[4]Kosmos, vol. v, 1881, p. 260 onwards.
[4]Kosmos, vol. v, 1881, p. 260 onwards.
[5]According to Poulton's report inNature, July 6, 1889, of 'Sykes, Natural Selection in the Lepidoptera,'Trans. Manchester Microscop. Soc.1897, p. 54.
[5]According to Poulton's report inNature, July 6, 1889, of 'Sykes, Natural Selection in the Lepidoptera,'Trans. Manchester Microscop. Soc.1897, p. 54.
PLATE I
To face Plate I
This comparative rarity is true of the imitators of the Heliconiidæ and their great mimicry ring of unpalatable species, and is very general. Thus, for instance, there is a series of palatable mimics of the beautiful blueEuplœæof the Indo-Malayan region (Pl. III, Figs. 25 and 27), but each of these mimics is rare compared with the hosts of the blue unpalatable company, for these immune butterflies also occur in many species, all similar toEuplœa midamusorbinotata(Pl. II, Figs. 1 and 3); and the same applies to the mimics of the Indo-Malayan Danaidæ. There are a great manyDanaisspecies, all of them resemblingDanais vulgaris(Pl. III, Fig. 20), which, when they occur together, form an inedible ring, and this ring is imitated by a whole series of edible species, each of which is comparatively rare. And there are no fewer than six species ofPapiliowhich resemble these Danaids to the point of being easily mistaken for them, while another rarePapilioeffectively copies the iridescence of the blueEuplœæ—a coloration so unusual in the genus that the species has received the name ofPapilio paradoxus.
But even in single species of butterflies immune through unpalatability there is usually a great abundance of individuals. ThusDanais chrysippus, which is distributed over the whole of Africa, is a very common butterfly wherever it can live at all; and in North America, in which country there are only two widely distributed species ofDanais, these often occur in enormous numbers. The beautiful largeDanais erippusCramer (Pl. I, Fig. 8), is distributed over almost all America, and in many places is not only frequent, but occurs in great swarms. Usually it peoples the broad, open stretches of the western prairies of the United States, but whenviolent winds blow, as they do there in September especially, the insects are driven together into the small wooded spots of the prairie, and then they cover the trees in incredibly large crowds, often so thickly that the leaves are entirely hidden, and the trees look brown instead of green. Millions of butterflies go to make up such swarms, which have been observed in many parts of the United States, even quite in the East, in New Jersey, and elsewhere.
Considering this extraordinary abundance of the immune species, it is not surprising that its palatable copy,Limenitis archippus(Pl. I, Fig. 9), should also be widely distributed in North America, and in many places it is not rare, but even abundant. The enormous majority ofDanais erippuswill protect the species which resembles it so closely, even though it is not rare. Any doubt as to this being a case of mimicry disappears in face of the fact that, in Florida, there flies a second very similar but much darker brown North AmericanDanais, and that it is accompanied there by an equally dark variety ofLimenitis archippus(L. eros).
To prove the correctness of the hypothesis of an actual process of selection—which we assume in our interpretation of mimicry—I mean the assumption that the disguise of the species seeking protection really deceives the enemy, and thus actually affords protection, I need only cite the evidence of an acute and experienced entomologist who was himself deceived by it. Seitz[6], to whom we owe many valuable biological observations on butterflies, relates that, while he was collecting in the neighbourhood of the town of Bahia, he was surrounded by swarms ofCatopsiliæ, similar to our lemon butterfly, especially the commonCatopsilia argante, but he took no notice of these, as he 'had already collected as many of them as he wanted.' It was only when he saw a pairin copulathat he caught them in his net. But to his extreme surprise he found that he had not caught aCatopsilia, but a butterfly of the family Nymphalidæ, one of thoseAnææwhose numerous species are distributed over South America. TheseAnææare dark, or beautifully bright on the upper surface, but on the under side are leaf-coloured, and one of them bears the nameAnæa opalina, because it is quite clear and pale, and of opal-like brilliance. The captive was nearly related tothis species. Seitz was so much surprised by the discovery that the male, which had quickly detached itself from the female, escaped him, and he could only make out that, 'as it flew away, it unfolded dark wings, which certainly bore little resemblance to those of the lemon butterfly.' In the hope of securing more of this rare booty he then hunted only forCatopsilia argante, without however securing another coveted specimen—he caught no moreAnœæ, which shows that in this case, too, the mimetic species was much rarer.
