INSECT COLOURING

Phanæus splendidulus, the glittering, the resplendent: this is the epithet selected by the official nomenclators to describe the handsomest Dung-beetle of the pampas. The name is not at all exaggerated. Combining the fire of gems with metallic lustre, the insect, according to the incidence of the light, emits the green reflections of the emerald or the gleam of ruddy copper. The muck-raker would do honour to the jeweller's show-cases.

For the rest, our own Dung-beetles, though usually modest in their attire, also have a leaning toward luxurious ornament. One Onthophagus decorates his corselet with Florentine bronze; another wears garnets on his wing-cases. Black above, the Mimic Geotrupes is the colour of copper pyrites below; also black in all parts exposed to the light of day, the Stercoraceous Geotrupes displays a ventral surface of a glorious amethyst violet.

Many other series, of greatly varied habits, Carabi,1 Cetoniæ, Buprestes, Chrysomelæ,2 rival and even surpass the magnificent Dung-beetles in the matter of jewellery. At times we encounter splendours which the imagination of a lapidary would not venture to depict. The Azure Hoplia,3 the inmate of the osier-beds and elders by the banks of the mountain streams, is a wonderful blue, tenderer and softer to the eye than the azure of the heavens. You could not find an ornament to match it save on the throats of certain Humming-birds and the wings of a few Butterflies in equatorial climes.

1 Cf. Chapter XIV. of the present volume.—Translator's Note.

2 Golden Apple-beetles.—Translator's Note.

3 A genus of Cockchafer. Cf. The Life of the Fly: chap. vii.—Translator's Note.

To adorn itself like this, in what Golconda does the insect gather its gems? In what diggings does it find its gold nuggets? What a pretty problem is that of a Buprestis' wing-case! Here the chemistry of colours ought to reap a delightful harvest; but the difficulties are great, it seems, so much so that science cannot yet tell us the why and the wherefore of the humblest costume. The answer will come in a remote future, if indeed it ever comes completely, for life's laboratory may well contain secrets denied to our retorts. For the moment, I shall perhaps be contributing a grain of sand to the future palace if I describe the little that I have seen.

My basic observation dates a long way back. I was at that time busy with the Hunting Wasps, following their larval development from the egg to the cocoon. Let us take an instance from my notes, which cover nearly all the game-hunters of my district. I will choose the larva of the Yellow-winged Sphex,4 which, with its convenient size, will furnish an easy object-lesson.

4 Cf. The Hunting Wasps: chap. iv.—Translator's Note.

Under the transparent skin of the larva, which has been recently hatched and is consuming its first Cricket, we soon perceive some fine white spots, which rapidly increase in size and number and eventually cover the whole body, except the first two or three segments. On dissecting the grub, we find that these spots have to do with the adipose layer, of which they form a considerable part, for, far from being scattered only on the surface, they run through its whole thickness and are present in such numbers that the forceps cannot seize the least fragment of this tissue without picking up a few of them.

Though perfectly visible without the help of a lens, these puzzling spots call for the microscope if we wish to study them in detail. We then find that the adipose tissue is made up of two kinds of vesicles: some, bright yellow and transparent, are filled with oily drops; the rest, opaque and starch-white, are distended with a very fine powder, which spreads in a cloudy trail when the vesicle containing it is broken on the object-slide. Intermingled without apparent order, the two kinds of bags are of the same shape and the same size. The first go to make up the nutritive reserves, the fatty tissue properly so-called; the second form the white dots which we will study for a moment.

An inspection under the microscope tells us that the contents of the white cells are composed of very fine, opaque grains, insoluble in water and of greater density. The use of chemical reagents on the object-slide proves that nitric acid dissolves these grains, with effervescence and without leaving the least residue, even when they are still enclosed in their vesicles. On the other hand, the true fatty cells suffer in no way when attacked by this acid; they merely turn a little yellower.

Let us take advantage of this property to operate on a larger scale. The adipose tissue taken from a number of larvæ is treated with nitric acid. The effervescence is as lively as if the reaction were taking effect on a bit of chalk. When it has subsided, some yellow clots are floating on the surface. These are easily separated. They come from the fatty substance and the cellular membranes. There remains a clear liquid containing the white granules in solution.

