CHAPTER IX.Details of Cœlenterata.
I. Hydrozoa.
A. Hydrida.
T
THE Hydras, as a rule, are not coloured in our sense of the term; that is to say, they are of a general uniform brown colour. But in one species,H. viridis, the endoderm contains granules of a green colour, which is said to be identical with the green colouring matter of leaves (chlorophyll). This does not occur in all the cells, though it is present in most. The green matter occurs in the form of definite spherical corpuscles, and these colour-cells define the inner layer of the integument (the endoderm), and render it distinct.[22]That portion of the endoderm which forms the boundary of the body-cavity has fewer green corpuscles, but contains irregular brown granules, thus roughly mapping out a structural region.
We thus see that even in so simple a body as the Hydra the colouring matter is distributed strictly according to morphological tracts.
B. Tubularida.The Tubularian Hydroids are the subject of an exhaustive and admirably illustrated monograph by Prof. J. Allman, from which the following details are culled. These animals are with few exceptions marine, and consist either of a single polypite or of a number connected together by a common flesh, or cœnosarc. Some are quite naked, others have horny tubes, into which, however, the polypites cannot retreat. The polypites consist essentially of a sac surrounded with tentacles; and one of their most striking charactersis their mode of reproduction. Little buds (gonophores) grow from the cœnosarc, and gradually assume a form exactly like that of a jelly-fish. These drop off, and swim freely about; and are so like jelly-fishes that they have been classed among them as separate organisms.
The Tubulariæ are all transparent; and in them we find structural colouration finely shown, the colour, as is usual in transparent animals, being applied directly to the different organs.
Writing of the colour, Prof. Allman says: "That distinct secretions are found among the Hydroida, and that even special structures are set aside for their elaboration, there cannot now be any doubt.
"One of the most marked of these secretions consists of a coloured granular matter; which is contained at first in the interior of certain spherical cells, and may afterwards become discharged into the somatic fluid. These cells, as already mentioned, are developed in the endoderm;[23]in which they are frequently so abundant as to form a continuous layer upon the free surface of this membrane. It is in the proper gastric cavity of the hydranth and medusa, in the spadix of the sporosac, and in the bulbous dilatations which generally occur at the bases of the marginal tentacles of the medusæ, that they are developed in greatest abundance and perfection; but they are also found, more or lessabundantly, in the walls of probably the whole somatic cavity, if we except that portion of the gastrovascular canals of the medusa which is not included within the bulbous dilatations.
"In the parts just mentioned as affording the most abundant supply of these cells, they are chiefly borne on the prominent ridges into which the endoderm is thrown in these situations; when they occur in the intervals between the ridges they are smaller, and less numerous.
"The granular matter contained in the interior of these cells varies in its colour in different hydroids. In many it presents various shades of brown; in others it is a reddish-brown, or light pink, or deeper carmine, or vermilion, or orange, or, occasionally, a fine lemon-yellow, as in the hydranth ofCoppinia arcta, or even a bright emerald green, as in the bulbous bases of the marginal tentacles of certain medusæ. No definite structure can be detected in it; it is entirely composed of irregular granules, irregular in form,and usually aggregated into irregularly shaped masses in the interior of the cells. It is to this matter that the colours of theHydroida, varying, as they do, in different species, are almost entirely due.
"The coloured granular matter is undoubtedly a product of true secretion; and the cells in which it is found must be regarded as true secreting cells. These cells are themselves frequently to be seen as secondary cells in the interior of parent cells, from which they escape by rupture, and then, falling into the somatic fluid, are carried along by its currents, until, ultimately, by their own rupture, they discharge into it their contents.
"We have no facts which enable us to form a decided opinion as to the purpose served by this secretion. Its being always more or less deeply coloured, and the fact of its being abundantly produced in the digestive cavity, might suggest that it represented the biliary secretion of higher animals. This may be its true nature, but as yet we can assert nothing approaching to certainty on the subject; indeed, considering how widely the cells destined for the secretion of coloured granules are distributed over the walls of the somatic cavity, it would seem not improbable that the import of the coloured matter may be different in different situations; that while some of it may be a product destined for some further use in the hydroid, more of it may be simply excretive, taking no further part in the vital phenomena, and intended solely for elimination from the system."[24]
Here we have very definite statements by a highly trained observer of the distribution of colour in the whole of these animals, and of the conclusions he draws from them.
Firstly as to the colour itself. We find it true colour—brown, pink, carmine, vermilion, orange, lemon-yellow, and even emerald green; a set of hues as vivid as any to be found in the animal kingdom. It is difficult to conceive these granules to be merely excrementitious matter; for in such simple creatures, feeding upon such similar bodies, one would hardly expect the excretive matter to be so diversified in tint. Moreover, excrementitious matter is not, as a rule, highly coloured, but brown. Thus, we see in the Rhizopods the green vegetable matter which has been taken in as food becomes brown as the process of assimilation goes on; and, indeed, colour seems almost always to be destroyed by the act of digestion.
