Chapter 10

Fig. 9.Hebomoja glaucippe, from India; under surface.A, in flight.B, in resting attitude.

In all the nocturnal Lepidoptera it is theupperside of the wing which is sympathetically coloured, if protective coloration has been developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ, the anterior wings are grey banded with darker zigzag lines, and mottled with many shades of black, grey, yellow, red, and even violet. As the anterior wings cover the body and the posterior wings like a roof, they make the resting insect very inconspicuous when it has settled on wooden fences, trunks of trees, or even old timber. When bright colours—red, yellow, or blue—occur in these mothsit is always on the posterior wings, which are covered when at rest. This can best be observed in the species of the genusCatocala.

Let us now, however, interrupt our survey of the facts for a moment, and let us inquire whether all the cases of protective colouring in Lepidoptera we have considered can be referred to natural selection, or whether it is not conceivable that other causes may have evoked them.

Fig. 10.Xylina vetusta, after Rösel.A, in flight.B, at rest.

The first thing to be said is that the Lamarckian principle of the inherited effects of use and disuse cannot here be taken into account, as the colours of the surface of the body do not exercise any active function at all; their effect is due simply to their presence, and it is for them quite indifferent whether and how often they have opportunity to protect their bearers from enemies, or whether no enemies ever chance to appear. It has frequently been suggested, too, that these colorations are associated with the differences in the strength of the illumination to which the different parts and surfaces are exposed. But this again is untenable, as is proved even by the dimorphism frequently occurring in caterpillars, for the green and the brown individuals are exposed to precisely the same light; and still more clearly by the sympathetic colouring, which is so exactly defined and yet so different on the under surface of the diurnal butterflies. Yet there are isolated cases in which it seems as if the direct influence of the light had brought about certain striking differences in the colouring of the parts of an insect, and I shall describe perhaps the prettiest of these cases, to which Brunner von Wattenwyl directed attention. It concerns one of the Orthoptera of Australia, a Phasmid,Tropidoderus childreni, Grey, which has a general colouring of leaf-green, but with singular deviations from it on certain areas of the body. In this insect the anterior wings which form the wing covers or elytra (Fig. 11,V) are so short that they scarcely cover the half of the long abdomen. Their place is taken by the anterior margin of the posterior wing (H. horn), which is hard and horny like the elytra, and in the resting position protects the whole abdomen. All these covering parts are grass-green, except at the places where they overlap; on these areas they have a faded look, andare yellowish instead of green. Brunner says of this: 'The phenomenon gives the impression that the more brilliant colour is a character due to daylight. If several sheets of white paper of unequal dimensions be placed one above the other, ... and exposed to the sun, after a short time silhouettes of the smaller sheets will appear on the larger ones, either in a lighter or in a darker colour. Probably this "fading" of the covered parts in the Phasmid also belongs to this "category of photographs."' This seems convincing, but analogous phenomena in other insects prevent our regarding the pretty comparison with the photograph as a sufficient explanation. If it were a question of a diurnal butterfly, such an assumption would have to be rejected on this ground alone, that the wing colouring is developed in the pupa, and appears perfect and unalterable as soon as the perfect insect emerges. But in the pupa the position of the wings is exactly the reverse of that seen in the resting attitude of a butterfly, that is, the protectively coloured under side of the wing is not turned towards the light but away from it. Moreover, in the pupa the anterior wings cover the posterior ones completely, no matter what the wing position may be later in the perfect insect. Furthermore, the thick and often darkly coloured sheath of the pupa prevents the light having any effect, and not a few species pass their pupal stage in such dark places—for instance, under stones, as in the case of many 'Blues'—that the light can hardly reach them. And if the light did exercise an influence, how could it produce such diverse coloration as the protective colours of diurnal butterflies, on the one side dark, even to blackness, on the other side, yellow, reddish, and even white and pure green; and how should the same rays of light call forth complicated colour patterns on one and the same surface, for instance, the white, sprinkled with green, of the Aurora butterfly (Anthocharis cardaminis)? Finally, we have only to remember that numerous nocturnal Lepidoptera pass through their pupa stage underground, although they exhibit brilliant as well as protective colours in the most appropriate distribution, to reject once for all the hypothesis that the influence of light plays any decisive rôle in determining the distribution of the colours on the wings of Lepidoptera.

