the sand on which they live ; it is impossible that they should not. Any creature not specially protected by nauseousness, or habit, or special device of some sort, must in the very nature of things, if it is to live at all, have some other protection, and that afforded by color and pattern is by far the most common. The world is made up of eaters and eaten, of devices to catch and devices to avoid being caught.

We may apply the same reasoning to two kinds of butterflies subject naturally to the same class of enemies ; that is, living in the same region and flying at the same time. If one has the slightest advantage over the other in the fight for life, by being, for instance, distasteful to one class of common enemies, so that these forbear to attack it after experiment or by instinct (the result of ancestral experiments), and there be among the less favored flock, here and there, an individual which, under circumstances favoring it, such as distance or shadow, may more often than its fellows be mistaken by the enemy for one of its distasteful neighbors through its possession of a little more than usual of a certain tint on a part of the wing, a little larger spot here, or more of the semblance of a band there, — how small soever this difference may be, it must, by the very laws of natural selection. be cherished, perpetuated, increased, by slow but sure steps. Nor is there any limit to its increase except its absolute deception of the enemy. So long as there is the slightest advantage in variation in a definite possible direction, the struggle for existence will compel that variation. Knowing what we now know of the laws of life, mimicry of favored races might even have been predicted.

It is to be presumed that the actual colors found in a mimicking butterfly are. with rare exceptions, such as existed somewhere in the ancestral form. In the case of our own mimicking Basilarchia, for example, whose orange ground tint is so totally at variance with the general color of the other normal members of the group, it will be observed that all the normal species possess some orange. Without this as a precedent fact, such perfect mimicry might perhaps never have arisen. Individuals among the normal species vary somewhat in this particular, so that it is easy to suppose that some of the original B. archippus, with more orange than usual, may have escaped capture, on occasion, from this cause. From such a small beginning, such as one may now see every year in B. astyanax, sprang doubtless the whole story, and at last we find a butterfly which has for a ground color of both surfaces of the wings an orange which is the exact counterpart of that of Anosia plexippus ; by reason of which, in all probability, it enjoys a freedom from molestation comparable to that attributed to plexippus, so that it ventures more into the open country than its allies, and thus gains a wider pasturage and surer subsistence.

It would seem, then, to be plain that all cases of protective coloring and mimetic form come under one and the same law, and have been produced by the same means (the survival of the best mocker), whether the object imitated be animal, vegetable, or mineral. The actual outcome is, indeed, vastly more surprising in some cases than in others, — in some " perfectly staggering,” as Bates says; yet though there be to all appearances a " palpably intentional likeness,”there is found to be no intention in the case so far as mocker and mocked are concerned, but the result of a natural selection against which neither could even strive, and of which neither was ever conscious.¹ The process has been a long one, so that in the case of parastatic mimicry, as that form which involves the copying of one’s fellows might be termed (or, if one prefers an English term, neighborly mimicry), we may readily presume far less difference between mocker and mocked when the mimicry between them first began than now exists between the mocked and the normal relatives of the mocker. It is argued, indeed, with great show of reason, that as the resemblance grew stronger the birds became more sharpsighted, which reflected again on the mimicry, and that thus the final departure from the normal type was intensified ; but this assumption is not necessary.

So far we have referred only to the first illustrations of mimicry given by Bates, those which present the simplest, though not the least striking forms, involving as they do the widest departure of mimetic butterflies from their normal type. Let us glance briefly at some other points.

