CHAPTER X_The Glow-Worm and Other Beetles

INSECT COLOURING

_Phanaeus 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] Cetoniae,

Buprestes, Chrysomelae,[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.

[Footnote 1: Cf. Chapter XIV. of the present volume.--_Translator's

Note_.]

[Footnote 2: Golden Apple-beetles.--_Translator's Note_.]

[Footnote 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.

[Footnote 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 larvae 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 larvae 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 larvae 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.

[Footnote 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 larvae.

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 larvae 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 Provencal 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 Locustidae 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. Reaumur 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 Epeira[6] 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.

[Footnote 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.

[Footnote 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 Scarabaeus 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 Phanaeus 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 Phanaeus

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.

[Footnote 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 Phanaeus, 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

Cetoniae. 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 Phanaeus 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.

[Footnote 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