The riddle of these granules was being presented to me for the first time; my predecessors had provided no physiological or anatomical data to guide me; great therefore was my joy when, after a little fumbling, I succeeded in hitting upon their characteristic feature.

The solution is evaporated in a small porcelain capsule, placed on the hot embers. On the residue I pour a few drops of ammonia, or else simply water. A glorious crimson colour at once makes its appearance. The problem is solved: the colouring-matter which has just formed is murexide; and consequently the powdery substance which filled the cells was none other than uric acid, or more precisely ammonium urate.

A physiological fact of this importance can hardly stand alone. Indeed, since this basic experiment I have discovered grains of uric acid in the adipose tissue of the larvæ of all the Hunting Wasps of our parts, as well as in the Bees at the moment of the nymphosis. I have observed them in many other insects, either in the larval or in the perfect state; but in this respect there is none to equal the grub of the game-hunting Wasp, which is all speckled with white. I think I see the reason.

Let us consider two larvæ which eat live prey: that of the Sphex and that of the Hydrophilus.5 Uric acid, the inevitable product of the vital transformations, or at all events one of its analogues, must be formed in both. But the Hydrophilus' larva shows no accumulation of it in its adipose layer, whereas the Sphex' is full of it.

5 The Great Water-beetle.—Translator's Note.

In the latter the duct through which the solid excretions pass is not yet in working order; the digestive apparatus, tied at the lower end, is not discharging an atom. The urinary products, being unable, for want of an open outlet, to flow away as formed, accumulate in the adipose tissue, which thus serves as a common store-house for the residues of the present and the plastic material of the future organic processes. Here something occurs analogous to what we see in the higher animals after the removal of the kidneys; the urea at first contained in the blood, in imperceptible quantities accumulates and becomes manifest when the means by which it is eliminated disappear.

In the larva of the Hydrophilus, on the other hand, the excretions enjoy a free outlet from the beginning; and the urinary products escape as and when formed and are no longer deposited in the adipose tissue. But during the intense labour of the metamorphosis, any excretion becomes impossible; the uric acid must and does collect in the adipose substance of the different larvæ.

It would be out of place, despite its importance, to pursue the problem of the uric residues any further. Our subject is coloration. Let us return to it with the evidence supplied by the Sphex. Her almost transparent larva has the neutral tint of fluid white of egg. Under its fine translucid skin there is nothing coloured, save the long digestive pouch, which is swollen a deep purple by the pulp of the consumed Crickets. But against this indefinite, vitreous background the opaque white uric cells stand out distinctly in their myriads; and the effect of this stippling is a sketchy but by no means inelegant costume. It is skimpy in the extreme, but at any rate it is something.

With the urinary broth of which its intestine is unable to get rid, the larva has discovered a means of making itself look a little smart. The Anthidia have shown us how, in their cotton-wool wallets, they manufacture a sort of jewellery with their ordure. The robe studded with grains of alabaster is a no less ingenious invention.

To beautify themselves cheaply by using up their own refuse is a very common method even among insects endowed with all that is wanted for evacuating waste matter. While the larvæ of the Hunting Wasps, unable to do better, stipple themselves with uric acid, there are plenty of industrious creatures that are able to make themselves a superb dress by preserving their excretions in spite of their own open sewers. With a view to self-embellishment, they collect and treasure up the dross which others hasten to expel. They turn filth into finery.

One of these is the White-faced Decticus (D. albifrons, FAB.), the biggest sabre-bearer of the Provençal fauna. A magnificent insect is this Grasshopper, with a broad ivory face, a full, creamy-white belly and long wings flecked with brown. In July, the season for the wedding-dress, let us dissect him under water.

The adipose tissue, which is abundant and yellowish white, consists of a lace of wide, irregular, criss-cross meshes. It is a tubular network swollen with a powdery matter which condenses into minute chalk-white spots, standing out very plainly against a transparent background. When crushed in a drop of water, a fragment of this fabric yields a milky cloud in which the microscope shows an infinite number of opaque floating atoms, without revealing the smallest blob of oil, the sign of fatty matter.

