The egg, which is usually laid on the upper side of a terminal leaf of bird's-foot trefoil (Lotus corniculatus) or on rest-harrow (Ononis spinosa), is whitish-green in colour, netted with glossy white.
The caterpillar is green, covered with short brownish hairs, with which are mixed some longer ones; it is wrinkled on the side, ridged on the back, and the line along the middle of the back is darker. Head black and glossy.
The chrysalis is green, with the head, wing-cases, and sometimes the hinder parts of the body, tinged with buff; thorax brighter green, rather shiny; a darker line down the centre of the body.
The plants mentioned, and especially rest-harrow, are known to be the food of the caterpillar, but eggs have also been found, in Scotland, on red clover, plantain, burnet saxifrage, and yarrow. The caterpillars are to be found, after hibernation, in April, and a second brood in June and July. Those feeding on rest-harrow seem to prefer the blossom.
This caterpillar is stated to form a cocoon, but the only approach to any such structure made by the seven individuals I had under observation was in the case of two caterpillars that pupated among leaves of Lotus, which were drawn together by the slenderest of threads. Four effected the change at the bottom of the cage and seemed to be quite free, one had climbed to the leno top of the cage and there spun a silken carpet under itself, which drew the leno together, and so formed a shallow cave in which the chrysalis rested. In every case the cast skin was attached to the tail, and so remained after the butterflies emerged.
The butterfly is to be found almost everywhere in the country, and its distribution extends throughout the United Kingdom, except, perhaps, the Shetland Isles. There appears to be only one flight in the north of Scotland and Ireland, and this occurs in June and July. In England there are two broods, and in some years probably three in the southern counties. It may be seen on the wing, in greater or lesser numbers, all through the season from May to September.
Abroad, the range extends over the whole of Europe to North Africa, and through Western and Northern Asia to Amurland and China.
The Common Blue, as well as the Chalk Hill and the Adonis Blues, are to be found, often commonly and sometimes in large numbers, in their favourite haunts. Each of them is subject to a considerable range of variation on the under side, and this seems to be of a similar character in all. Very striking aberrations are, perhaps, not often obtained, but still many modifications are to be found, and the possibility of a really good thing turning up, induces one to give attention to the business of overhauling these butterflies. A very good method of conducting this kind of work is to first ascertain the places where they chiefly congregate, and then to visit there on dull days or late in the afternoon, when the butterflies are asleep or, at all events, resting. They can then be easily examined as they sit on the long grass stems, etc. (Plate [27]), but only the under sides can be viewed in this way. So to avoid passing over a good upper-side variety, it will be needful to take each specimen between the finger and thumb of the right hand, seizing the closed wings gently, but firmly, near their base, and then quickly secure the thorax from underneath with thumb and index finger of the left hand, when the upper as well as the under side becomes available for inspection. There is no reason whatever to damage the insects in any way, and those that are not required may be set free again none the worse for their short detention. Work against the wind, and to avoid a second interview, turn rejected specimens to the rear.