Still, it by no means follows that this colour, even if it isproduced for the sake of decoration, as we suggest, may not owe its direct origin to the process of digestion. The digestive apparatus is the earliest developed in the animal kingdom, and in these creatures is by far the most important; the cœlenterata being, in fact, little more than living stomachs. If, then, colouration be structural, what is more likely than that the digestive organs should be the seat of decoration in such transparent creatures?
Secondly, as to the distribution of the colour. We find it "frequently forming a continuous layer upon the free surface of" the endoderm, in the "spadix of the sporosac," and in the "bulbous terminations" of the canals, that colour is best developed. In other words, the colour is distributed structurally, and is most strongly marked where the function is most important.
Prof. Allman gives no hint that the colour may be purely decorative, and is naturally perplexed at the display of hues in such vigour; but if this be one of the results of the differentiation of parts, of the specialization of function, then we can, at least, understand why we find such brilliant colour in these creatures, and why it is so distributed.
As an illustration of theTubulariawe have selectedSyncoryne pulchella,Fig. 2, Pl. VI., and its medusa,Fig. 1. The endoderm of the spadix of the hydranths is of a rich orange colour, which becomes paler as it descends towards the less highly organized stem. Medusæ are seen in various stages of development, and one, mature and free, is shown. In these the manubrium, and the bulbous terminations of the canals are also seen to be coloured orange.
In these medusæ we find the first appearance of sensory organs. They consist of pigment-cells enclosed in the ectoderm, or outside covering; and are singular as presenting the first true examples of opaque colouring in the animal kingdom. They are associated with nerve cells attached to a ring of filamentous nerve matter, surrounding the base of the bell. In some important respects the pigment differs from that in other parts of the animal. It is more definite in structure; and the whole ocellus is "aggregation of very minute cells, each filled with a homogeneous coloured matter."[25]These ocelli, and similar sense organs, calledlithocysts, are always situated over the bulbous termination of the canals. The pigment is black (as in this case), vermilion, or deep carmine.
Plate VI.SYNCORYNE PULCHELLA.
Plate VI.
SYNCORYNE PULCHELLA.
The dependence of colour upon structure is thus shown to hold good throughout these animals in a most remarkable manner, and the acceptance of the views here set forth gives us an insight into the reasons for this colouration which, as we have seen, did not arise from the study of the question from the ordinary point of view.
C. Sertularida.These animals are very similar to the last, but they are all compound, and the polypites can be entirely withdrawn within the leathery investment or polypary. Their mode of reproduction is also similar, and their colouration follows the same general plan. Being so like the preceding order, it is unnecessary to describe them.
B. Siphonophora.
The Siphonophora are all free-swimming, and are frequently called Oceanic Hydrozoa. They are divided into three orders, viz.:—
a. Calycophoridæ.b. Physophoridæ.c. Medusidæ.
a. Calycophoridæ.b. Physophoridæ.c. Medusidæ.
a. Calycophoridæ.These animals have a thread-like cœnosarc, or common protoplasm, which is unbranched, cylindrical, and contractile. They are mostly quite transparent, but where colour exists it is always placed structurally. Thus, inDiphyesthe sacculi of the tentacles are reddish, inSphæronectesthey are deep red, and inAbylathe edges of the larger specimens are deep blue.[26]
b. Physophoridæ.These creatures are distinguished by the presence of a peculiar organ, the float, orpneumatophore, which is a sac enclosing a smaller sac. The float is formed by a reflexion of both the ectoderm and endoderm, and serves to buoy up the animal at the surface of the sea. The best known species is the Physalia, or Portuguese Man-o'-War.
Prof. Huxley, in his monograph on the Oceanic Hydrozoa, gives many details of the colouration; and, not having had much opportunity of studying them, the following observations are taken from his work. It will be seen that the Physophoridæ illustrate the structural distribution of colour in a remarkable manner.
Stephanomia amphitridis, the hydrophyllia, colourless, and sotransparent as to be almost imperceptible in water, cœnosarc whitish, enlarged portions of polypites, pink or scarlet, sacs of tentacles scarlet.
The enlarged portion of the polypites is marked with red striæ, "which are simply elevations of the endoderm, containing thread-cells and coloured granules." The small polypites do not possess these elevations, and are colourless.
Agalma breve, like a prismatic mass of crystal, with pink float and polypites.
