Chemical and physical nature of the pigment.—Researches in this difficult field of inquiry have been made by Landois (1864), Sorby (1871), Meldola (1871), by Krukenberg (1884), and more recently by Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental importance in dealing with mimicry and protective coloration, the primary causes of which appear to be due to the action of physical and chemical agents.
Over twenty years ago Meldola observed that the yellow pigment of the sulphur-yellow butterfly (Gonopteryx rhamni) was soluble in water, and showed that its aqueous solution had an acid reaction.
Besides the yellow uranidin found by Krukenberg in different beetles and lepidopterous pupæ, still other coloring-matters, which are very constant in different species are readily recognized by the spectroscope. “Thus there appear in the brownish yellow lymph of Attacus pernyi, Callosamia promethea and Telea polyphemus, after saponification of the precipitated soap readily effected by ether, or incompletely or not removed by benzine, a chlorophane-like lipochrome; and in the yellowish green lymph of Saturnia pyri and of Platysamia cecropia besides this pigment still another whose spectrum shows a broad band on D, but which disappears with the addition of acetic acid or ammonia, as also after a long heating of the lymph up to 66° C.”
Coste, and more especially Urech, have shown that many of the pigments may be dissolved out of the scales by means of chemical reagents, giving colored solutions, and leaving the scales white or colorless. They have also shown that some of these pigments may be changed in color by the action of reagents, and then restored to their original color by other reagents. They have proved that reds, yellows, browns, and blacks are always due to pigments, and in a few cases greens, blues, violets, purples, and whites, and not, as is usually the case, to structural conditions, such as striæ on the scales (Mayer). They confined themselves solely to the chemical side of the problem, not considering the structure of the scales themselves.
Urech has also discovered a beautiful smaragd-green coloring-matter in the wings (not in the scales) of the pupa of Pieris brassicæ. It is not chlorophyll, and Urech suggests that it may be either the germinal substance of the pigments of the scales or its bearer. It is not the pigment of the blood.
Urech has also demonstrated that in many Lepidoptera the color of the urine which is voided upon emergence from the chrysalis is similar to the principal color of the scales.
Hopkins has worked on the pigments within the scales of butterflies. The yellow pigment in Gonopteryx rhamni is a derivation of uric acid, and he calls it lepidotic acid. Its aqueous solution is strongly acid to litmus, and must be bad-tasting to birds.
Hopkins has dissolved the red pigment from the border of the hind wing of Delias eucharis, an Indian butterfly, in pure water, finding as the result a yellow solution; but if the solution be evaporated to dryness, the solid residue of pigment is red once more. He has obtained from this pigment of eucharis a silver compound which contains a percentage of metals exactly equal to that from the pigment of G. rhamni. (Nature, April 2, 1892.)
“The scales of the wings of the white butterflies (Pieridæ) are also shown by Hopkins to contain uric acid, this substance practically acting as a white pigment in these insects. A yellow pigment, widely distributed in the same family, is shown to be a derivative of uric acid, and its artificial production as a by-product of the hydrolysis of uric acid is demonstrated. That this yellow pigment is an ordinary excretory product of the butterfly is indicated by the fact that an identical substance is voided from the rectum on emergence from the pupa. These excretory pigments, which have well-marked reactions, are apparently confined to the Pieridæ, and are not found in other Rhopalocera. This fact shows that when a Pierid mimics an insect belonging to another group, the pigments of the mimicked and mimicking insects, respectively, are chemically quite distinct. Other pigments existing, not in the scales, but between the wing-membranes, are shown to be of use for ornament.” (Proc. Royal Soc., London, 1894.)
Griffiths (1892) claims that the green pigment found in several species of Papilio, Hesperia, and Limenitis, also in Noctuidæ, Geometridæ, and Sphingidæ likewise consists of a derivative of uric acid, which he calls lepidopteric acid. By prolonged boiling in HCl it is converted into uric acid.