Plate IV.
SPOTS and STRIPES.
We will now take an illustration of the formation of stripes or bars from spots, and in doing so must call attention to the rarity of true stripes in butterflies. By a true stripe I mean one that has even edges, that is, whose sides are uninfluenced by structure. In all our British species such as P. machaon, M. artemis, M. athalia, V. atalanta, L. sibilla, A. iris, and some of the Browns, Frittilaries and Hair-streaks, which can alone be said to be striped, the bands are clearly nothing more than spots which have spread up to the costæ, and still retain traces of their origin either in the different hue of the costæ which intersect them, or in curved edges corresponding with the interspaces of the costæ. This in itself is sufficient to indicate their origin. But in many foreign species true bands are found, though they are by no means common. Illustrations are given in [Plate IV]., of two Swallow-tails, Papilio machaon, [Fig. 11], and P. podalirius, [Fig. 12], in which the development of a stripe can readily be seen.
In machaon the dark band inside the marginal semi-lunar spots of the fore-wings retain traces of their spot-origin in the speckled character of the costal interspaces, and in the curved outlines of those parts. In podalirius the semi-lunar spots have coalesced into a stripe, only showing its spot-origin in the black markings of the intersecting costæ; and the black band has become a true stripe, with plain edges. Had only such forms as this been preserved, the origin of the spots would have been lost to view.
It may, however, be said, though I think not with justice, that we ought not to take two species, however closely allied, to illustrate such a point. But very good examples can be found in the same species. A common German butterfly, Araschnia Levana, has two distinct varieties, Levana being the winter, and prorsa the summer form; and between these an intermediate form, porima, can be bred from the summer form by keeping the pupæ cold. Dr. Weismann, who has largely experimented on this insect, has given accurate illustrations of the varieties. [Plate V.] is taken from specimens in our possession. In the males of both Levana, [Fig. 4], and prorsa, [Fig. 1], the hind-wing has a distinct row of spots, and a less distinct one inside it, and in the females of both these are represented by dark stripes. In porima we get every intermediate form of spots and stripes, both in the male and female, and as these were hatched from the same batch of eggs, or, are brothers and sisters, it is quite impossible to doubt that here, at least, we have an actual proof of the change of spots into stripes.
