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.