[6]In citing this observation of Seitz, I do not mean to assert that there is true mimicry betweenAnæa opalina, or its allied species in Bahia, and theCatopsilia, though I regard this as extremely probable, because of the marked dimorphism between the male and the female, in conjunction with the very striking resemblance of the female to theCatopsilia. The example was given only to show how very deceptive such resemblances may be. To assert with confidence that it is a case of mimicry we should require to know thatCatopsiliais immune, and on that point we have as yet no information.
[6]In citing this observation of Seitz, I do not mean to assert that there is true mimicry betweenAnæa opalina, or its allied species in Bahia, and theCatopsilia, though I regard this as extremely probable, because of the marked dimorphism between the male and the female, in conjunction with the very striking resemblance of the female to theCatopsilia. The example was given only to show how very deceptive such resemblances may be. To assert with confidence that it is a case of mimicry we should require to know thatCatopsiliais immune, and on that point we have as yet no information.
PLATE II
To face Plate II
We see, then, that the need for protection in butterflies has a great influence on their external appearance, especially as regards their colour and marking. First, because the resting insect frequently has the visible surfaces sympathetically coloured, and also, because there are numerous species, indeed whole families, which contain nauseous, perhaps even actually poisonous, juices, and these have been subject to a double process of selection, directed towards the increase of the nauseousness, and at the same time towards acquiring as conspicuous a dress as possible. Thus the whole surface of these butterflies became gaily coloured, and often—as in many of the tropical nocturnal Lepidoptera which fly by day, the Agaristidæ, Euschemidæ, and Glaucopidæ—quite glaringly bright. We thus understand the striking or at least readily recognizable colours of the Heliconiidæ, the Euplœæ, the Danaidæ, and the Acræidæ. Finally, the unpalatable species influence many others which are edible, since the latter strive to resemble an immune species; and how considerable the variations and colour transformations thus induced can be is shown by the Whites of the genusPerhybris(Pl. II, Figs. 16 and 17) andArchonias, in which the male has wholly or partially retained the primitive dress of the Whites, and in which, side by side with wholly mimetic species, other species occur in which both sexes exhibit the garb of the Whites unaltered. Such cases tell decidedly against the often expressed view that mimetic species must have had from the outset a great resemblance to the model; they show rather that very great deviations in form, but more especially in colour, have been brought about solely by the necessity for mimetic adaptation, and that they have come about only slowly and step by step, as the different grades of resemblance to the model in different species of the same genus clearly show.
Lepidoptera are by no means the only insects which exhibit the phenomenon of mimicry, nor are insects the only animals in which it occurs; and unpleasant taste and odour are not the only protective characters; there are many others, as, for instance, among insects, the hardness of the chitinous cuticle.
One of the most beautiful examples of mimicry was discovered by Gerstäcker, not in free nature, but in the entomological collection at Berlin. There he found beside a green, metallic weevil-beetle, one of the Pachyrhynchidæ from the Philippines, two other insects with the same metallic sheen and very similar form of body. They had been put in beside the weevil as duplicates, but more careful observation showed that they were delicate Gryllidæ, which mimicked the hard beetles so deceptively that even the practised eye of the entomologist was misled by them. Later on it was shown that these Gryllids live in the Philippines beside the weevils, and even on the same leaves with them, and that the beetles are protected from the attacks of birds and other enemies by the extraordinary hardness of their cuticle. The case is especially remarkable because in general the Gryllidæ have no metallic shimmer, and the form of body must have been considerably altered to make them resemble the beetle. The usually broad head of the Gryllids is in this case narrower, the usually flat wing-covers are arched and pear-shaped, and the legs have become quite beetle-like. The security enjoyed by the weevil must be very perfect, for it is mimicked by three other species of beetle in the Philippines.