Here again we have ammonium urate. Treated with nitric acid, the adipose tissue of the Decticus produces an effervescence similar to that of chalk and yields enough murexide to redden a tumblerful of water. A strange adipose body, this bundle of lace crammed with uric acid without a trace of fatty matter! What would the Decticus do with nutritive reserves, seeing that he is near his end, now that the nuptial season has arrived? Delivered from the necessity of saving for the future, he has only to spend in gaiety the few days left to him; he has only to adorn himself for the supreme festival.

He therefore converts into a paint-factory what at first was a warehouse for storing up foodstuffs; and with his chalk-like uric pulp he lavishly daubs his belly, which turns a creamy white, and smears it on his forehead, his face, his cheeks, until they assume the appearance of old ivory. All those parts, in fact, which lie immediately under the translucid skin are covered with a layer of pigment which can be turned into murexide and is identical in nature with the white powder of the adipose lace.

Biological chemistry can hardly offer a simpler and more striking experiment than this analysis of the Decticus' finery. To those who have not this curious Grasshopper handy, I recommend the Ephippiger of the Vines, who is much more widely distributed. His ventral surface, which also is of a creamy white, likewise owes its colour to a plastering of uric acid. In the Grasshopper family many other species of smaller size and requiring more delicate handling would give us the same results in varying degrees.

White, slightly tinged with yellow, is all that the urinary palette of the Locustidæ shows us. A caterpillar, the Spurge Hawk-moth's, will take us a little farther. Dappled red, black, white and yellow, its livery is the most remarkable in our part of the country. Réaumur in fact calls it la Belle. The flattering title is well-deserved. On the black background of the larva, vermillion-red, chrome-yellow and chalk-white figure side by side in circles, spots, freckles and stripes, as clearly marked as the glaring patches of a harlequin's dress.

Let us dissect the caterpillar and apply the lens to its mosaic. On the inner surface of the skin, except in the portions coloured black, we observe a pigmentary layer, a coating here red, there yellow or white. We will cut a strip from this coat of many colours, after depriving it of its muscular fibres, and subject it to the action of nitric acid. The pigment, no matter what its hue, dissolves with effervescence and afterwards yields murexide. Here again, then, it is to uric acid, present, however, in small quantities in the adipose tissue, that the caterpillar's rich livery is due.

The black parts are an exception. Unassailable by nitric acid, they retain their sombre tint after treatment as before, whereas the portions stripped of their pigment by the reagent become almost as transparent as glass. The skin of the handsome caterpillar thus has two sorts of coloured patches.

Those of an intense black may be likened to dyers' products: they are completely impregnated with the colouring matter, which is part and parcel of the molecular constitution and cannot be isolated by the nitric solvent. The others, red, yellow or white, are actually painted: on a translucid sheet is a wash of urinary pigment, which is discharged by the minute ducts issuing from the adipose layer. When the action of the nitric acid has ceased, the transparent circles of the latter stand out against the black background of the former.

Yet one more example taken from a different order. As regards elegance of costume, the Banded Epeira6 is the most highly favoured of our Spiders. On the upper surface of her corpulent belly alternate, in transversal bands, bright black, a vivid yellow like that of yolk of egg and a dazzling white like that of snow. The black and yellow also show underneath, but arranged differently. The yellow, in particular, forms two longitudinal ribbons, ending in orange-red on either side of the spinnerets. A pale purple is faintly diffused over the sides.

6 Cf. The Life of the Spider: chaps. ii., vii., xi. and xiii.—Translator's Note.

Examined from the outside with the lens, the black parts reveal nothing out of the common. The black is homogeneous and everywhere of equal depth. On the other hand, in the coloured portions, we see little polygonal, granular masses, forming a close-meshed network. By cutting round the circumference of the abdomen with a pair of scissors, the horny integument of the dorsal surface may readily be removed in one piece, without any shreds of the organs which it protected. This large strip of skin is transparent in the zones that correspond with the white bands in the natural state; it is black or yellow on the black or yellow bands. These last indeed owe their colouring to a layer of pigment which the point of a paintbrush will easily loosen and remove.