A new element enters when we find that neighborly mimicry is sometimes confined to a single sex of a butterfly; that is to say, one sex is of the normal color of its allies, while the opposite sex departs widely therefrom, and is found to resemble closely another and a nauseous butterfly of the same region. Now, as mimicry is clearly only a protective device, or rather outcome, we should naturally inquire whether either sex was more in need than the other of protection from those foes against which mimicry could avail anything. Plainly, it would be the female, since, were she lost before oviposition, just so many eggs would be lost with her; and besides this, her heavier, more sluggish flight — a necessity from her burden of eggs — makes her an easier prey to insectivorous creatures against which mimicry is aimed. Accordingly, we find many instances in which the female is mimetic and the male normal. Probably they are far more numerous than we imagine, and many of the exceedingly common differences between the sexes, which since Darwin’s day we have been wont to set down to sexual selection, doubtless are to be attributed to something of this nature. But there is no known case of neighborly mimicry confined to the male sex. On the other hand, some of the most vivid and striking examples of mimicry are to be found confined to the females. There is one example brought forward by Trimen which is the most surprising yet published, where not only have two kinds of African swallow-tail butterflies, one with, the other without, tails, long supposed to he widely distinct species, been proved to be male and female, the female departing from the type to mimic a Euplœid butterfly, butthe male is found to have no less than three distinct wives, each mimicking a different kind of Euplœid characteristic of the region inhabited by mocker and mocked, and each very different from the husband : while an allied male, formerly thought to be the same as the preceding, keeps a similar harem, similarly mimetic of species of Euplœinæ prevailing in its districts, and, besides, has in one place at least a concubine which is not at all mimetic. Surely the play of mimicry can go little farther.

But in all this arises a new difficulty. How is it that mimetic qualities, which in a given locality breed so true, are inherited by one sex only ? Why do the males escape ? Here the question is, not, Why are the females mimetic ? but rather, Why are the males not mimetic? To this no satisfactory answer has yet been given. It has been attributed to sexual selection, the females being supposed to be of a conservative frame of mind, and admitting no variation in their consorts ; but this it would be difficult to prove, or, it seems to me, to render very probable.

This, however, is the view of it taken by Belt, who remarks that “ it is supported by the fact that many of the males of the mimetic Leptalides have the upper half of the lower wing of a pure white, whilst all the rest of the wings is barred and spotted with black, red, or yellow, like the species they mimic. The females have not this white patch, and the males usually conceal it by covering it with the upper wing, so that I cannot imagine its being of any other use to them than as an attraction in courtship, when they exhibit it to the females, and thus gratify their deepseated preference for the normal color of the older [tribe] to which the Leptalides belong.”

Stilt another difficulty besets the subject,— a difficulty in part recognized by Bates. It has been the subject of much discussion, but on the principles supported above is far more easily disposed of. Bates found not only that the distasteful Heliconoid butterflies were mimicked by those which were in evident need of protection, from the fact of their being greedily eaten by insectivorous animals, but that there were cases of mimicry quite as close among the Heliconoid butterflies themselves. Many instances of the same kind have since been recognized in other parts of the world. Here both mocked and mockers were protected by nauseousness, and it was by no means clear to him how any advantage, the fundamental cause of variation of this kind, was to be gained by such imitation. The resemblance was so close that, according to his own words, " species belonging to distinct genera have been confounded, owing to their being almost identical in colors and markings ; in fact, many of them can scarcely be distinguished except by their generic characters.” Bates himself was inclined to look upon these, not as cases of parastatic mimicry, but as due “to the similar adaptation of all to the same local, probably inorganic conditions.”

But this vague explanation has not been satisfactory to others, and Wallace and Meldola, and particularly Fritz Müller, have followed the matter, and shown that, if the mimicked species possesses the slightest advantage in the mere point of numbers over the mimicking, this advantage is sufficient to produce the mimicry concerned. It is highly probable, from the experiments of Fritz Müller and the observations of Belt, that the Heliconoid butterflies are simply distasteful, not poisonous, to insectivorous animals. Müller has even figured a considerable number of examples of a single species found by him (in this instance belonging to the Acræinæ a closely allied nauseous group) in which the wings had evidently been seized by insectivorous birds, for they show great gaps in their wings, such as a bill would make upon them. By such seizures many of the distasteful butterflies doubtless perish, and Meldola shows very clearly by mathematical analysis that a resemblance between two species so close that the experimental seizures would be divided between them in the ratio of their numbers gives an advantage decidedly in favor of the scarcer species. Or, as Wallace puts it, " if two species, both equally distasteful, closely resemble each other, then the number of individuals sacrificed is divided between them in the proportion of the squares of their respective numbers.” If the rarer species is only one tenth as numerous, it will benefit in the proportion of one hundred to one.

