FIG. 25.—Wings of Ituna Ilione, female.
Wings of Thyridia megisto, female.
The figures above of the fore and hind wings of two of these mimicking species, from Dr. Fritz Müller's original paper in Kosmos, will serve to show the considerable amount of difference, in the important character of the neuration of the wings, between these butterflies, which really belong to very distinct and not at all closely allied genera. Other important characters are—(1) The existence of a small basal cell in the hind wings of Ituna which is wanting in Thyridia; (2) the division of the cell between the veins 1b and 2 of the hind wings in the former genus, while it is undivided in the latter; and (3) the existence in Thyridia of scent-producing tufts of hair on the upper edge of the hind wing, while in Ituna these are wanting; but in place of them are extensible processes at the end of the abdomen, also emitting a powerful scent. These differences characterise two marked subdivisions of the Danaoid Heliconinae, each containing several distinct genera; and these subdivisions are further distinguished by very different forms of larvae, that to which Ituna belongs having from two to four long threadlike tentacles on the back, while in that containing Thyridia these are always absent. The former usually feed on Asclepiadeae, the latter on Solanaceae or Scrophulariaceae.
The two species figured, though belonging to such distinct and even remote genera, have acquired almost identical tints and markings so as to be deceptively alike. The surface of the wings is, in both, transparent yellowish, with black transverse bands and white marginal spots, while both have similar black-and white-marked bodies and long yellow antennae. Dr. Müller states that they both show a preference for the same flowers growing on the edges of the forest paths.[105]
We will now proceed to give the explanation of these curious similarities, which have remained a complete puzzle for twenty years. Mr. Bates, when first describing them, suggested that they might be due to some form of parallel variation dependent on climatic influences; and I myself adduced other cases of coincident local modifications of colour, which did not appear to be explicable by any form of mimicry.[106] But we neither of us hit upon the simple explanation given by Dr. Fritz Müller in 1879.
His theory is founded on the assumed, but probable, fact, that insect-eating birds only learn by experience to distinguish the edible from the inedible butterflies, and in doing so necessarily sacrifice a certain number of the latter. The quantity of insectivorous birds in tropical America is enormous; and the number of young birds which every year have to learn wisdom by experience, as regards the species of butterflies to be caught or to be avoided, is so great that the sacrifice of life of the inedible species must be considerable, and, to a comparatively weak or scarce species, of vital importance. The number thus sacrificed will be fixed by the quantity of young birds, and by the number of experiences requisite to cause them to avoid the inedible species for the future, and not at all by the numbers of individuals of which each species consists. Hence, if two species are so much alike as to be mistaken for one another, the fixed number annually sacrificed by inexperienced birds will be divided between them, and both will benefit. But if the two species are very unequal in numbers, the benefit will be comparatively slight for the more abundant species, but very great for the rare one. To the latter it may make all the difference between safety and destruction.
To give a rough numerical example. Let us suppose that in a given limited district there are two species of Heliconidae, one consisting of only 1000, the other of 100,000 individuals, and that the quota required annually in the same district for the instruction of young insectivorous birds is 500. By the larger species this loss will be hardly felt; to the smaller it will mean the most dreadful persecution resulting in a loss of half the total population. But, let the two species become superficially alike, so that the birds see no difference between them. The quota of 500 will now be taken from a combined population of 101,000 butterflies, and if proportionate numbers of each suffer, then the weak species will only lose five individuals instead of 500 as it did before. Now we know that the different species of Heliconidae are not equally abundant, some being quite rare; so that the benefit to be derived in these latter cases would be very important. A slight inferiority in rapidity of flight or in powers of eluding attack might also be a cause of danger to an inedible species of scanty numbers, and in this case too the being merged in another much more abundant species, by similarity of external appearance, would be an advantage.
The question of fact remains. Do young birds pursue and capture these distasteful butterflies till they have learned by bitter experience what species to avoid? On this point Dr. Müller has fortunately been able to obtain some direct evidence, by capturing several Acraeas and Heliconidae which had evidently been seized by birds but had afterwards escaped, as they had pieces torn out of the wing, sometimes symmetrically out of both wings, showing that the insect had been seized when at rest and with the two pairs of wings in contact. There is, however, a general impression that this knowledge is hereditary, and does not need to be acquired by young birds; in support of which view Mr. Jenner Weir states that his birds always disregarded inedible caterpillars. When, day by day, he threw into his aviary various larvae, those which were edible were eaten immediately, those which were inedible were no more noticed than if a pebble had been thrown before the birds.
The cases, however, are not strictly comparable. The birds were not young birds of the first year; and, what is more important, edible larvae have a comparatively simple coloration, being always brown or green and smooth. Uneatable larvae, on the other hand, comprise all that are of conspicuous colours and are hairy or spiny. But with butterflies there is no such simplicity of contrast. The eatable butterflies comprise not only brown or white species, but hundreds of Nymphalidae, Papilionidae, Lycaenidae, etc., which are gaily coloured and of an immense variety of patterns. The colours and patterns of the inedible kinds are also greatly varied, while they are often equally gay; and it is quite impossible to suppose that any amount of instinct or inherited habit (if such a thing exists) could enable young insectivorous birds to distinguish all the species of one kind from all those of the other. There is also some evidence to show that animals do learn by experience what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J. Bursch that very young chickens peck at insects which they afterwards avoid. Lizards, too, often seized larvae which they were unable to eat and ultimately rejected.
Although the Heliconidae present, on the whole, many varieties of coloration and pattern, yet, in proportion to the number of distinct species in each district, the types of coloration are few and very well marked, and thus it becomes easier for a bird or other animal to learn that all belonging to such types are uneatable. This must be a decided advantage to the family in question, because, not only do fewer individuals of each species need to be sacrificed in order that their enemies may learn the lesson of their inedibility, but they are more easily recognised at a distance, and thus escape even pursuit. There is thus a kind of mimicry between closely allied species as well as between species of distinct genera, all tending to the same beneficial end. This may be seen in the four or five distinct species of the genus Heliconius which all have the same peculiar type of coloration—a yellow band across the upper wings and radiating red stripes on the lower,—and are all found in the same forests of the Lower Amazon; in the numerous very similar species of Ithomia with transparent wings, found in every locality of the same region; and in the very numerous species of Papilio of the "Aeneas" group, all having a similar style of marking, the resemblance being especially close in the females. The very uniform type of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of the same character.[107] In all these cases the similarity of the allied species is so great, that, when they are on the wing at some distance off, it is difficult to distinguish one species from another. But this close external resemblance is not always a sign of very near affinity; for minute examination detects differences in the form and scalloping of the wings, in the markings on the body, and in those on the under surface of the wings, which do not usually characterise the closest allies. It is to be further noted, that the presence of groups of very similar species of the same genus, in one locality, is not at all a common phenomenon among unprotected groups. Usually the species of a genus found in one locality are each well marked and belong to somewhat distinct types, while the closely allied forms—those that require minute examination to discriminate them as distinct species—are most generally found in separate areas, and are what are termed representative forms.