The Chalk Hill Blue (Lycæna corydon).
Although this butterfly (Plate [109]) is, in England, fairly constant in the matter of colour, and, as regards the male especially, differences in tint are noticeable when series from various localities are ranged side by side. Silvery-blue perhaps best expresses the general colour of the male on the upper surface, sometimes very pale, and sometimes faintly tinged with greenish. The blackish border on the outer margin of the fore wings varies in width and in intensity; often there are indications of eyed spots on this margin, and occasionally these spots are quite distinct, although the whitish rings are not always clearly outlined. The black border on the outer margin of the hind wings is often narrow and external to a series of white-edged black spots, but sometimes it is broad and obscures the spots; orange markings rarely appear on this margin, but such aberrations have been taken on the Dorset coast. The fringes are white chequered with blackish on the fore wings, but with seeming continuation of the veins through those of the hind wings. The female is sooty-brown above, with a black discal spot on the fore wings, and sometimes on the hind wings also, and these spots may be ringed with blue or bluish-white; the outer marginal borders are hardly darker, and those on the fore wings are limited by a wavy pale line, which may be faintly or strongly marked with orange, but orange marking on these wings is rather the exception than the rule; on the outer margin of the hind wings there are some black spots, edged outwardly with white and inwardly with orange. The fringes are white chequered with brown, and those of the fore wings are tinged with brown. There are generally some blue scales at the base of the fore wings and over a larger portion of the basal area of the hind wings, but occasionally the whole discal area of the hind wings (Fig. [7,] Plate [117]), or of all the wings, var. syngrapha (Fig. [8,] Plate [117]), is of the male colour. The former is from Eastbourne and the latter from Wiltshire. They are rather uncommon varieties, but intermediate forms are more often met with in the same localities as well as in other parts of England where the species occurs.
On the coast of Dorsetshire a very unusual form occurs. The border of the outer margin is white instead of the usual black or blackish; the inner limit of this border is, on the fore wings, defined by a dusky shade, and the black nervules break up the border into six spots; on the hind wings four or five of the white spots are centred with black dots. The female has a similar border, but on the hind wings it is inwardly edged with orange. It has been named var. fowleri, and I have seen one example of this form without black dots in the marginal white spots of the hind wings. On the under side variation is on somewhat similar lines to that adverted to in the last species. On Plate [109,] Fig. [8] represents the typical under side of the male, and Fig. 7 that of the female. It will be noticed that the male is greyer than the female. Some of the ordinary aberrations are shown on the same plate, and some rarer ones will be found on Plate [118,] and of these Fig. [12,] if without the basal spot on the fore wings, would represent var. lucretia.
For figures of the early stages see Plate [108;] that of the caterpillar is after Buckler. The egg is flat on the top, with a slightly darker pit in the centre (the micropyle); the sides are rounded, netted, and studded, and the colour whitish-green. The above short description was taken from one of a few eggs of this butterfly sent me in August last by Mr. Ovenden, and the same egg has been figured.
Mr. Frohawk has described the egg more fully in the Entomologist for 1900. With reference to the egg-laying of the butterfly he writes: "On August 13th, 1900, I watched several females in the act of depositing, on various stems of the usual stunted herbage to be found growing on chalk downs. They frequently crawled among the plants for a distance of about a couple of feet, occasionally curving the abdomen downwards among the small plant-stems and grasses, and here and there depositing an egg. I therefore dug up portions of the turf, potted it, and placed a couple of females on each lot; they deposited ova on the 14th and 15th, on the stems of various plants; a few were laid upon the brown dead trefoil leaves, as well as on the living leaves; but the site generally chosen is the intermingled stems of both plants and grasses. Another female, placed upon a similar pot of plants, deposited about fifty ova on September 10th, nearly all being placed upon the stems, and a few upon the under side of the leaves of rock-rose; in all cases the eggs are deposited singly."
The caterpillars do not hatch out until the following spring. According to Buckler and Hellins, the only difference between the caterpillar of this butterfly and that of the next species, Adonis, is that the latter "has its ground colour deeper green, with the hairs or bristles black, while Corydon has the ground colour of a lighter, brighter green (a green with more yellow in its composition), and the hairs light brown."
The butterfly is common and often abundant in July and August, chiefly the latter month, on chalk downs in Oxfordshire, Buckinghamshire, Berkshire, Kent, Surrey, and Sussex; it is also found in the Newmarket district of Cambridgeshire and on one chalk hill in Norfolk, according to Barrett, who adds: "on the oolite as well as the chalk in Wilts, Dorset, Gloucestershire, and Somerset; and on limestone at Grange and Silverdale in North Lancashire, in Lincolnshire, Westmoreland, and Cumberland. It has also been taken in Essex, Hants, Cornwall, and in one locality in Glamorganshire."
Mr. Sydney Webb has stated that a dwarf form occurs pretty regularly in a valley about two miles east of Dover, but that it only appears to be found at odd times in other parts of England.
Abroad, the species is found in Central Europe, also in the Pyrenees, Aragonia, and the Balkan Peninsula.