As for the white bands, their origin is this: once the skin has been removed, the dorsal surface of the abdomen, whose graceful mosaic is not in any way disturbed, reveals a layer of polygonal white spots, distributed in belts, here densely and there less so. The denser belts correspond with the white bands. It is their magnificent opaque white granulations which, seen through the transparent skin, form the snow-white stripes in the live Spider.

Treated with nitric acid on the microscopic slide, they do not dissolve nor produce effervescence. Uric acid then is not present in this case; and the substance must be guanine, an alkaloid known to be the urinary product of the Spiders. The same is true of the yellow, black, purple or orange pigment that forms a coating under the skin. In short, by utilizing, in a different chemical combination, the waste products of animal oxidization, the magnificent Spider rivals the magnificent caterpillar; she beautifies herself with guanine as the other does with its uric acid.

Let us abridge this dry subject; let us be content with these few data, which could if necessary be corroborated by many others. What does the little that we have learnt teach us? It tells us that the materials rejected by the organism, guanine, uric acid and other dross from life's refinery, play an important part in the coloration of the insect.

Two cases are distinguishable, according as the colour is dyed or simply painted. The skin, itself colourless and transparent, is in places illumined by a coloured varnish, which can be removed by a touch with a paintbrush. Here we have paint, the result of the urinary compound laid on the inner surface of the covering, just as the chromatic ingredients of our glass-painters are laid on our stained-glass windows.

At other places the skin is coloured in its very substance; the colouring-matter forms an integral part of it and can no longer be swept away with a camel-hair brush. Here we have a dyed fabric, represented in our windows by the panes of coloured glass which the crucible decorates uniformly with this or that tint, by means of the incorporated metallic oxides.

Whereas, in these two cases, there is a profound difference in the distribution of the chromatic materials, is this true of their chemical nature as well? The suggestion is hardly admissible. The worker in stained glass dyes or paints with the same oxides. Life, that incomparable artist, must even more readily obtain an infinite variety of results by uniformity of method.

It shows us, on the back of the Spurge Caterpillar,7 black spots jumbled up with other spots, white, yellow or red. Paints and dyes lie side by side. Is there on this side of the dividing line a paint-stuff and on the other side a dye-stuff, absolutely different in character from the first? While chemistry is not yet in a position to demonstrate, with its reagents, the common origin of the two substances, at least the most convincing analogies point to it.

7 The caterpillar of the Spurge Hawk-moth.—Translator's Note.

In this delicate problem of the insect's colouring, one single point thus far comes within the domain of observed facts: the progressive advance of chromatic evolution. The carbuncle of the Dung-Beetle of the Pampas suggested the question. Let us then inquire of his near neighbours, who will perhaps enable us to advance a step farther.

Newly stripped of his cast-off nymphal skin, the Sacred Beetle possesses a strange costume, bearing no resemblance to the ebony black which will be the portion of the mature insect. The head, legs and thorax are a bright rusty red; the wing-cases and abdomen are white. As a colour, the red is almost that of the Spurge Caterpillar, but it is the result of a dye on which nitric acid has no effect as a detector of urates. The same chromatic principle must certainly exist in a more elaborate form and under a different molecular arrangement in the skin of the abdomen and the wing-cases which will presently replace white by red.

In two or three days the colourless becomes the coloured, a process whose rapidity implies a fresh molecular structure rather than a change of composition. The building-stone remains the same, but is arranged in a different order; and the structure alters in appearance.

The Scarabæus is now all red. The first brown stains show themselves on the denticulations of the forehead and fore-legs, the sign of an earlier maturity in the implements of labour, which are to acquire an exceptional hardness. The smoky tinge spreads more or less all over the insect, replaces the red, turns darker and finally becomes the regulation black. In less than a week the colourless insect turns a rusty red, next a sooty brown and then an ebony black. The process is completed; the insect possesses its normal colouring.