But it is otherwise withTropidoderus. In this case the wings grow gradually during the slow growth of the animal, which takes place in full light, and the wings of the young insect probably lie one above the other, in exactly the same position, and cover the same places as in the full-grown form; we might, therefore, from the facts of the case, admit the possibility that the yellow of the covered portions is due to the exclusion of light.

Fig. 11.Tropidoderus childreni, after Brunner von Wattenwyl, in flying pose.Vanterior wing.H. häut, membranous part of posterior wing.H. horn, horny portion.

But as soon as the conditions that obtain among Lepidoptera are also taken into consideration we recognize the insufficiency of the interpretation suggested, for among butterflies we have precisely the same phenomenon—sharp limitation of the protective colouring to the parts visible in the resting position, a fact which, in the case of the said butterflies, admits of no other interpretation than that of natural selection. Let us therefore see if we cannot, in the case ofTropidoderus, arrive at some better understanding of the phenomenon than that implied in the theory of direct light-influence. Obviously, the yellow parts of the animal do not require to be green, since they are not visible in the sitting position, and the locust in flight could not by any device be made invisible. It therefore only remains to be explained why the yellow parts are not colourless, and why they are not also green. We cannot at present answer with any confidence; it is possible that the colouring matter which causes the green only becomes green under the influence of direct sunlight, and otherwise remains yellow; it is possible, too, that, as in Lepidoptera (see Fig. 9), the full protective colour is only developed by natural selection in the places which are visible in the sitting position, andthat the covered parts take on any indifferent colour, which might be readily afforded by the metabolism of the insect. But this much is certain, that the covered parts would be green, if that were advantageous to the survival of the species, just as the under surface of the wings of some diurnal butterflies is green. Had it been required, the green colour would have resulted in the course of natural selection, just as it has resulted in the most different parts of the most diverse insects, even in those whose development takes place entirely removed from the influence of light. Therein lies the difference between our interpretation and that of Brunner von Wattenwyl: without natural selection no explanation of this case is possible.

Fig. 12.Notodonta camelina, after Rösel.A, in flight.B, at rest.

Hitherto I have spoken only of the diurnal butterflies in which the anterior wings show an extension of the protective colouring which marks the whole surface of the posterior wings, and it was always the tips of the anterior wings that were thus coloured. But among the nocturnal Lepidoptera there are corresponding cases, in which a little tip of the posterior wing forms the continuation of the protective surface of the anterior wing. Some species ofNotodontaand allied genera show in the posterior corner of the otherwise whitish posterior wings a little grey spot, and a hair tuft which in colour, and—when it is big enough—in marking, exactly resembles the protectively coloured anterior wings (Fig. 12). The 'why' is at once clear, when one looks at the insect in the resting position, for only this little corner of the wing projects beyond the covering anterior wing. This has been regarded as telling against natural selection, for such a little spot could not possibly, by its colour, turn the scale as to the life or death of the individual, and so could not be selected. But one might say the same of the tip of the anterior wing in the diurnal forms, although there the protective surface is larger, often much larger. But who is to decide how large an exposed, unprotected spot must be in order to attract the attention of an enemy on the look out for food? Orwho can prove that the best and most familiar protective colouring really protects its possessors? What if, after all, it is all a game, a joke, which the Creator is playing with us poor mortals? Did not a trustworthy observer recently watch carefully, and see how a pair of sparrows daily cleared a wooden fence on which moths of the genusCatocalaand other species of nocturnal Lepidoptera, excellently furnished with protective colours, were wont to settle by day? They did their work thoroughly, and hardly overlooked a single individual. But who has a right to see anything more in this than—what surely goes without saying—that the best protective colouring is not an absolute protection, and never preserves all from destruction, but always only some, and it may be very few.

How else could there be such a high ratio of elimination, and such a constancy in the number of individuals of a species on any unchanging area? These sparrows had simply made full use of an experience, probably acquired by chance to begin with, and their vision had become sharpened for this particular species on the almost similarly coloured wooden fence, just as that of the expert butterfly collector does. It certainly does not follow from this that the protective colouring was useless, nor can we regard the harmony between the protruding tip of the anterior or posterior wing and the large protectively coloured surface of the covering wing as of no importance. On the contrary, if the tips were white or conspicuously coloured like the rest of the posterior wing, they would assuredly attract the sharp eye of hungry enemies to the spot, and so betray the victim. Instead of this, the spot in question is not only dark, but, in the case ofNotodonta, is furnished with a tuft of hairs, which, in the insect's resting position (Fig. 12,B), lies on the back, and looks like a dark, somewhat curved projecting tooth, in front of which there stands another, quite similar, which arises from the anterior wing, and behind there are other seven, rather smaller, dark teeth of the same kind, springing from the outer edge of the anterior wing. Taken altogether, they mimic the dentated edge of a withered leaf, and thus, in spite of their diverse origins, form a unified picture, and one with a considerable protective value. How is it possible to doubt that each of these hair-tufts has arisen under the influence of natural selection, and that its absence or imperfect development might result in the discovery and elimination of the insect concerned?