Athorybia rosacea, float pink, with radiating dark-brown striæ, made up of dots; polypites lightish red, shading to pink at their apices; tentacles yellowish or colourless, with dark-brown sacculi; thread-cells dark brown.
Rhizophysa filiformis, pink, with deep red patch surrounding the aperture of the pneumatocyst.
Physalia caravilla, bright purplish-red, with dark extremities, and blue lines in the folds of the crest; polypites violet, with whitish points, larger tentacles red, with dark purple acetabula, smaller tentacles blue, bundles of buds reddish.
P. pelagica, in young individuals pale blue, in adult both ends green, with highest part of crest purple, tentacles blue, with dark acetabula; polypites dark blue, with yellow points.
P. utriculus.Prof. Huxley describes a specimen doubtfully referred to this species very fully, as follows:—
"The general colour of the hydrosoma is a pale, delicate green, passing gradually into a dark, indigo blue, on the under surface.
"The ridge of the crest is tipped with lake, and the pointed end is stained deep bluish-green about the aperture of the pneumatocyst.
"The bases of the tentacles are deep blue; the polypites deep blue at their bases, and frequently bright yellow at their apices; the velvetty masses of reproductive organs and buds on the under surface are light green."
He further remarks that the tentacles have reniform thickenings at regular intervals, and "the substance of each thickening has a dark blue colour, and imbedded within it are myriads of close-set, colourless, spherical thread-cells."
It would not be possible to find a more perfect example of regional colouration. Not only is each organ differently coloured,but the important parts of each organ, like the ridge of the crest, the bases of the tentacles, and the thread-cell bearing ridges of the tentacles, are also emphasized with deep colour.
Velella.This beautiful creature, which sometimes finds its way to our shores, is like a crystal raft fringed with tentacles, and having an upright oblique crest, or sail. The margins of the disk and crest are often of a beautiful blue colour, and the canals of the disk become deep blue as they approach the crest. The polypites may be blue, purple, green, or brown.
C. Medusidæ.The structure and colouration of the true Medusæ are so like that of the medusiform larvæ of the other Hydrozoa, that they need not be particularly described.
D. Lucernarida.Of this sub-class we need only cite theLucernariathemselves; which are pretty bell-shaped animals, having the power of attaching themselves to seaweeds, etc., and also of swimming freely about. Round the margin are eight tufts of tentacles, opposite eight lobes, the membrane between the lobes being festooned. InL. auricula, a British species, the membrane is colourless and transparent, the lobes bright red, or green, and the tentacles blue.
As a group the Hydrozoa display regional colouration in a very perfect manner.
II. Actinozoa.
It is not necessary to trace the colouration through all the members of this group, but we will trace the variation of colour through two species of anemonies, which have been admirably studied by Dr. A. Andres.[27]The first column shows the general hue, the second the tints of that hue which are sufficiently marked to form varieties as cochineal red, chocolate, bright red, rufous, liver-coloured, brown, olive, green and glaucous. The third column gives the spotted varieties, from which it will be seen that the chocolate, liver, and green coloured forms have each coloured varieties. It will be seen that the range of colour is very great, passing from pale pink, through yellowish-brown to blue-green.
Varieties of Actinea Cari.
The following brief descriptions illustrate the distribution of the colour:—
Actinea Cari.
Uniform varieties (Homochroma).
In this table the varieties above mentioned are further particularized. The column is the stalk or body, the tentacles are the arms, the gonidia the eye spots, and the zone the line round the base. It will be noticed that these regions are often finely contrasted in colour.
Bunodes gemmaceusis another variable form, and the following varieties are recognised.
Heterochroma.
α. Ocracea(type), peristome ochre yellow, zone black, tentacles grey, with blue and white spots.
β.Pallida, peristome whitish grey unbanded, tentacles with white spots.
γ.Viridescens, peristome greenish white unbanded, tentacles with white spots and rosy shades.
δ.Aurata, column at base golden, peristome intenser yellow with crimson flush, tentacles grey with ochreous and white spots.
ε.Carnea, column at base flesh coloured, peristome rosy, tentacles rosy, with white spots.
Homochroma.
ζ.Rosea, like ε, but with rosy tubercles.
η.Nigricans, peristome blackish, with blue and green reflexions (riflessi).
A few other examples may be given, all of which can be studied in Dr. André's magnificently coloured plates.
Aiptasia mutabilisis yellow brown, the tentacles spotted in longitudinal rows, the spots growing smaller towards the tip, thus affording a perfect example of the adaptation of colour to structure.
Anemonia sulcatahas normally long light yellow pendulous tentacles tipped with rose, but a variety has the column still yellow but the tentacles pale green, tipped with rose.
