The caterpillar when full grown, is whity-brown in colour with brownish lines. The head is brown, tinged with pink, and the tail-like points on the last ring are pink. The head, as well as the body, is clothed with short hair.
The chrysalis is also whity-brown with a pinkish tinge, browner speckling on the wing cases, and the body is marked down the back with yellow.
Hellins says, "It hibernates when very small, becomes full fed in June, and changes to a pupa without suspending itself in any way, or making a cocoon; I think it would hide itself, as my examples did; I found they had got among the thick moss with which I had furnished the bottom of their cage, and apparently made little hollows for themselves by turning round."
Cock's-foot grass (Dactylis glomerata) and cat's-tail grass (Phleum pratense) are given as food-plants, but the caterpillars in confinement seem to eat any kind of grass that is supplied.
The butterfly is found in most of the Midland counties and in nearly all of the Southern ones, but is especially common on the chalk downs of the South-west. It does not occur in Ireland or Scotland, and seems to be absent from the Northern counties of England except Yorkshire. In the last-named county it was supposed to be extinct, but during the past ten years it has been observed at Sledmere, and near Scarborough and Helmsley. It is also reported to be not uncommon in three localities not far from York.
Pl. 72.
Milkweed Butterfly.
Egg, natural size and enlarged; caterpillar and chrysalis (after Smith).