Animals can also be protected from attack by the possession of dangerous weapons. To this class belong insects with poisonous stings, like the bees, wasps, and ants, and in some degree also the ichneumon-flies. We cannot wonder, therefore, that these dreaded species find imitators. In this case it is not of so much importance that the copy should be rarer than the model, for anything that looks like a dangerous insect will be avoided, since close investigation is in this case attended with danger. So we find that hornets, wasps, and bees are frequently imitated by other insects, by beetles, flies, and butterflies; and these must derive a certain advantage, even when the resemblance is only a general one. Many Longicorns, which visit flowers, are striped black and yellow, like a wasp, and so are many flies, like the species ofSyrphus, and so on. The LongicornNecydalis majorbears a strong resemblance to a large ichneumon-fly; it has the same long-drawn-out body, the same swellings on the femur and tibia, the curved antennæ, the glossy brown colour, and its wing-covers are quite short, leaving the wings free, so that the deception is very complete.
Bees, too, are sometimes so well imitated that they are hardly to be distinguished from their mimics, not in flight only, but also when visiting flowers. The best and commonest mimic of our honey-bee is a perfectly harmless fly of the same size and colour, the drone-fly (Eristalis tenax). The two are often to be seen together on thesame flowering shrub, as, for instance, in autumn, on the Japanese buckwheat of our gardens (Polygonum sieboldii), both busily seeking for honey. I once noticed a boy catching the flies with a net in order to imprison them, but a bee stung him severely in the finger. He immediately abandoned the chase, and gave up the flies, perceiving the dangers of confusion. So the animal enemies ofEristaliswill often prefer to leave it in peace rather than run the risk of being stung.
PLATE III
To face Plate III
There is still another relation between two species which can be induced by mimicry—namely, parasitism, when, for instance, the so-called cuckoo-bees and parasitic humble-bees deceptively resemble in colour, arrangement of hair, and form of body, the species into whose nests they smuggle their eggs, to have them brought up at the expense of the bee or humble-bee in question. In the same way, among the numerous parasites of ant nests, there are some which copy the ants themselves, and so secure themselves from molestation, although they devour the ants' eggs and pupæ. Thus, among the hosts of South American driver-ants (Eciton prædator) there lives a predaceous beetle of the family Staphylinæ, which has received the nameMimecitonbecause it resembles the ant in form and in the nature of the external surface, though not in colour, which is to be explained by the fact that this ant has no compound eyes, and is therefore almost blind, or at any rate cannot see colours.
I should never come to an end were I to attempt to exhibit the great wealth of observations now available in regard to mimicry. But this at least may be added, that isolated cases of mimicry have been found even among Vertebrates. Thus, according to Wallace, the red-and-black striped poisonous coral snake of South America (Elaps) is most realistically imitated by a non-poisonous snake (Erythrolampus) of the same region. Among birds, Wallace cites a few cases which may be regarded as mimicry, but none are known among mammals, which is not to be wondered at when we consider how very much less numerous in individuals the species are which live together on one area, and how much less likely it is that two species should be, to begin with, so near each other in size, habit, and form that the process of natural selection could bring about a deceptive degree of resemblance. Without doubt it is among insects that the conditions for mimicry are especially favourable, partly because of the enormous number of species which live together and have interrelations on the same area, even in our latitudes and much more so in the tropics, and also because of their usually great fecundity, and their rapid multiplication, both of which are factorsfavourable to starting and continuing the processes of natural selection. Furthermore, we have to take into account the hosts of enemies which depend wholly or in great part on insects for food, and destroy them in enormous numbers, eliminating them in inverse proportion to the perfection of their adaptation. Finally, there is the extreme susceptibility of many insects to injury. This makes it very desirable that they should have some disguise sufficient to protect them from even the first attempt at an attack, since that would in many cases prove fatal.