These cases seem to me particularly beautiful proofs of the productive efficiency of selection. The wing is protected just as far as it protrudes from beneath the other—not a millimetrefurther! How should it be otherwise, when the colouring of the parts just beside these is indifferent for the species, so that any variations in these parts in the direction of protective colouring never survive to be transmitted and accumulated?

It is precisely this restriction to what is absolutely necessary that is the surest sign, here and elsewhere, that the character in question has been brought about by natural selection. And if this is the only possible, and at the same time quite sufficient explanation of the remarkably well-defined colour deliminations in all Lepidoptera, there can be no reason why we should try to drag in any other factor to explain the case ofTropidoderus, the less so as here again selection alone can account for the green of the exposed surfaces; and furthermore, the modification, common in other Phasmidæ, of the most anterior green stripe of the posterior wing into a firm cover protecting the soft abdomen, also points to natural selection; the cover-wings proper have here become too short, and so the edge of the posterior wing has been modified into a hard rib, which protects the soft body of the insect (Fig. 11,H. horn). No differences in illumination, and nodirecteffect of any external influence whatever could have brought that about.

How much more I might adduce in this connexion! The manifold diversity of colour and form adaptation is so great among insects, to which protection from their enemies is so necessary, and especially among butterflies, that I should never come to an end if I were to try to give even an approximate idea of it. Let us, therefore, turn now from such cases to a higher—the highest—grade of adaptation, that in which there is not only a mimicry of special and complex coloration, but in which the whole animal has become like some external object, and is thereby secured from discovery.

We must first consider the case of our lappet moth (Gastropacha quercifolia), which in its copper-red colour and in the remarkable shape and dentated edges of its wings, and finally in the quite extraordinary clucking-hen-like attitude of the wings when at rest, greatly resembles some dry oak-leaves lying one above the other.

Not unlike this is a 'shark' moth found in this country,Xylina obsoleta, which, as the name indicates, looks when at rest like a broken bit of half-rotten wood (Fig. 10, p. 77). It 'feigns death,' as we commonly say, that is, it draws the legs and antennæ close to the body, and does not move; indeed, one may lift it up and throw it on the ground without its betraying by a single twitch that it lives. Only after it has been left undisturbed for some time does it showsigns of life again, and makes off hastily, to find a better hiding-place. The colouring of this moth is so curiously mingled—brown, whitish, black, and yellow—and traced with acute-angled lines and curves, that one cannot distinguish it at sight from a bit of rotten wood. I experienced that myself once when, passing a hedge, I thought I saw aXylinasitting on the ground, and picked it up to examine it. I thought it was a bit of wood, and, disappointed, I threw it down again on the grass, but then I felt uncertain, and picked it up once more—to find that it was a moth after all[1]!

[1]Rösel says in this connexion: 'The marvellous form of this Papilio preserves it from injuries, for, when he hangs freely on a trunk of a tree, he would be taken ten times sooner for a piece of bark than for a living creature. By day, too, he is so little sensitive, that if he be thrown down from his resting-place he falls to the ground as if lifeless, and remains lying motionless. One may also throw him into the air, or turn him about, and he will rarely give a sign of life. I have impaled many of them on needles, without seeing any sign of sensitiveness on their part. This is the more remarkable that these birds (sic), after they have submitted to all the torment and misery one can inflict on them, without showing any sign of feeling, will, whenever they are left in peace and have no further disturbances to fear, quickly creep off to a dark corner and attempt to conceal themselves from future attacks.'—Insektenbelustigungen, Nürnberg, 1746, vol. i. p. 52.

This case ofXylinais hardly less remarkable, and its likeness to the mimicked object is scarcely less wonderful than that of the often discussed mimicry of a leaf, with stalk, midrib, and lateral veins, by many of the forest butterflies of South America and India.

