This butterfly occasionally multiplies immensely, and is given to migrating in vast armies to distant settlements, sometimes crossing the sea to effect this purpose. Here is an extract from a Kentish newspaper, describing an occurrence of this phenomenon:—
"One of the largest flights of butterflies ever seen in this country, crossed the Channel from France to England on Sunday last. Such was the density and extent of the cloud formed by the living mass, that it completely obscured the sun from the people on board our Continental steamers, on their passage, for many hundreds of yards, while the insects strewed the decks in all directions. The flight reached England about twelve o'clock at noon, and dispersed themselves inland and along shore, darkening the air as they went. During the sea-passage of the butterflies, the weather was calm and sunny, with scarce a puff of wind stirring; but an hour or so after they reached terra firma, it came on to blow great guns from the S. W., the direction whence the insects came."
A contemporary account states that these were the small white butterflies (Pieris Rapæ).
The smaller butterfly with more dusky markings, formerly known as P. Metra, has been recently proved to be merely a variety of Rapæ, a Mr. J. F. Dawson having reared a brood of caterpillars all exactly similar in appearance, which eventually produced every variety of P. Rapæ and P. Metra.
Mr. Curtis, in his "Farm Insects," mentions the capture, near Oldham in Lancashire, of a male specimen, which had all the wings of a bright yellow colour.
Most juvenile butterfly hunters, unblest by scientific knowledge of insect life, imagine that this and the last owe their difference in size simply to their being old and young individuals of the same name; forgetting—or, rather, never having heard—that butterflies never grow in the slightest degree after once getting their winged form; only as caterpillars do they grow.
The male is distinguished from the female by having only one round black spot, or sometimes none, on each upper wing, whilst the female is spotted as in the engraving. The under side of the hind wings is dull yellow, lightly powdered with black scales.
The butterfly is seen during nearly the whole of the summer, and is found almost everywhere.