Bunodes rigidushas the column green, with rows of crimson tubercles, the tentacles are flesh-coloured, except the outer row which are pearly; the peristome is green, with brown lips.
CHAPTER X.The Colouration of Insects.
I
IN the decoration of insects and birds, nature has exerted all her power; and amongst the wealth of beauty here displayed we ought to find crucial tests of the views herein advocated. It will be necessary, therefore, to enter somewhat into detail, and we shall take butterflies as our chief illustration, because in them we find the richest display of colouring. The decoration of caterpillars will also be treated at some length, partly because of their beauty, and partly because amongst them sexual selection cannot possibly have had any influence.
Butterflies are so delicate in structure, so fragile in constitution, so directly affected by changes of environment, that upon their wings we have a record of the changes they have experienced, which gives to them a value of the highest character in the study of biology. In them we can study every variation that geographical distribution can effect; for some species, like the Swallow-tail (Papilio machaon) and the Painted Lady (Cynthia cardui), are almost universal, and others, like our now extinct Large Copper (Lycæna dispar), are excessively local, being confined to a very few square miles. From the arctic regions to the tropics, from the mountain tops to the plains, on the arid deserts and amidst luxuriant vegetation, butterflies are everywhere to be found.
Before entering into details, it will be as well to sketch some of the broad features of butterfly decoration. In the first place they are all day-fliers, and light having so strong an influence upon colour, there is a marked difference in beauty between them and the night-flying moths. A collection of butterflies viewed side by side with a collection of moths brings out this fact more strongly thanwords can describe, especially when the apparent exceptions are considered; for many moths are as brightly coloured as butterflies. These will be found to belong either to day-flying species, like the various Burnets (Zygæna), Tiger Moths (Arctia), or evening flyers like the Hawk Moths (Sphyngidæ.) The true night-flying, darkness-loving moths cannot in any way compare with the insects that delight in sunshine. We see the same thing in birds, for very few nocturnal species, so far as we are aware, are brilliantly decorated.
Another salient feature is the difference that generally exists between the upper and lower surfaces of the wings. As a rule, the upper surface is the seat of the brightest colour. Most butterflies, perhaps all, close their wings when at rest, and the upper wing is generally dropped behind the under wing, so that only the tip is visible. The under surface is very frequently so mottled and coloured as to resemble the insect's natural surroundings, and so afford protection. It does not follow that this protective colouring need be dull, and only when we know the habit of the insect can we pronounce upon the value of such colouring. The pretty Orange-tip has its under wings veined with green, and is most conspicuous in a cabinet, but when at rest upon some umbelliferous plant, with its orange tip hidden, these markings so resemble the environment as to render the insect very inconspicuous. The brilliantArgynnis Lathonia, with its underside adorned with plates of metallic silver, is in the cabinet a most vivid and strongly-marked species; but we have watched this insect alight among brown leaves, or on brown stones, outside Florence, where it is very common, and find that these very marks are a sure protection, for the insect at rest is most difficult to see, even when it is marked down to its resting-place.
But some butterflies have parts of the under surface as gaily decorated as the upper; and this not for protection. This may be seen to some extent in our own species, for instance in the orange-tip of the Orange-tip, and the red bar in the upper wing of the Red Admiral (V. atalanta). If we watch these insects, the conviction that these are true ornaments is soon forced upon us. The insect alights, perhaps alarmed, closes its wings, and becomes practically invisible. With returning confidence it will gradually open its wings and slowly vibrate them, then close them again, and lift the upper wing to disclose the colour. This it will do many times running, and the effect of the sudden appearance and disappearance of the brighthues is as beautiful as it is convincing. None can doubt the love of display exhibited in such actions.
The delicacy of their organization renders butterflies peculiarly susceptible to any change, and hence they exhibit strong tendencies to variation, which make them most valuable studies. Not only do the individuals vary, but the sexes are often differently coloured. Where two broods occur in a season they are sometimes quite differently decorated, and finally a species inhabiting widely different localities may have local peculiarities.
We can thus study varieties of decoration in many ways, and we shall treat of them as follows:—
1.Simple Variation, in which the different individuals of a species vary in the same locality.2.Local Variation, in which the species has marked peculiarities in different localities.3.Sexual Dimorphism, in which the sexes vary.4.Seasonal Dimorphism, in which the successive broods differ.
1.Simple Variation, in which the different individuals of a species vary in the same locality.
2.Local Variation, in which the species has marked peculiarities in different localities.
3.Sexual Dimorphism, in which the sexes vary.
4.Seasonal Dimorphism, in which the successive broods differ.