Fig. 13.Kallima paralecta, from India,right under side of the butterfly at rest.K, head.Lt, maxillary palps.B, limbs.V,anterior wing.H, posterior wing.St, 'tail'of the latter, corresponding to the stalk ofthe leaf.gl1andgl2, transparent spots.Aufl,eye-spots.Sch, mould-spots.

The best known of these is the IndianKallima paralecta, which, when it settles, is deceptively like a dead leaf, or rather like a dry or a half-withered one, on which brown alternates with red, and on which there are one or two translucent spots, without scales, presumably representing dewdrops. The upper surface of this butterfly is simply marked, but gorgeously coloured—blue-blackwith a reddish yellow, or bluish white band—and quite constant. The under surface, on the other hand, although it always resembles a dead leaf, shows very varied ground colours, being sometimes greyish, sometimes yellowish, or reddish yellow, or even greenish. Often it shows the lateral veining of the leaf quite as distinctly as in Fig. 13, but often quite indistinctly, and the black, mouldy spots (Sch) of our figure may be more strongly marked, or they may be absent. It would seem as if the mimicry of different kinds of leaves was here aimed at—so to speak—just as in the case of the varied and numerous species of the South American genusAnæa, which usually live in the woods, and are all more or less leaf-like, but each species is like a different leaf, or like a leaf in a different condition, dry, moist, or decomposing. It is simply astounding to see this diversity of leaf mimicry, and the extraordinary faithfulness with which the impression of the leaf is reproduced. But it is by no means always the venation which causes the resemblance, for this is often inconspicuous; the high degree of deceptiveness is due to the silvery-clear yellow, dark yellow, red-brown to dark black-brown ground-colouring, which is never quite uniform, and over which there usually spreads a whitish ripple, combined with the remarkable imitation of the sheen of many leaves. The upper side of this butterfly is almost always conspicuously decorated with violet, dark blue or red, but always without any relation to the under surface. Not in all, but in many of the species of this genus, we find the round, translucent mirrors on the wing already mentioned in the case ofKallima, and in some species quite remarkable means are made use of to make the resemblance to a leaf thoroughly deceptive. ThusAnæa polyxo, when sitting, looks like a leaf out of the edge of which a caterpillar has eaten a little piece; in reality there is nothing missing from the wing, but on the front margin of the anterior wing a semicircular spot of a bright, soft, yellow colour stands out so sharply from the rest of the chestnut-brown wing surface, that it has the effect of a hole in the leaf.

Fig. 14.Cœnophlebia archidona, from Bolivia, in itsresting attitude.mr, midrib of the apparent leaf.st, the apparent stalk.

A modern opponent of the selection theory (Eimer) has suggested that the marking of the lateral veins, and other resemblances to a leaf inKallima, represent nothing more than the pattern which was present in any case, inherited from ancestors, and which in the course of time arranged itself in a particular manner according to internal developmental laws. Not selection—that is, adaptation to surroundings—but the internal developmental impulse has brought about the resemblance to the leaf. It is astonishing how a preconceived idea can blind a man and weaken his judgment! It goeswithout saying that the adaptations do not start from atabula rasa, but from what is already present; of course, natural selection makes use of the markings inherited from ancestors; it takes what already exists, and alters or extends it as suits best. Thus it is easy to prove that the clear mirrors (Fig. 13,gl1andgl2) on the wings ofKallimahave arisen from a modification of the nuclei of eye-spots, just as the dark mould-spots which often occur, frequently develop in association with the inherited eye-spots; not always however, for many such accumulations of black scales occur in spots on which there has never been an eye-spot. Thus, too, the 'midribs' of the butterfly have in part arisen from a gradual displacing, extending, and altering of the direction of inherited stripes as, for instance, is clearly recognizable in the posterior wing of Fig. 13, but sometimes they are new formations. But the veining of a leaf is never found on the wing of any butterfly of a species which has not the habit of resting among leaves, or which has not had it at one time, and it never corresponds to the natural marking of any genus which does not live in forests. This impression of leaf-venation has obviously arisen from quite different patterns of markings, and it has been reached now by one way, now by another. We can see this from the fact that, in different butterflies, it lies in quite different positions on the wing. In theKallimaspecies the stalk of the leaf lies in the tail of the posterior wing, the tip of the midrib lies near the tip of the wing; inCœnophlebia archidonait is exactly reversed, the tip of the anterior wing (Fig. 14) is prolonged, and forms the stalk, while a broad, dark, stripe, the midrib (mr), runs from there across the middle of both wings, and seems to give off two or three lateral ribs running outwards. If it be asked whether this butterfly always sitsdown so artistically that the 'upward turning leaf-stalk is in juxtaposition to a twig,' we may answer that a bird flying fast is not likely to look to see whether every leaf in the profusion of foliage in the primitive forests is properly fastened to its stalk or not, any more than we should do in the case of a painted bush, on which many a leaf has the appearance of floating in the air, just as in nature, or in its faithful copy, the photograph.

