The Naturalist on the River Amazons

A large number of species are accompanied in the districts which they inhabit by other species which counterfeit them. For example, in districts where an /¢homia abounds, a Leftalzs will, as remarked by Bates in his letter quoted above, be found mingled in the same

• Transactions of the Linnean Soctety of London, xxiii., part 3, pp. 495-566, 1862, With Plates. Tt Zap 502.

flock, so like the /thomza, that the most experienced insect-hunter cannot detect the difference till he compares the structure of the mimicker and the mimicked. (Sometimes the members of one species counterfeit each other.) The mimicked are distinguished from the mimickers by their having the “same family facies,” whilst the mimickers are unlike their nearest allies, being “ perverted, as it were, to produce the resemblance from the normal facies of the genus or family to which they severally belong.” * Both the genuine and the counterfeit forms fly in the same parts of the forest, and generally in company, but “the Leptalzdes are exceedingly rare; they cannot be more than as I to 1000 with regard to the /thomie.” f

“Some of the imitations by insects of inanimate and living objects are very singular, and may be mentioned in this place. Many caterpillars of moths, but sometimes the cases only which are manufactured and inhabited by the caterpillars, have a most deceptive likeness to dry twigs and other objects. Moths themselves very frequently resemble the bark on which they are found, or have wings coloured and veined like the fallen leaves on which they lie motionless. The accidental general resemblance between the shape of moths’ wings and leaves here gives nature the groundwork for much mimeticanalogy. It has been pointed out by Réssler that the buff-tip moth, when at rest, is intended to represent a broken piece of lichen-covered branch,—the coloured tips of these wings, when they are closed, resembling a section of the wood. Other moths are deceptively like the excrement of birds on leaves. I met witha species of phytophagous beetle (chlamys pilula) on the Amazons, which was indistinguishable by the eye from the dung of caterpillars on foliage. These two latter cases of imitation should be carefully considered by those who would be inclined to think that the object of mimetic analogies in nature was simply variety, beauty, or ornament: nevertheless these are certainly attendants on the phenomena; some South American Casszd@ resemble glittering drops of dew on the tips of leaves, owing to their burnished pearly gold colour. Some species of Longicorn Coleoptera have precisely the colour and sculpture of the bark of the particular species of tree on which each is found. It is remarkable that other species of the same small group of Longicornes (phacellocera buguetit, cyclopeplus batesiz) counterfeit, not inanimate objects, like their near kindred just cited, but other insects, in the same way as the Leféalzdes do the ffeliconide.

• Tbid., p. 504. } Lbid., p. 505.

x] MEMOIR.

“ Amongst the living objects mimicked by insects are the predaceous species from which it is the interest of the mimickers to be concealed. Thus, the species of Scaphura (a genus of crickets) in South America resemble in a wonderful manner different sand wasps of large size, which are constantly on the search for crickets to provision their nests with. Another pretty cricket, which I observed, was a good imitation of a tiger beetle, and was always found on trees frequented by the beetles (odontocheile). There are endless instances of predaceous insects being disguised by having similar shapes and colours to those of their prey; many spiders are thus endowed; but some hunting spiders mimic flower-buds, and station themselves motionless in the axils of leaves and other parts of plants to wait for their victims.

“The most extraordinary instance of imitation 1 ever met with was that of a very large caterpillar, which stretched itself from amidst the foliage of a tree which I was one dayiexamining, and startled me by its resemblance to a small snake. The first three segments behind the head were dilatable at the will of the insect, and had on each side a large black pupillated spot, which resembled the eye of the reptile; it was a poisonous or viperine species mimicked, and not an innocuous or colubrine snake; this was proved by the imitation of keeled scales on the crown, which was produced by the recumbent feet, as the caterpillar threw itself backwards. The Rev. Joseph Greene, to whom 1 gave a description, supposes the insect to have belonged to the family (Votodontide, many of which have the habit of thus bending themselves. I carried off the caterpillar and alarmed every one in the village where I was then living to whom I showed it. It unfortunately died before reaching the adult state. “ A similar series of mimetic analogies occurs in the Old World, between the Asiatic and African Danaide, or representatives of the Fleliconid@, and species of other families of butterflies and moths. No instance is known in these families of a tropical species of one hemisphere counterfeiting a form belonging to the other. A most remarkable case of mimicry has been recorded by Mr. Trimen ina Papilio of South Africa (P. Cenea), whose male wears to deception the livery of one species of Danazs, whilst the female resembles a quite different one. Mimetic analogies, however, are not confined to the Lepidoptera; most orders of insects supply them; but they are displayed only by certain families. Many instances are known where parasitic bees and two-winged flies mimic in dress various

ILLUSTRATIONS OF MIMICRY BETWEEN BUTTERFLIES.