Diagram of Butterfly's WingFig. 3. Diagram of Butterfly's Wing.A.Upper Wing.B.Lower Wing.a.Costal Margin.b.Hind Margin.c.Inner "d.Anal Angle.e.Costa.f.Costal nervure.g.Sub-costal do.g1-4.Branches of do.h.Median nervure.i.Sub-median do.j.Discoidal Cell.k.Discoidal Veins.
Diagram of Butterfly's WingFig. 3. Diagram of Butterfly's Wing.A.Upper Wing.B.Lower Wing.a.Costal Margin.b.Hind Margin.c.Inner "d.Anal Angle.e.Costa.f.Costal nervure.g.Sub-costal do.g1-4.Branches of do.h.Median nervure.i.Sub-median do.j.Discoidal Cell.k.Discoidal Veins.
Fig. 3. Diagram of Butterfly's Wing.A.Upper Wing.B.Lower Wing.a.Costal Margin.b.Hind Margin.c.Inner "d.Anal Angle.e.Costa.f.Costal nervure.g.Sub-costal do.g1-4.Branches of do.h.Median nervure.i.Sub-median do.j.Discoidal Cell.k.Discoidal Veins.
Fig. 3. Diagram of Butterfly's Wing.
In order fully to understand the bearing of the following remarks it is necessary to know something of the anatomy and nomenclature of butterflies.Fig. 3is an ideal butterfly. The wing marginsare described as theCostal, which is the upper strong edge of the wing, theHindmargin, forming the outside, and theInnermargin, forming the base. The nervures consist of four principal veins; theCostal, a simple nervure under the costa, theSub-costal, which runs parallel to the costal and about halfway to the tip emits branches, generally four in number; theMedianoccupying the centre of the wing and sending off branches, usually three in number, and theSub-medianbelow which is always simple. There are thus two simple nervures, one near the costal the other near the inner margin, and between them are two others which emit branches. Between these two latter is a wide plain space known as thediscoidal cell. Small veins called thediscoidalpass from the hind margin towards the cell, and little transverse nervures, known as sub-discoidal, often close the cell. By these nervures the wing is mapped out into a series of spaces of which one, the discoidal cell, is the most important.
The nervures have two functions, they support and strengthen the wing, and being hollow serve to convey nutritive fluid and afterwards air to the wing.
The wings are moved by powerful muscles attached to the base of the wings close to the body and to the inside of the thorax, all the muscles being necessarily internal. "There are two sets which depress the wings; firstly a double dorsal muscle, running longitudinally upwards in the meso-thorax;[28]and, secondly, the dorso-ventral muscles of the meso- and meta-thorax,[29]which are attached to the articulations of the wings above, and to the inside of the thorax beneath. Between these lie the muscles which raise the wings and which run from the inner side of the back of the thorax to the legs."[30]When we consider the immense extent of wing as compared with the rest of the body, the small area of attachment, and the great leverage that has to be worked in moving the wings, it is clear that the area of articulation of the wing to the body is one in which the most violent movement takes place. It is here that the waste and repair of tissue must go on with greatest vigour, and we should, on our theory, expect it to be the seat of strong emphasis. Accordingly we commonly find it adorned with hairs, and in a vast number of cases the general hue is darker than that of the rest of the wing, and so far as we have been able to observe, never lighter than the bodyof the wing. Even in the so-called whites (Pieris) this part of the wing is dusky, and instances are numerous onPlate IV.
The scales, which give the colour to the wings, deserve more than a passing notice. They are inserted by means of little stalks into corresponding pits in the wing-membrane, and overlap like tiles on a roof; occasionally the attachment is a ball and socket (Morphinæ), in which case it is possible the insect has the power of erecting and moving its scales. The shapes are very numerous, but as a rule they are short. To this there is a remarkable exception on the wings of the males of certain butterflies, consisting of elongated tufted prominences which appear to be connected with sense-organs. They are probably scent-glands, and thus we find, even in such minute parts as scales, a difference of function emphasized by difference of ornamentation, here showing itself in variety of forms; but, as we have said, ornamentation in form is often closely allied to ornamentation in colours. In some butterflies, indeed, these scales are aggregated into spots, as inDanais, and have a different hue from the surrounding area.
The scales are not simple structures, but consist of two or more plates, which are finely striated. The colouring matter consists of granules, placed in rows between the striæ, and may exist upon the upper surface of the upper membrane (epidermal), or the upper surface of the under or middle plate (hypodermal), or the colour may be simple diffraction colour, arising from the interference of the lightwaves by fine striæ.
Dr. Haagen, in the admirable paper before mentioned, has examined this question thoroughly, and gives the results set forth in the following table:—
Epidermal Colours.Metallic blues and greensBronzeGoldSilverBlackBrownRed (rarely)}Persistent after death.Hypodermal Colours.BlueGreenYellowMilk-whiteOrange andshades betweenRed}Fading after death.