Even so do the Copres, the Gymnopleuri,8 the Onites, the Onthophagi and many others behave; even so must the jewel of the pampas, the Splendid Phanæus set to work. With as much certainty as though I had him before my eyes at the moment when he divests himself of his nymphal swaddling-bands, I see him a dull red, rusty or crimson, excepting on the wing-covers and the abdomen, which are at first colourless and presently turn the same colour as the rest. In the Sacred Beetle this initial red is followed by black; the Phanæus replaces it by the brilliance of copper and the reflections of the emerald. Ebony, metal, the gem: have they the same origin here then? Evidently.

8 Cf. The Sacred Beetle and Others: chap. viii.—Translator's Note.

The metallic lustre does not call for a change of nature; a mere nothing is enough to produce it. Silver, when very finely subdivided by the methods whereof chemistry knows the secret, becomes a dust as poor to look at as soot. When pressed between two hard bodies, this dirty powder, which might be dried mud, at once acquires the metallic sheen and again becomes the silver which we know. A mere molecular contact has wrought the miracle.

Dissolved in water, the murexide derived from uric acid is a magnificent crimson. Solidified by crystallization, it rivals in splendour the gold-green of the Cantharides. The widely-used fuschine affords a well-known example of like properties.

Everything, then, appears to show that the same substance, derived from urinary excretions, yields, according to the mode in which its ultimate particles are grouped, the metallic red of the Phanæus, as well as the white, the dull red and the black of the Sacred Beetle. It becomes black on the dorsal surface of the Stercoraceous Geotrupes and the Mimic Geotrupes; and, with a quick change, it turns into amethyst under the belly of the first and into copper pyrites under the belly of the second. It covers the back of Cetonia floricola with golden bronze and the under surface with metallic purple. According to the insect, according to the part of the body, it remains a dingy compound or sparkles with reflections even more vivid and varied than those possessed by the metals.

Light seems irrelevant to the development of these splendours; it neither accelerates nor retards them. Since direct exposure to the sun, owing to the excess of heat, is fatal to the delicate process of the nymphosis, I shaded the solar rays with a screen of water contained between slips of glass; and to the bright light thus moderated in temperature I daily, throughout the period of chromatic evolution, subjected a number of Sacred Beetles, Geotrupes and Cetoniæ. As standards of comparison I had witnesses of whom I kept some in diffused light and others in complete darkness. My experiments had no appreciable result. The development of the colours took place in the sunlight and in the dark alike, neither more rapidly nor more slowly and without difference in the tints.

This negative result was easy to foresee. The Buprestis emerging from the depths of the trunk in which he has spent his larval life; the Geotrupes and the Phanæus leaving their natal burrows possess their final adornments, which will not become richer in the rays of the sun, at the time when they make their appearance in the open air. The insect does not claim the assistance of the light for its colour chemistry, not even the Cicada,9 who bursts her larval scabbard and changes from pale green to brown as easily in the darkness of my apparatus as in the sunlight, in the usual manner.

9 Cf. The Life of the Grasshopper: chaps. i. to v.—Translator's Note.

The chromatics of the insect, having as its basis the urinary waste products, might well be found in various animals of a higher order. We know of at least one example. The pigment of a small American lizard is converted into uric acid under the prolonged action of boiling hydrochloric acid.10 This cannot be an isolated instance; and there is reason to believe that the reptilian class daubs its garments with similar products.

10 A. B. Griffiths, Transactions of the Académie des sciences, 26 November, 1894.—Author's Note.

From the reptile to the bird is no great distance. Then the Wood-pigeon's iridescent hues, the eyes on the Peacock's tail, the Kingfisher's sea-blue, the Flamingo's carmine are more or less closely connected with the urinary excretions? Why not? Nature, that sublime economist, delights in these vast antitheses which upset all our conceptions of the values of things. Of a pinch of common charcoal she makes a diamond; of the same clay which the potter fashions into a bowl for the Cat's supper she makes a ruby; of the filthy waste products of the organism she makes the splendours of the insect and the bird. The metallic marvels of the Buprestis and the Ground-beetle; the amethyst, ruby, sapphire, emerald and topaz of the Humming-bird; glories which would exhaust the language of the lapidary jeweller: what are they in reality? Answer: a drop of urine.