On Insect Colours¹

Abstract

II. NOW it is necessary to explain the “reversion effects” of red, so frequently alluded to. I am tempted to give a detailed account of the experiments made in this connection, but the length to which this article has already run warns me that I must be very brief indeed: and I will therefore content myself with giving simply the broad results.² All reds and pinks (always omitting the last four in the table), are turned orange or yellow instantly by acids. When nitric acid is used, this effect is permanent; and whether the yellowed wing be dried, or washed, the yellow is immovable. I have kept such wings for five or six months, and they were as yellow as possible at the end of that time. In the case of all other acids,³ the yellow is permanent only so long as the wing is actually acid: directly the acid is removed, the original red returns; and thus a wing may be alternately yellowed and “reverted” time after time. This reversion to the original red may be produced either by long exposure to the air, allowing the last traces of acid to drain off; or instantly by neutralizing the residual acid with a drop of ammonia, or by copious washing. It must therefore be understood that, with the exception of those cases in which nitric acid has been used, the permanency of the artificial yellow is entirely dependent upon the presence of acid: remove the acid, and the yellow vanishes. Accordingly, I have suggested the following explanation. Let us denote the molecule of red pigment by X; when any acid, except nitric, is added, I assume that this forms with X a so-called molecular compound: for instance, on treating with hydrochloric acid, we should get the hydrochloride of X, viz. X.(H.Cl)_n; and it is evident that these hydro-chlorides, hydrosulphates, &c., of X are yellow, although the original X is red. To all these facts, of course, there are ample analogies known to chemistry. Next, for the resuscitation of red. We must suppose—what is certainly to all appearance very clear—that these molecular compounds are very unstable; an easily understandable fact; and that consequently the addition of even excess of water is sufficient to decompose them, removing the acid molecule, and thus restoring the pigment X to its original condition. Far more rapidly does this resuscitation occur if a drop of ammonia be used, this at once combining with the acid and liberating the X molecule. In the case of resuscitation produced by slow air drying, the action apparently would be in some cases due to gradual evaporation, or to some process of oxidation—anyhow producing dissociation of the molecular salt of X. Finally, in the case of nitric acid, it is clear that this acid does not form a molecular compound, but, as we might expect, exercises a permanently destructive action on the original pigment. Admitting that red has been developed from yellow, it is not surprising that it may be easily reconverted permanently into yellow by such a reagent as nitric acid. Before quitting this topic, I may point out that the cyanide reaction of the yellows is very suggestive indeed as to the kind of process by which the red pigment is developed from yellow.