A form of variation in the female, and most frequent perhaps in Irish specimens, is a tendency of the spots on the upper side of the fore wings to spread and run together, and so form an interrupted band.
Specimens with a distinct creamy tint on the wings are sometimes met with, but such varieties, as well as yellow ones (var. flava, Kane), are probably more often obtained in Ireland and Scotland than in England. Occasionally male specimens of the second brood have two black spots on the disc of the wing. Some forms of this butterfly have been named, and these will now be referred to.
Sabellicæ (Petiver), Stephens, has been considered as a species distinct from P. napi, L. Stephens ("Brit. Entom. Haust.," I. Pl. iii., Figs. 3, 4) figured a male and a female as sabellicæ, which he states differs from napi in having shorter and more rounded yellowish-white wings. No locality or date is given in the text (p. [21]) for the specimens figured; but referring to another example which he took at Highgate on June 4, he says that it agrees with his Fig. [2.] Probably, however, it was his second figure that he intended, the Fig. [4] of the plate, which is a female. This is rather more heavily marked with dusky scales than is usual in specimens of the first brood, at least in England, although it agrees in this respect with some Irish June examples. Fig. [3] represents a male which certainly seems to be referable to the spring form. Most authors give sabellicæ; as belonging to the summer flight, but this does not seem to be correct.
Var. napææ is a large form of the summer brood, occurring commonly on the Continent, in which the veins on the under side of the hind wings are only faintly shaded with greenish-grey. Occasionally specimens are taken in this country in August, which both from their size and faint markings on the under side seem to be referable to this form.
Var. bryoniæ is an Alpine form of the female, and in colour is dingy yellow or ochreous, with the veins broadly suffused with blackish grey, sometimes so broadly as to hide the greater part of the ground colour. This form does not occur in any part of the British Islands, but some specimens from Ireland and from the north of Scotland somewhat approach it.
All the early stages are shown on Plate [10.]
The egg is of a pale straw colour when first laid, but it soon turns to greenish, and as the caterpillar within matures, the shell of the egg becomes paler. The ribs seem to be fourteen in number.
The eggs are laid singly on hedge garlic (Sisymbrium alliaria) and other kinds of plants belonging to the Cruciferæ. The egg in the illustration was laid on a seed-pod of hedge garlic, but the caterpillar that hatched from it was reared on leaves of garden "nasturtium" and wallflower.
The caterpillar when full grown is green above, with black warts, from which arise whitish and blackish hairs. There is a darker line along the back, and a yellow line low down on the sides. Underneath the colour is whitish-grey. The spiracular line is dusky, but not conspicuous, and the spiracles are blackish surrounded with yellow. It has been stated that caterpillars fed upon hedge garlic and horseradish produce light butterflies, and that those reared on mignonette and watercress produce dark butterflies. Barrett mentions having reared a brood of the caterpillars upon a bunch of watercress placed in water and stood in a sunny window, but he does not refer to anything peculiar about the butterflies resulting therefrom. He states, however, that from eggs laid in June the earliest butterfly appeared within a month, and the remainder by the middle of August, only one remaining in the chrysalis until the following June.