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Ch. XXI.]

MIMETIC INSECTS.

MIMETIC INSECTS

381

each season being generally all that are taken. It is usually found on the leaves of young trees, from twelve to twenty feet from the ground. I have taken the rather heavy-bodied female by throwing a stone at it and causing it to fall within reach, but the male is more active on the wing, and it was long before I obtained a specimen.

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Amongst the insects of Chontales none are more worthy of notice than the many curious species of Orthoptera that look like green and faded leaves of trees. I have already described one species that resembles a green leaf, and so much so that it even deceived the acute senses of the foraging ants; other species, belonging to a closely

related genus (Pterochroza), imitate leaves in every stage of decay, some being faded-green, blotched with yellow; others, as in the species figured, resemble a brown withered leaf, the resemblance being increased by a transparent hole through both wings that looks like a piece taken out of the leaf. In many butterflies that resemble leaves on the under side of their wings, the wings being raised and closed together when at rest so as to hide the bright colours of the upper surface, there are similar transparent spots that imitate holes; and

MOSS INSECT.

others again are jagged at the edge, as if pieces had been taken out of them. Many chrysalides also have mirror-like spots that resemble holes; and one that I found hanging from the under side of a leaf had a real hole through it, formed by a horn that projected from the thorax and doubled back to the body, leaving a space between. Another insect, of which I only found two specimens, had a wonderful resemblance to a piece of moss, amongst which it concealed itself in the daytime, and was not to be distinguished except when accidentally shaken out. It is the larval stage of a species of Phasma. Ch. XXI.] EXPLANATION OF MIMETIC FORMS.

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The extraordinary perfection of these mimetic resemblances is most wonderful. I have heard this urged as a reason for believing that they could not have been produced by natural selection, because a much less degree of resemblance would have protected the mimetic species. To this it may be answered, that natural selection not only tends to pick out and preserve the forms that have protective resemblances, but to increase the perceptions of the predatory species of insects and birds, so that there is a continual progression towards a perfectly mimetic form. This progressive improvement in means of defence and of attack may be illustrated in this way. Suppose a number of not very swift hares and a number of slow-running dogs were placed on an island where there was plenty of food for the hares but none for the dogs, except the hares they could catch; the slowest of the hares would be first killed, and the swifter preserved. Then the slowest-running dogs would suffer, and having less food than the fleeter ones, would have least chance of living, and the swiftest dogs would be preserved; thus the fleetness of both dogs and hares would be gradually but surely perfected by natural selection, until the greatest speed was reached that it was possible for them to attain. I have in this supposed example confined myself to the question of speed alone, but in reality other means of pursuit and of escape would come into play and be improved. The dogs might increase in cunning, or combine together to work in couples or in packs by the same selective process; and the hares on their part might acquire means of concealment or stratagem to elude their enemies; but, on both sides, the improvement would be progressive until the highest form of excellence was reached. Viewed in this light, the wonderful perfection of mimetic forms is a natural consequence of the selection of the individuals that, on the one side, were more and more mimetic, and on the other (that of their enemies) more and more able to penetrate through the assumed disguises. It has doubtless happened in some cases that species, having many foes, have entirely thrown off some of them through the disguises they have been brought to assume, but others they still cannot elude.

Since Mr. Bates first brought forward the theory of mimetic resemblances its importance has been more and more demonstrated, as it has been found how very largely animal life has been influenced in form and colour by the natural selection of the varieties that were preserved from their enemies, or enabled to approach their prey, through the resemblance they bore to something else. So general are these deceptive resemblances throughout nature, that it is often difficult to determine whether sexual preferences or the preservation of mimetic forms has been most potent in moulding the form and coloration of species, and in some the two forces are seen to be opposed in their operation. Thus in some butterflies that mimic the Heliconidæ, the females only are mimetic, the males retaining the normal form and coloration of the group to which they belong. In such cases it appears as if the females have not been checked in gradually assuming the disguise they wear, and it is important that they should be protected, as they are more exposed to destruction while seeking for places to deposit their eggs; but that both sexes should not have inherited the change in form and colour when it would have been beneficial to

Ch. XXI.]

DEPARTURE FOR ENGLAND.

385

both can only be explained, I think, on the supposition that the females had a choice of mates and preferred those that retained the primordial appearance of the group. This view 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, and 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 excepting as an attraction in courtship, to exhibit to the females, and thus gratify a deep-seated preference for the normal colour of the order to which the Leptalides belong.

I finally left the mines September 6th, 1872, on my way to England. I was accompanied through the forest by several of the mining officials. Though glad to return to Europe, it was not without some feeling of regret that I rode for the last time through the forest where I had so often wandered during the years I had been at Santo Domingo. The woods had become as familiar to me as home scenes. No more should I see the white-headed ruby humming-bird come darting down the brook, chasing away the green-throat from its bathing-place; no more watch the flocks of many-coloured birds hunting the insects in the forests, or admire the wonderful instincts of the tropical ants. I listened with pleasure to the last hoarse cries of the mot-mots, and tried to impress on my memory the curious forms of vegetation

-the palms, the gigantic arums, the tangled lianas, and perching epiphytes.

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