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tropics, and remembering that in early tertiary times, nearly the whole world was in the same favourable condition as regards temperature (vegetation, according to Heer, extending to the poles), and must have supported a vast number of species and genera that were destroyed during the glacial period, we must suppose, that, in that great variety of forms, it sometimes occurred that two species belonging to distinct orders somewhat resembled each other in form or colouration, and that the resemblance was gradually increased, when one species had special means of protection, by the other being benefited the more nearly it approached it in appearance.
It is to be remarked that the forms imitated have always some kind of defence against insectivorous birds or mammals; they are provided with stings or unpleasant odours or flavours, or are exceedingly swift in flight; excepting where inanimate nature is imitated for concealment. Thus 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 dragon-flies to their young, and although the Heliconii swarmed in the neighbourhood 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 beetle, or butterfly, given to him, and I used to bring to 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
in his hand and dropped them quietly again after a few moments. There could be no doubt, however, from the monkey's actions, that they were distasteful to him. 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.
Amongst the beetles there is a family that is just as much mimicked as the Heliconii are amongst the butterflies. These are the Lampyridæ, to which the fireflies belong. Many of the genera are not phosphorescent, but all appear to be distasteful to insectivorous mammals and birds. I found they were invariably rejected by the monkey, and my fowls would not touch them.
The genus Calopteron belonging to this family is not phosphorescent. In some of the species, as in C. basalis, (Klug), the wing-covers are widened out behind in a peculiar manner. This and other species of Calopteron are not only imitated in their colour and markings by other families of beetles but also in this peculiar widening of the elytra. Besides this, the Calopteron when walking on a leaf raises and depresses its wing cases, and I observed exactly the same movement in a longicorn beetle (Evander nobilis, Bates), which is evidently a mimetic form of this genus. In addition to being mimicked by other families of beetles, Calopteron is closely resembled by a species of moth (Pionia lycoides, Walker). This moth varies itself in colour; in one of the varieties it has a central black band across the wings, when it resembles Calopteron vicinum (Deyrolle), in another this black band is wanting, when it resembles C.basalis. Professor Westwood has also pointed out to me that the resemblance to the beetle is still further increased in the moth by raised lines of scales running lengthwise down the thorax.
The phosphorescent species of Lampyridæ, the fireflies, so numerous in tropical America, are equally distasteful, and are also much mimicked by other insects. I found different species of cockroaches so much like them in shape and colour that they could not be distinguished without examination. These cockroaches, instead of hiding in crevices and under logs like their brethren, rest during the day exposed on the surface of leaves, in the same manner as the fireflies they mimic.
Protective resemblances amongst insects are so numerous and wide-spread, and they have been so ably described by Bates and Wallace, that I shall only mention a few of the most noticeable examples that came under my attention, and which have not been described by other authors. Amongst these were the striking modifications of some beetles belonging to the Mordellidæ. These, in their normal form, are curious wedge-shaped beetles, which are common on flowers, and leap like fleas. In some of the Nicaraguan species the body is lengthened, and the thorax and elytra coloured, so as to resemble wasps and flies. In the Mordellidæ the head is small, and nearly concealed beneath the large thorax; and in the mimetic forms the latter is coloured so as to resemble the large head and eyes of the wasp or fly imitated. The species that resembles a wasp moves its antennæ restlessly, like the latter insect.
The movements, as well as the shape and colour of the insect imitated, are mimicked. I one day observed what appeared to be a hornet, with brown semi-transparent wings and yellow antennæ. It ran along the ground vibrating its wings and antennæ exactly like a
hornet, and I caught it in my net, believing it to be one. On examining it, however, I found it to belong to a widely different order. It was one of the Hemiptera, Spiniger luteicornis (Walk.), and had every part coloured like the hornet (Priocnemis) that it resembled. In its vibrating, coloured wing-cases it departed greatly from the normal character of the Hemiptera, and assumed that of the hornets.
All the insects that have special means of protection, by which they are guarded from the attacks of insectivorous mammals and birds, have peculiar forms, or strongly contrasted, conspicuous colours, and often make odd movements that attract attention to them. There is no attempt at concealment, but, on the contrary, they appear to endeavour to make their presence known. The long narrow wings of the Heliconii butterflies, banded with black, yellow, and red, distinguish them from all others, excepting the mimetic species. The banded bodies of many wasps, or the rich metallic colours of others, and their constant jerky motions, make them very conspicuous. Bees announce their presence by a noisy humming. The beetles of the genus Calopteron have their wing-cases curiously distended, and move them up and down, so as to attract attention; and other species of Lampyridæ are phosphorescent, holding out danger signals that they are not eatable. The reason in all these cases appears to be the same as Mr. Wallace has shown to hold good with banded, hairy, and brightly coloured caterpillars. These are distasteful to birds, and, in consequence of their conspicuous colours, are easily known and avoided. If they were like other caterpillars, they might be seized and injured before it was known they were not fit for food.*
Amongst the mammals, I think the skunk is an example of the same kind. Its white tail, laid back on its black body, makes it very conspicuous in the dusk
* In a paper on “Mimicry, and other Protective Resemblances amongst Animals," first published in the Westminster Review, July 1867, afterwards in “ Natural Selection,” Wallace has elaborately discussed this question. My observations are supplemental to his and to the original ones of Bates.