The hypodermal colours are usually lighter than the epidermal, and are sometimes changed by a voluntary act. Hypodermal and epidermal colours are, of course, not peculiar to insects; and, as regards the former, it is owing to their presence that the changing hues of fishes, like the sole and plaice, and of the chameleon are due.
The great order Lepidoptera, including butterflies and moths, seems to the non-scientific mind to be composed of members which are pretty much alike, the differences being of slight importance; but this is not in reality the case, for the lepidoptera might, with some accuracy, be compared to the mammalia, with its two divisions of the placental and non-placental animals. Comparing the butterflies (Rhopalocera) to the placental mammals, we may look upon the different families as similar to the orders of the mammalia. Were we as accustomed to notice the differences of butterflies as we are to remark the various forms of familiar animals, we should no longer consider them as slight, but accord to them their true value. When in the mammalia we find animals whose toes differ in number, like the three-toed rhinoceros and the four-toed tapir, we admit the distinction to be great, even apart from other outward forms. So, too, the seal and lion, though both belonging to the carnivora, are readily recognized as distinct, but the seals may easily be confounded by the casual observer with the manatees, which belong to quite a different order.
Thus it is with the Lepidoptera, for from six-legged insects, whose pupæ lie buried beneath the soil, like most moths, we pass to the highest butterflies, whose fore-legs are atrophied, and whose pupæ hang suspended in the open air; and this by easy intermediate stages. Surely, if six-legged mammals were the rule, we should look upon four-legged ones as very distinct; and this is the case with the butterflies. It is necessary to make this clear at starting, in order that we may appreciate to its full value the changes that have taken place in the insects under study.
Butterflies (Rhopalocera) are grouped into four sub-families, as under:—
1.Nymphalidæ, having the fore-legs rudimentary, and the pupæ suspended from the base of the abdomen.2.Erycinidæ, in which the males only have rudimentary fore-legs.3.Lycænidæ, in which the fore-legs of the males are smaller than those of the females, and terminate in a simple hook.4.Papilionidæ, which have six perfect pairs of legs, and in which the pupæ assume an upright posture, with a cincture round the middle.
1.Nymphalidæ, having the fore-legs rudimentary, and the pupæ suspended from the base of the abdomen.
2.Erycinidæ, in which the males only have rudimentary fore-legs.
3.Lycænidæ, in which the fore-legs of the males are smaller than those of the females, and terminate in a simple hook.
4.Papilionidæ, which have six perfect pairs of legs, and in which the pupæ assume an upright posture, with a cincture round the middle.
It may, at first sight, appear curious that the imperfect-leggedNymphalidæshould be placed at the head of the list, but this is based upon sound reasoning. The larva consists of thirteen segments, and, in passing to the mature stage, the second segment alone diminishes in size, and it is to this segment that the first pair of legs is attached. Looking now to the aerial habits of butterflies, we can understand how, in the process of evolution towards perfect aerial structure, the legs, used only for walking, would first become modified; and, naturally, those attached to the segment which decreases with development would be the first affected. When this is found to be combined with an almost aerial position of the pupæ, we see at once how such insects approach nearest to an ideal flying insect. It is a general law that suppression of parts takes place as organisms become specialized. Thus, in the mammalia, the greatest number of toes and teeth are found in the lowest forms and in the oldest, simplest fossil species.
A butterfly is, indeed, little more than a beautiful flying machine; for the expanse of wing, compared with the size of the body, is enormous.
CHAPTER XI.The Colouration of Insects.
(Continued.)
General Scheme of Colouring.So various are the patterns displayed upon the wings of butterflies, that amidst the lines, stripes, bars, dots, spots, ocelli, scalloppings, etc., it seems at first hopeless to detect any general underlying principle of decoration; and this is the opinion that has been, and is still, held by many who have made these insects a special study. Nevertheless, we will try to show that beneath this almost confused complexity lie certain broad principles, or laws, and that these are expressed by the statement that decoration is primarily dependent upon structure, dependent upon the laws of emphasis and repetition, and modified by the necessity for protection or distinction.
To render this subject as plain as possible, British species will be selected, as far as possible, and foreign ones only used when native forms do not suffice.
The body of by far the greater number of species is either darker or of the same tint as the mass of the wings; and only in rare cases lighter. When the body has different tints, it is generally found that the thorax and abdomen differ in colour, and in many cases the base of the thorax is emphasized by a dark or light band.
On the wings the functional importance of the parts attached to the body is generally darker, perhaps never lighter, than the ground of the wing, and is frequently further emphasized by silky hairs. This has already been sufficiently pointed out.
The wing area may be divided into the strong costal margin, the hind margin, the nervules, and the spaces; and, however complex the pattern may be, it is always based upon these structure lines.
