The Naturalist in Nicaragua

by Thomas Belt

Concealment, while useful at times to all animals, is absolutely essential to some; and it is wonderful in what different ways it is attained. In cases of "cryptic resemblance to surroundings" the shape, colouration, or markings are such as to conceal an animal by rendering it difficult to distinguish from its immediate environment. In most cases the effect is PROTECTIVE; but in snakes, spiders, mantids, and other preying animals it is termed AGGRESSIVE, since it enables these animals to stalk their prey undetected. It is probable that this power, when possessed by a vertebrate animal, nearly always bears the double meaning, as in the green tree frog, where the colouration is protective so far as it provides concealment from snakes, which are particularly fond of these frogs, and aggressive in that it allows flies and other insects to approach without suspicion.

There may be either General Resemblance to surrounding objects or Special Resemblance to definite objects. The plain sandy colour of desert animals, the snow white of the inhabitants of the arctic regions, the inconspicuous hues of nocturnal animals, the stripes of the tiger and the zebra, the spots of the leopard and the giraffe have all a cryptic effect which at a very short distance renders the creatures invisible amid their natural surroundings. Nor is it necessary in order to attain this invisibility that the colouring should be really dull and plain. It all depends upon the habitat. Mr. Wallace has described "a South American goatsucker which rests in the bright sunshine on little bare rocky islets in the upper Rio Negro where its unusually light colours so closely resemble those of the rock and sand that it can scarcely be detected till trodden upon." A little observation will supply large numbers of instances of such protective colouration.

It is, however, in the insect world that this principle of adaptation of animals to their environment is most fully and strikingly developed. "There are thousands of species of insects," says Mr. Wallace again, "which rest during the day clinging to the bark of dead or fallen trees; and the greater portion of these are delicately mottled with grey and brown tints, which though symmetrically disposed and infinitely varied, yet blend so completely with the usual colours of the bark, that at two or three feet distance they are quite undistinguishable."

In protective resemblances at their highest state of perfection the colouring is not constant but, as Professor Poulton puts it in his delightful book on "The Colours of Animals", "can be adjusted to harmonise with changes in the environment or to correspond with the differences between the environment of different individuals." The seasonal change of colour in northern animals is a well-known instance of the former, and the chameleon's alterations of hue of the latter.

Besides General Resemblance, in which the general effects of surrounding colours are reproduced, we have Special Resemblance, in which the appearance of a particular object is copied in shape and outline as well as in colour. Numerous instances will be found in this book, and a "Leaf Insect" and a "Moss Insect" are illustrated. But the classic example is the butterfly from the East Indies so graphically described by Mr. Wallace, Kallima paralekta, which always rests among dead or dry leaves and has itself leaf-like wings spotted over with specks to imitate the tiny fungi growths on the foliage it resembles. "It sits on a nearly upright twig, the wings fitting closely back to back, concealing the antennae and head, which are drawn up between their bases. The little tails of the hind wings touch the branch and form a perfect stalk to the leaf, which is supported in its place by the claws of the middle pair of feet which are slender and inconspicuous. The irregular outline of the wings gives exactly the perspective effect of a shrivelled leaf." The wonderful "stick insects" in like manner mimic the twigs of the trees among which they lurk. Nor need we go abroad in search of examples, for among our own insects are countless instances of marvellous resemblances to the inanimate or vegetable objects upon which they rest. One of the most interesting is that of the geometer caterpillars, which are very plentiful, and any one can observe them for himself even in a London garden. They support themselves for hours by means of their posterior legs, forming an angle of various degrees with the branch on which they are standing and looking for all the world like one of its twigs. The long cylindrical body is kept stiff and immovable, with the separations of the segments scarcely visible, and its colour is obscure and similar to that of the bark of the tree. Kirby and Spence tell of a gardener mistaking one of these caterpillars for a dead twig, and starting back in great alarm when, on attempting to break it off, he found it was a living animal.

Sometimes concealment is secured by the aid of adventitious objects. Many lepidopterous larvae live in cases made of the fragments of the substances upon which they feed; and certain sea-urchins cover themselves so completely with pebbles, shells, and so forth, that one can see nothing but a heap of little stones. Perhaps, however, the most interesting instance is the crab described by Mr. Bateson, which "takes a piece of weed in his two chelae and, neither snatching nor biting it, deliberately tears it across, as a man tears paper with his hands. He then puts one end of it into his mouth, and after chewing it up, presumably to soften it, takes it out in the chelae and rubs it firmly on his head or legs until it is caught by the peculiar curved hairs which cover them. If the piece of weed is not caught by the hairs, the crab puts it back in his mouth and chews it up again. The whole proceeding is most human and purposeful."

There is another class of colours in which not concealment but conspicuousness is the object aimed at. Such colours are borne by animals provided with formidable weapons of defence (the sting of the wasp, for example), or possessed of an unpleasant taste or offensive odour, and their foes come by experience to associate this form of colouring with disagreeable qualities and avoid the animals so marked. Belt was the first to account, in this way, for the conspicuous colouration of the skunk; and it is now believed that startling colours and conspicuous attitudes are intended to assist the education of enemies by enabling them to learn and remember the animals which are to be avoided. The explanation of warning colours was devised by Mr. Wallace to account for the brilliancy in the tints of certain caterpillars which birds find disagreeable, and the subject has been principally studied by experiments upon such caterpillars. But examples of warning colours are recognised, among many others, in the contrasted black and yellow of wasps, bees, and hornets, the bright red, black, and yellow bands of the deadly coral snakes, and the brilliantly coloured frog of Santo Domingo which hops unconcernedly about in the daytime in his livery of red and blue—"for nothing will eat him he well doth know."

But—and here comes in the principle to which the term "mimicry" is now restricted—if warning colours are helpful to noxious animals, then defenceless animals acquiring these colours will share in the protection afforded by them. And so we find a deceptive similarity between animals occurring in the same district, but not closely related, in which the mimicked form is unpalatable or has an odour repulsive to birds and lizards. It must, of course, be understood that the mimicry is unconscious, the result, as in the cases of cryptic resemblance, having been brought about by natural selection—the less perfect the mimicry the more liable are the individuals to be attacked, and the less chance have they of reproducing their kind.

This imitation was first accounted for by Mr. Bates in the case of the Heliconidae, a group of showy, slow-flying abundant butterflies possessing "a strong pungent semi-aromatic or medicinal odour which seems to pervade all the juices of their system." It does not follow, of course, that what seems to us a disagreeably smelling fluid should prove distasteful to the palate of a lizard or a bird. But careful observation of the butterflies convinced both Bates and Wallace that they were avoided, or at any rate not pursued, by birds and other creatures; and Belt found that they were rejected by his tame monkey which was very fond of other insects. So their conspicuous wings, with spots and patches of yellow, red, or white upon a black, blue or brown ground, may fairly be considered an example of warning colouration—though Mr. Thayer has with great ingenuity and acumen endeavoured to show that the markings are effective for concealment and that their value as warning marks is doubtful. Now, says Mr. Beddard, "in the same situations as those in which the Heliconias are found there also occur, more rarely, specimens of butterflies minutely resembling the Heliconias, but belonging to a perfectly distinct family—the Pieridae. They belong to the two genera Leptalis and Euterpe, consisting of numerous species, each of which shows a striking likeness to some one particular species of Heliconia. This likeness is not a mark of near affinity; it affects no important character, but only the shape and colouration of the wings."

The particular resemblance here described was the origin of the theory of Protective Mimicry, the conditions under which it occurs being, according to Mr. Wallace: