The Milkweed Butterfly (Anosia Plexippus)
The butterfly figured on Plate 120 is brownish-orange, with black veins and margins on all the wings. White spots are arranged in double rows on the black outer margin of each wing, and there are seven other rather larger white spots on the black apical patch of the fore wings. The male has a patch of black scales, covering the scent pouch, close to vein 2 on the hind wings.
The egg is long, oval in shape, with over twenty low upright ridges and many cross-lines; is of a pale green colour; and is laid singly on the food-plant of the caterpillar (various kinds of milkweed, especially the commonest kind, Asclepias cornuti), and usually upon the under surface of the upturned apical leaves near the middle. The egg state lasts only about four days (Scudder). The caterpillar has the head smooth and rounded, yellow, conspicuously banded with black. Body cylindrical, tapering a little in front, naked, but with two pairs of long and very slender black thread-like filaments, one pair, the longer, on the second thoracic, the other on the eighth abdominal segment. The body is white, with numerous slender black and yellow, and especially black, transverse stripes, repeated with considerable regularity on each of the segments, so that there are nowhere any broad patches of colour (Scudder).
The chrysalis is stout and not elongated, largest in the middle of the abdomen; where it is transversely ridged; elsewhere it is smooth and rounded, with no striking prominences, but with little conical projections at most of the elevated points, like those which half encircle the body at the abdominal ridge, all of a golden colour except the latter, which are situated in a tri-coloured band, black in front, nacreous in the middle, and gilt behind (Scudder).
According to Dr. Holland, “the butterfly is considered to be polygoneutic, that is to say, many broods are produced annually; and it is believed by writers, that with the advent of cold weather these butterflies migrate to the South [in America], the chrysalids and caterpillars which may be undeveloped at the time of the frosts are destroyed, and that when these insects reappear, as they do every summer in North America, they represent a wave of immigration coming northward from the warmer regions of the Gulf States. It is not believed that any of them hibernate in any stage of their existence. This insect sometimes appears in great swarms on the eastern and southern coasts of New Jersey in late autumn. The swarms pressing southward are arrested by the ocean.” Within quite recent years it seems to have effected a settlement in Australia, “and has thence spread northward and westward, until in its migrations it has reached Java and Sumatra, and long ago took possession of the Philippines. Moving eastward on the lines of travel, it has established a more or less precarious foothold for itself in Southern England.... It is well established at the Cape Verde Islands, and in a short time we may expect to hear of it as having taken possession of the continent of Africa, in which the family of plants upon which the caterpillars feed is well represented.”
So far as is shown by the published records, the actual number of specimens of the Milkweed, or, as it is sometimes called, Monarch butterfly, seen or caught in England between 1876, in which year it was first observed in this country, and the present time, does not much exceed thirty, and about one-third of these were obtained in September, 1885. In 1876 single specimens were captured at Neath, S. Wales; Hayward’s Heath and Keymer, Sussex; and Poole, Dorset. In 1896 single specimens were reported as seen at Lymington, Hants, in May; Newlands Corner, Surrey, in July; and the Lizard, Cornwall, in September. The years in which the butterfly has been noticed in Britain are 1876, 1881, 1884, 1885, 1886, 1887, 1890, and 1896. It was first observed on the Continent in 1877, when, according to Barrett, a specimen was taken in La Vendée, France. In 1886, when half a dozen were recorded from England, single specimens were obtained in Guernsey, and at Oporto and Gibraltar. “More recently,” Barrett states, “Mr. H.W. Vivian found it, I believe not uncommonly, in the Canaries, and very kindly brought me a specimen.”
There seems to be no question that the species is migratory in its habits, but exactly how it reaches this country is not definitely known. Neither is it known whether the species, having arrived, is able to reproduce its kind here. From the fact of its recurrence in England for four years in succession, the possibility of its breeding in this country might be assumed. One objection to any such inference, however, is that it is a many-brooded species, but, with the exception of two records in 1896, all British specimens were captured or seen in August, September, or October, and none seem to have been observed in the earlier months of those years in which the autumnal butterflies were obtained.
The Milkweeds (Asclepias) are not indigenous plants, but, as pointed out by the late Mr. J. Jenner Weir, A. purpurescens and A. tuberosa are hardy in this country. He endeavoured to ascertain whether these plants, or either of them, were grown in any of the gardens in the Cornish locality where four fresh specimens were captured in September, 1885. I do not find that the desired information was furnished. Recently I have ascertained that A. cornuti, which grows to a height of four feet, is used as a border plant in some parts of England. It is commonly known as Swallow-wort, and is esteemed for its fragrant pale purple flowers.
We now come to the Satyrinæ, which, as regards the number of species belonging to it, is a very large sub-family. In Great Britain, however, there are but eleven species, and although some of these are rather local, none are really scarce, and most are common.