This butterfly is by no means an abundant insect, though it is widely distributed, and in some places plentiful. Its chief haunts are woods, and we may mention among its favoured localities Epping Forest, Monk's Wood in Cambridgeshire, the wooded parts of South Devon and Dorset, New Forest, Colchester, and Peterborough.
The perfect insect is on the wing from July to October, and the eggs are deposited in the autumn on the twigs of its food plant—the blackthorn (Prunus spinosa). These do not hatch till the following spring. Toward the end of June the caterpillar is fully fed.
The colour of the caterpillar is light green, with two white stripes down the back, and two others along the sides. There are also two small oblique whitish lines on each side of each segment.
The chrysalis is smooth, and of a pale brown colour.
The Black Hairstreak, or White-letter Hairstreak (Thecla W-album)
The first of the above two popular names has been applied to this species on account of the very deep brown colour of the upper side, which colour is often a near approach to black. The second is due to the W-shaped bend of the white streak of the hind wings. The ground colour of the under side ([Plate VI], fig. 4) is greyish brown, with a bright orange band, spotted with black near the hind margin of the hind wings.
W-album is a somewhat rare insect, but is occasionally seen in plenty in a few localities, Cambridgeshire, Berkshire, Epping, Colchester and Suffolk being among its chief resorts. It is out on the wing in July, and should be looked for in wooded country where the common elm (Ulmus campestris) and the wych elm (U. montana), its food plants, exist.