The caterpillar when full grown, as figured on Plate 1, is bright green with an orange-spotted black band on each ring of the body, and blackish tinged with bluish between the rings. The head is yellow striped with black. When it first leaves the egg-shell, which it eats, the caterpillar is black with a noticeable white patch about the middle of the body. After the third change of skin it assumes the green colour, and at the same time a remarkable =V=-shaped fleshy structure of a pinkish or orange colour is developed. This is the _osmaterium_, and is said to emit a strong smell, which has been compared to that of a decaying pine-apple. The organ, which is extended in the figure of the full-grown caterpillar, is not always in evidence, but when the caterpillar is annoyed the forked arrangement makes its appearance from a fold in the forepart of the ring nearest the head. Other food-plants besides milk parsley are angelica (_Angelica sylvestris_), fennel (_Foeniculum vulgare_), wild carrot (_Daucus carota_), etc. From eggs laid in May or June caterpillars hatch in from ten to twelve days, and these attain the chrysalis state in about six or seven weeks. If the season is a favourable one, that is fine and warm, some of the butterflies should appear in August, the others remaining in the chrysalids until May or June of the following year; a few may even pass a second winter in the chrysalis. Caterpillars from eggs laid by the August females may be found in September, nearly or quite full grown, and chrysalids from October onwards throughout the winter. They are most frequently seen on the stems of reeds, but they may also be found on stems or sprays of the food-plants, as well as on bits of stick, etc. It would, however, be practically useless to search for the late chrysalids as the reeds are usually cut down in October, when the fenmen keep a sharp look-out for them, and few are likely to escape detection in any place that would be accessible to the entomologist. [Illustration: _Pl._ 2. =Swallow-tail Butterfly.= 1 _male_; 2 _female_.] [Illustration: _Pl._ 3. =Black-veined White Butterfly.= _Eggs, natural size and enlarged; caterpillar and chrysalis._] On Plate 1 three forms of the chrysalis are shown. The figures are drawn from specimens collected in Wicken Fen in October, 1905. Occasionally a much darker, nearly black, form is found. This butterfly was known to Petiver and other early eighteenth-century entomologists as the Royal William. There is every reason to believe that at one time it was far more widely distributed in England than it now is. Stephens, writing in 1827, states that it was formerly abundant at Westerham, and gives several other localities, some very near to London.