The caterpillar is rather stout for its length. It has a dark greyish head, which is covered with short bristles. The ground colour of the body varies from greyish-green and ochreous-grey to blackish, and in the darker colour is generally freckled with paler, sometimes yellowish. There is a black line along the back, often edged with yellowish, and sometimes much broken up; the lines on the sides are yellowish, but not always distinct; the line below the yellow-ringed black spiracles, however, is generally broad and yellowish in colour. Although thistles (_Carduus_) appear to be the plants most frequently eaten by these caterpillars, they have sometimes been found feeding upon mallow (_Malva_), burdock (_Arctium_), viper's bugloss (_Echium_), and even nettle (_Urtica_). They commence life by fixing up the edges of a leaf so as to form a sort of pocket in which to conceal themselves, but as they eat away the fleshy part of the leaf their retreat is easily detected. The hiding-place, or dining-room, of a full-grown caterpillar is shown on the plate; change to the chrysalis is often effected in a somewhat similar structure. The chrysalis is grey, ochreous-grey, or greenish; shaded or striped with brownish. The raised points are burnished, and according to the way light falls on them appear golden or silvery. This metallic effect is also seen on other parts of the chrysalis, but chiefly on the back. This butterfly is a notorious migrant. Its proper home is probably in Northern Africa, and there it, at times, becomes so exceedingly numerous that emigration is possibly a necessity in the interests of future generations of the species. Whatever the cause of their leaving may be, there is no doubt about the fact that the butterflies do quit the land of their birth in great swarms. Almost any part of the world may become the dumping-ground of this surplus stock. Our own islands are frequently favoured in this way, and it is most likely that if this were not so, this pretty butterfly would not be so common throughout Great Britain as it is in some years. The natural habit of the species is to go on reproducing its kind throughout the year, and those individuals that arrive here most certainly endeavour to do this in their new home. Unfortunately our climate is not, as a rule, a suitable one for those caterpillars which hatch from the egg late in the season, and although some may complete their growth, and even attain the perfect state, the butterfly, so far as is known, does not hibernate as do the Tortoiseshells and the Peacock. It may therefore be assumed that the specimens seen in May or June of any year are not native born, but early immigrants, and that it is from such aliens that the caterpillars and butterflies observed later in the year are descended.