Exactly the same argument can be applied to examples of mimicry between two species where neither is distasteful. These cases, though less conspicuous, are probably more numerous than those of which we have been speaking ; for, on the principles that we have laid down, any advantage which one species has over another will be attacked by that other in every possible way; and if there be elements in the structure or markings which admit of a closer resemblance between the two and this resemblance will lessen the disadvantage under winch the weaker species labors, then in the very nature of things that resemblance must follow, unless other opposing elements intervene. For here, at least, the relative abundance of the species concerned is an essential element. It has been thought by some to be also an essential element of all mimicry ; but not only is there no sufficient reason for holding Such a view, excepting in cases like those last quoted, but it has been asserted by no less keen an observer than Fritz Müller himself, and agreed to by others, that the mimicked species is not always more abundant than its counterfeit ; indeed, the mimicking and the mimicked species have been found to vary in their relative numbers in different localities, sometimes the one, sometimes the other, preponderating. But with regard to mimicry of one distasteful butterfly by another, there may also enter another element; for it is hardly to be believed that all distasteful butterflies are equally objectionable to all birds, and it is obvious that the more distasteful the butterfly is to its rapacious foes, by so much more has it the advantage in the struggle for life; so that mimicry of one distasteful butterfly by another less distasteful is scarcely more surprising than the mimicry of a nauseous butterfly by one that has not this quality.

Only one further difficulty remains, and this is that, in a few instances, an insect has been found differing so peculiarly from its congeners as to leave no doubt in the mind that it differs in the direction of mimicry when no exact prototype can be found. For example, the butterfly of one of the Nymphalinæ, with normal dark colors and a definite pattern, will vary altogether from that pattern and coloring, to take on the livery peculiar to the Euplœinæ, a group very extensively imitated, when there is found in the regions inhabited by this supposed mimicking species no Euplœid which it in any way specially resembles. In this case but two explanations have been offered : one that the mimicked butterfly has not yet been found, another that it has for some cause become extinct. But with the extinction of the mimicked form we should expect speedy extinction of the mimicking, and it would seem more probable that these were cases of general mimicry in process of formation toward some specific type. At any rate, we need to know more definitely about these instances before we can properly discuss them. They have never been collated.

In support of the general theory of mimicry, it may be said that cases are far more common in the tropics than in temperate regions, even relatively ; and so, too, are insectivorous animals. The accounts of travelers in the tropics constantly mention the attacks of birds upon butterflies, while instances of butterflies being seen pursued by birds are vastly more rare in the temperate regions. I have never seen one. In the tropics, moreover, the birds are aided by a great number of other insectivorous animals, such as lizards. In our own country, therefore, we should not look for many instances of mimicry of any decided type. The most striking is unquestionably that of Basilarchia archippus, which mimics Anosia plexippus, and the closely related case of Basilarehia eros and Tasitia berenice, the last two butterflies largely supplanting the first two on the peninsula of Florida. In both these instances the mimicry is enjoyed by both sexes. A third case is found in the less close but still striking mimicry of Basilarchia astyanax by the female of Semnopsyehe diana, an instance the more remarkable as the mimicked species belongs to the same genus as our two other mimicking forms.

When we take a general view of mimicry as exhibited by one butterfly for another, how strange it seems ; and what an interesting illustration it is of the adaptability and pliancy of natural forces, that for the evident protection of one species in the struggle for existence so exact and beautiful a resemblance should be brought about! Consider for a moment that the subjects of mimicry are at the final stage of life ; they have already passed through nearly all the dangers to which the species as a species is subjected, — so rudely subjected that they are indeed but a centesimal, or even less, rarely or never more, of those brought into the world with them. During the early period of their life they were exposed to vastly more dangers than they can now experience. At times they were absolutely helpless, without the power of movement. They are now endowed with powers of flight sufficient to thwart the purpose of many a foe; yet it is in just this period that these special and extraordinary provisions for their safety and for the accomplishment, so far as the species is concerned, of the end of their life are given them. All this has been brought about for the sole purpose of prolonging their aerial life for the exceedingly few days which are necessary for pairing and the deposition of eggs. The more we contemplate so strange and perfect a provision, and the means by which it is accomplished, the more are we impressed with the capabilities of natural selection, and begin to comprehend how powerful an element it has been in the development of the varied world of beauty about, us.

Samuel H. Scudder.