Fig. 15.Cærois chorinæus, from the lower Amazon, in itsresting attitude.V, anterior wing.H, posterior wing.mr,midrib of the apparent leaf.sr, lateral veins.st, hint ofa leaf-stalk.

Quite different from the leaf-marking either ofCœnophlebiaorKallimais that of one of the Satyrides of the lower Amazon valley,Cærois chorinæus(Fig. 15). If one spreads this butterfly out in the usual way it does not look in the least like a leaf, and one only sees a number of curiously placed disconnected stripes on the under surface of the wing. But if the wings be folded together to correspond with the sitting position of the butterfly, there appears the figure of a leaf, of which, however, only half is present, and whose midrib (mr) runs obliquely forward from the inner angle of the posterior wing. Here, again, it is not difficult to guess that this straight stripe has arisen, by displacement and straightening, from a curved line inherited from some remote ancestor, and it is these precise changes which are the work of the adaptive processes of natural selection. The same applies to the lateral ribs (sr), which are here four in number.

But even the division of the wing surface by a single dark line, such as that which crosses the middle of the posterior wing ofHebomoja(Fig. 9), an Indian butterfly, heightens not inconsiderably the resemblance of the resting butterfly to a leaf, a resemblance whichhas already been shown in the form and colour. Indeed, even the sharp division of the wing surface into a darker inner and a lighter outer portion, which occurs in many species ofAnæa, gives a very vivid impression of a leaf crossed by a midrib.

It is not without a purpose that I have lingered so long over the leaf-butterflies. I wished to make it clear that we have by no means to do with a few exceptional cases, but with a great number, in all of which resemblance to a leaf has been aimed at, although it has been attained in varying degrees, and by very diverse ways. Whoever surveys this wealth of fact must certainly receive the impression, that, wherever it was advantageous to the existence of the species, the evolution of such a deceptive resemblance has also been possible. In any case one cannot but be convinced that it is not a case of chance resemblance, as some naturalists have recently tried to maintain.

But I have not yet quite finished my outline-survey of the facts, for I must not omit to mention that, in the evergreen tropical forests, there are also large nocturnal Lepidoptera, which mimic leaves, sometimes green ones, sometimes brown, dead ones.

Fig. 16.Phyllodes ornata, from Assam.Upper surface with leaf-like marking onlyon the anterior wing, which is the onlypart visible when at rest; 2/3 nat. size.

Fig. 16 gives a good picture, reduced to two-thirds, of such a species,Phyllodes ornata, from Assam. The posterior wings are conspicuously coloured in deep black and yellow; in the resting position they are covered by the anterior wings, and these are red-brown with black markings which precisely and clearly mimic the ribs of a leaf. The midrib begins near the tip of the anterior wing, but breaks off half-way across the wing at two silvery white spots, similar to those in many of the diurnal forms, which also mimic decaying leaves. Three pairs of side veins go off backwards and forwards with remarkable regularity from the midrib, almost at the same angle, and parallel to one another, and three more are indicated by vague shading. Then the midrib begins again in the internal half of the wing, though only represented by a broad shading. The whole suggests two torn, rotten leaves, one partly covering the other; and the deception will certainly be perfect when the moth rests on the ground or among decaying leaves.

That all these extremely favourable protective colorations findtheir explanation in the slow and gradually cumulative effects of natural selection cannot be disputed; it is beyond doubt that they cannot be explained, so far as we know, in any other way.