In the majority of insects the costal margin is marked with strong colour. This may be noticed inPapilio Machaon,P. merope,Vanessa antiopa, and the whites inPlate IV. The extreme tip of the fore-wings is nearly always marked with colour, though this may run into the border pattern. This colour is dark or vividly bright, and we know no butterfly, not even dark ones, that has a light tip to the wings. Sometimes, it is true, the light bead-border spots run to the tip, but these are not cases in point. The development of tips has been traced inChapter VI., and need not be repeated.
The hind margin of both wings is very commonly emphasized by a border, of whichV. Antiopa,Pl. III. Fig. 3, is a very perfect example.
The border pattern may consist of one or more rows of spots, lines, bands, or scallops;[31]and there is frequently a fine fringe, which in many cases is white, with black marks, and to which the term bead-pattern may be applied.
A definite relation subsists in most cases between the shape of the hind margin and the character of the border-pattern. The plain or simple bordered wings have plain border patterns, and the scalloped wings have scalloped borders; or rather scalloped borders are almost exclusively confined to scalloped wings. In our English butterflies, for instance, out of the 62 species:—
33 have plain margins to the wings. In all the border is plain, or wanting.20 have the fore-wings plain, and the hind-wings scalloped, and in all the hind-wings are scalloped and the fore-wings plain, or with slightly scalloped border-patterns.9 have scalloped margins and scalloped border-patterns.
33 have plain margins to the wings. In all the border is plain, or wanting.
20 have the fore-wings plain, and the hind-wings scalloped, and in all the hind-wings are scalloped and the fore-wings plain, or with slightly scalloped border-patterns.
9 have scalloped margins and scalloped border-patterns.
Another relation between structure and pattern is found in those insects which have tailed hind-wings, for the tail is very frequently emphasized by a spot, often of a different colour from the rest of the wing as in the Swallow-Tails, PlatesIV.andV.
Yet another point may be noticed. In each wing there is a space, the discoidal cell,jFig. 3, at the apex of which several nervures join, forming knots. These are points at which obstacles exist to the flow of the contents, and they are almost always marked by a distinct pattern. We thus have a discoidal spot in very many butterflies, innearly all moths; and in the other orders of winged insects the decoration is even more pronounced, as any one may see who looks at our dragon-flies, wasps, bees, or even beetles.
In some insects the decoration of the body is very marked, as in our small dragon-flies, the Agrions. In one species, for example,A. Puella, the male is pale blue banded with black, and the female bronze black, with a blue band on the segment, bearing the sexual organ; the ovipositors are also separately decorated. The male generative organs are peculiar, in that the fertilizing fluid is conveyed from one segment to a reservoir at the other end of the abdomen. Both the segments bearing these organs are marked by special decoration. The peculiar arrangement of the sexual organs in dragon-flies is very variable, and certain segments are modified or suppressed in some forms, as was shown by J. W. Fuller.[32]In every case the decoration follows the modification. In the thorax of dragon-flies, too, the principal muscular bands are marked out in black lines. This distinct representation of the internal structure is beautifully shown inÆschnaandGomphina, and in the thorax ofCicada, as shown by Dr. Haagen in the paper quoted in the last chapter.
We may, then, safely pronounce that the decoration of insects is eminently structural.
Simple Variation.Cases of simple variation have been already cited in our description of spots and stripes, and it only remains to show that in this, as in all other cases, the variation is due to a modification of original structural decoration.
To take familiar examples. Newman, in his British Butterflies, figures the varieties of the very common Small Tortoiseshell (Vanessa urticæ). In the normal form there is a conspicuous white spot on the disc of the fore-wings, which is absent in the first variety, owing to the spreading of the red-brown ground colour. This variety is permanent on the Mediterranean shores. In variety two, the second black band, running from the costa across the cell, is continued across the wing. The third variety, Mr. Newman remarks, is "altogether abnormal, the form and colouring being entirely altered." Still, when we examine the insect closely, we find it is only a modification of the original form. The first striking difference is in the margin of the wings, which in the normal form is scalloped with scallop-markings, whereas, in the variety the margins are much simpler, and the border pattern closely corresponds with it, havinglost its scalloping. In the fore-wing some of the black bands and spots are suppressed or extended, and the extensions end rigidly at nervules. The dark colouring of the hind-wings has spread over the whole wing. We thus see that the decoration, even in varieties called abnormal, still holds to structural lines, and is a development of pre-existing patterns.