(Reproduced from Plate 55, Trans. Linn. Soc. XXIII. Pt. 3) by kind permission of the Linnæan Society.

1.—Leptalis Theonoë. 1a.—Ithomia Flora. 2.—Leptalis Theonoë, var. Melanoë. 2a.—Ithomia Onega. 3.—Leptalis Theonoë, var. Lysinoë. 3a.—Stalachtis Duvalisa.

4.—Leptalis Theonoë, var. Ega. 5.—Leptalis Theonoë, var. Ega. 6.—Leptalis Theonoë, var. Ega. 7.—Leptalis Theonoë, var. St. Paulo. 8.—Leptalis Theonoë, var. Ega. 9.—Leptalis Theonoë, var. St. Paulo. 6a.—Ithomia Illinissa.

industrious or nest-building bees, at whose expense they live, in the manner of the cuckoo. I found on the banks of the Amazons many of these cuckoo bees and flies, which all wore the livery of working bees peculiar to the country.” *

These “ protective resemblances,” or imitation of external appearances, are, so far as the end effected is concerned, identical with the phenomena of “ protective coloration,” or the adaptation of animals in their colours and markings to surrounding objects, whereby they are alike concealed from foes and prey.

Such, to take a few more or less familiar examples, are the sandy colours of desert animals; the white colour of certain arctic animals in winter; the sombre colour of nocturnal animals; the green plumage of parrots and other tropical birds; the “ mottled blacks and browns of woodcocks and snipes which assimilate to the hues of dead leaves and grass”; the vertical stripes of the tiger, whose haunts are the jungle, with its long yellow grass and its intervals of deep shadows; the waving twig-like form of green snakes suspended from branches; the transparency of many marine animals; the curious concealment of certain crabs which cover the upper portion of their bodies with pieces of seaweed; the chameleon which, under the influence of nervous stimuli received through the eye, contracts its pigment-bearing cells, and changes to the colour of the object on which it rests; the adaptation of birds to the various colours of the foliage amidst which they live; the modifications of the females, which are often more mimetic than the males, due to the greater perils attending their functions of egg-carrying and hatching; and, to end citations from a well-nigh exhaustless list, the protective coloration of the eggs themselves.

Whether there be adaptation of colour of an organism to its habitat, whereby it is concealed; or imitation of one species by another species, whereby it is rendered conspicuous; the phenomena are so nearly allied in the object attained, which is the securing of some advantage,t that the agent in each case may reasonably be assumed to be the same. “T think,” Bates says, “it will be conceded that all these various

• Trans. of Linn. Soc., pp. 506; 508-9. + For a convenient summary, Grant Allen’s paper on ‘“Mimicry,” in the latest edition of the Zzcyclop. Britannica, may be consulted; and, amongst more recent authorities, Poulton’s Colours of Animals, Beddard's Animal Coloration, and Hudson's Naturalist zn La Plata, ch. viii. { “There is no luxury in Nature. Nothing can arise unless it be useful.”—Weismann’s Lssays upon Heredity, vol. ii., pp. 27, 57.

q kinds of imitative resemblances belong to the same class of pheno-; mena, and are subject tothe same explanation. All are adaptations, either of the whole outward dress or of special parts, having in view the welfare of the creatures that possess them. Every species in nature may be looked upon as maintaining its existence by virtue of some endowment enabling it to withstand the host of adverse circumstances by which it is surrounded. The means are of endless. diversity. Some are provided with special organs of offence, others. have passive means of holding their own in the battle of life. Great fecundity is generally of much avail, added to capabilities, active or passive, of wide dispersion; so that when the species is extirpated in one part of its area of distribution, the place is refilled by migration of individuals from another part. A great number have means of concealment from their enemies, of one sort or other. Many are enabled to escape extermination, or obtain subsistence, by disguises of various kinds; amongst these must be reckoned the adaptive resemblance of an otherwise defenceless. species to one whose flourishing race shows that it enjoys peculiar advantages.” * Now what Bates did, as Darwin says in his review of the paper,t was to give to “these facts (of adaptive resemblance) the requisite touch of genius, and hit on the final cause of all this. mimicry.” “ The explanation seems to be quite clear on the theory of natural selection, as recently expounded by Mr. Darwin in the Ovigin of Species. The local varieties, or races, cannot be supposed to have been formed by the direct action of physical conditions on the individuals, because in limited districts, where these conditions are the same, the most widely contrasted varieties are found existing together, and it is inexplicable how they could have produced the nice adaptations which these diverse varieties exhibit. Neither can these adapted races, as before remarked, have originated in one generation by sorts, or a single act of variation in eachcase. It is. clear, therefore, that some other active principle must be here at work to draw out, as it were, steadily in certain directions, the suitable variations which arise, generation after generation, until forms have resulted which, like our races of Leptal:s Theonoé, are considerably different from their parent as well as their sister forms.