If, however, it were possible for a species of butterfly living in the forest and among leaves to become, through natural selection, in any degree, and in a continually increasing degree, like a leaf, surely many insects living in the woods, and especially in the tropical woods, would also have followed such an advantageous path of variation—at least, so we should be inclined to think. And this is indeed the case; numerous insects, of different orders, if they are as large as a leaf, have taken on the colour, form, and usually also the markings, of a leaf. Thus green and also decaying and dead leaves are most realistically imitated by many tropical Locustidæ. BesidesTropidoderus, figured onp. 79, aPterochroaof South Brazil affords a particularly fine illustration of this, for not only does the ground-colour, brown or green, harmonize with that of a dead or fresh leaf, but, at the same time, all sorts of details are marked on the insect, which help to heighten the deceptive impression. Even the outline of the wings is leaf-like, and leaf-veins are marked on the wing-covers with the most beautiful distinctness, and finally there is, especially in the light-green individuals, a spot at the wing tip which, by means of a mixture of brown, yellow, reddish, and violet colour-tones, mimics a decaying spot with astonishing fidelity. Here, again, the origin of this special adaptation can be clearly recognized, for the vaguely concentric arrangement of the colours indicates that, in the ancestors of the species, an eye-spot had occurred on this area, of the same kind as we still see on the posterior wing, which is covered in the resting position. Thus we can again look back on the history of the species and conclude that the dissolution and degeneration of the eye-spot began at the time when the leaf resemblance was evolved, and this was probably caused by some change of habitat, which we can now no longer guess at.

Many species of leaf-like Orthoptera, both in the Old and New World, have tough, green, parchment-like wing-covers which bear a remarkable resemblance to the thick Magnolia-like leaves of tropical plants. Along with these we must also mention the 'walking leaf,' which has been well known for centuries. In its case, not the wing-covers alone, but the head and thorax, and even the legs, are of the colour and shape of a leaf.

The stick-insects, too, must not remain unnoticed; those quaint inhabitants of warm countries, whose elongated brown body looks like a knotted twig, and whose long legs, likewise stick-like, are stretchedout irregularly at different angles to the body, and usually remain motionless when the insect is resting. These creatures are vegetarian, and generally keep so still, that even the naturalist who is on the look-out for them may easily overlook them. Even such an experienced student of insects as Alfred Russel Wallace was deceived, for a native of the Phillipines once brought him a specimen as a 'walking-stick' insect, which he rejected, saying that this time it was no animal but really a twig, until the native showed him that it was an insect whose likeness to a twig was increased by the fact that it bore on its back a ragged green growth, which looked exactly like a liverwort (Jungermannia), which occurs on the twigs of the trees in that region.

We must also notice here the thorn-bugs, which are numerous on the prickly shrubs of tropical deserts and plateaux, especially in Mexico. These bear on the relatively very small body two or three large spines, which make them look like a part of the thorny bush on which they sit. But this masking by mimicry of thorns is not confined to insects, it is seen in lizards as well, notably inMoloch horridus, a lizard that lives in the Australian bush, and is covered all over with thorn-like scales.

These examples should be enough to show that mimicry of the usual surroundings on the part of animals which are in need of protection, or are wont to lurk on the watch for their prey, are not isolated exceptions, chance resemblances, or, as they used to be called, 'freaks of nature,' but that, on the contrary, they are the rule, depending on natural causes, and always occurring when these causes are operative. That such protective resemblances seem to be much more frequent in warmer climates than with us is probably a fallacy due to the fact that the number of species (especially of insects) is very much greater there, and that many insect types have their representatives of considerable size of body, which not only makes them more conspicuousto us, but makes some protective device in relation to their enemies or victims much more necessary.

But we must here take account of one more example which occurs in our fauna in many modifications: the caterpillars of Geometridæ. Many of these soft and easily injured caterpillars resemble closely, in colour and shade, the bark of the tree or shrub on which they live (Fig. 17). At the same time they have the habit, when at rest, of stretching themselves out straight and stiff, so that they stand out free, at an acute angle from the branch, thus seeming like one of its lateral twigs. In many species the resemblance is heightened by the extraordinary pose of the head (K) and of theclaw-like feet (F), which, partly pressed close to the head, partly standing out from it, give the anterior end of the caterpillar the appearance of two terminal buds, while various little pointed, knotlike warts, scattered over the body, represent the sleeping buds of the little twig. Who has not at one time or other taken such a caterpillar for a little branch, and not inexpert observers only, but even trained naturalists? Many a time I have not been able to make quite sure of what I had before me until I touched it!

Fig. 17.Caterpillar ofSelenia tetralunaria, seated on a birch twig.K, head.F, feet.m, tubercle, resembling a 'sleeping bud'; nat. size.

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