Lepidoptera The Black Hairstreak, Or White-letter Hairstreak Butterfly – 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.
The eggs are laid on the twigs of these trees late in the summer, and the young caterpillars do not appear till the following spring. The chrysalis may be found attached to an elm twig or leaf about the end of June.
The caterpillar is pale green. The ridges along the back are tinged with yellow, and there are two fine oblique white lines on each side of each segment.