No one can have examined large series of any species without being impressed with the modification of patterns in almost every possible way. For instance, we have reared quantities ofPapilio Machaon, and find great differences, not only in the pattern, but in the colour itself. A number of pupæ from Wicken Fen, Cambridgeshire, were placed in cages, into which only coloured light could fall, and though these experiments are not sufficiently extended to allow us to form any sound conclusions as to the effect of the coloured light, we got more varieties than could be expected from a batch of pupæ from the same locality. The tone of the yellow, the quantity of red, the proportion of the yellow to the blue scales in the clouds, varied considerably, but always along the known and established lines.
The variations in the colour of Lepidoptera has been most admirably treated by Mr. J. Jenner Weir in a paper, only too short, read before the West Kent Natural History Society.[33]He divides variations into two sections, Aberrations or Heteromorphism, and constant variations or Orthopæcilism, and subdivides each into six classes, as under:—
In some cases, he remarks, variations are met with which may with equal propriety be classed in either section.
Albinism he finds to be very rare in British species, the only locality known to him being the Outer Hebrides. This reminds us of Wallace's remark upon the tendency to albinism in islands. Xanthism, he finds to be more plentiful, and quotes the common Small Heath (Cænonympha pamphilus) as an illustration. In these varieties we have simply a bleaching of the colouring matter of the wings, and therefore no departure from structural lines. Melanism arises from the spreading of large black spots or bars, or, as inBiston betularia, a white moth peppered with black, dots by the confluence of small spots; for this insect in the north is sometimes entirely black. It is singular that insects have a tendency to become melanic in northern and alpine places, and this is especially the case with white or light coloured species. (SeePlate IV., Fig. 17) It has recently been suggested that this darkening of these delicate membranous beings in cold regions is for the purpose of absorbing heat, and this seems very probable.[34]
Of ordinary spots it is merely necessary to remark, that they are all cases in our favour. Thus, inSatyrus hyperanthuswe have "the ordinary round spots ... changed into lanceolate markings"; this takes place also inC. davus. The other cases of aberration do not concern us.
When, however, we come to the cases in which a species has two or more permanent forms, it is necessary to show that they are in all cases founded on structure lines. The patterns, as shown inPlate V., Figs. 1-13, are always arranged structurally, and the fact that not only are intermediate forms known, as inAraschnia porima,Plate V., Fig. 6, but that the various forms are convertible into one another, would in itself be sufficient to show that in these cases there is no departure from the general law. InGrapta interrogationis,Plate V., Figs. 8-10, we see in the central figure one large spot above the median nervure, in the left-hand form this is surmounted by another spot above the lowest sub-costal branch, and in the right-hand figure this latter spot is very indistinct. We have here a perfect gradation, and the same may be said of the colouration of the lower wings. Take again the three forms ofPapilio Ajaxin the same plate,Figs. 11-13, and we have again only modifications of the same type.
In local varieties, as in seasonal forms, we have again nothing more than developments of a given type, as is well shown inPlates IV.&V., Figs.13-18&1-13.
When, however, we come to mimetic forms, whether they mimic plants, as inPlate I., or other species, as inPlates II.&III., a difficulty does seem to arise.
The leaf butterfly (Kallima inachus),Plate I., offers no trouble when we view the upper surface only with its orange bands, but its under surface, so marvellously like a dead leaf that even holes and microscopic fungi are suggested, does seem very like a case in which structure lines are ignored. Take, for instance, the mark which corresponds to the mid-ribs, running from the tail to the apex of the upper wing; it does not correspond to any structure line of the insect. But if we take allied and even very different species and genera of Indian and Malayan butterflies, we shall find every possible intermediate form between this perfect mimicry and a total lack of such characters. To cite the most recent authority, the various species of the Genera Discophora, Amathusia, Zeuxidia, Thaumantis, Precis, &c., figured so accurately in Distant's Rhopalocera Malayana, will give all the steps.
In the cases of true mimicry, as inFigs. 1-3, Plates II.&III., where insects as different as sheep from cats copy one another, we find that of course structure lines are followed, though the pattern is vastly changed. ThePapilio merope,Fig. 1, Plate II., which mimicsDanais niavius,Fig. 3, does so by suppressing the tail appendage, changing the creamy yellow to white—a very easy change, constantly seen in our own Pieridæ—and diffusing the black. A similar case is seen inFigs. 4-5, Plate III., where a normally white butterfly (Panopœa hirta) mimics a normally dark one of quite a different section. Here again the change is not beyond our power of explanation. Where a Papilio likemeropemimics a brown species likeDanais niavius, we have a still greater change in colour, but not in structural pattern.
If we ascribe to these insects the small dose of intelligence we believe them to possess, we can readily see how the sense of need has developed such forms.
Local varieties present no difficulty under such explanation. The paramount necessity for protection has given the Hebridran species the grey colour of the rocks, and the desert species their sandy hue.