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. [Illustration: FIG. 84.--THE WHITE-LETTER HAIRSTREAK.] _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. _The Dark Hairstreak_ (_Thecla Pruni_) The upper side of this butterfly is very much like that of _W-album_, but it may be distinguished by the presence of a few orange spots near the anal angle of the hind wings. The colouring of the under side (Plate VI, fig. 5) is also very similar, except that the white lines of the wings are thinner and less distinct than in the last species, and do not exhibit the W-shaped bend. The orange band of this surface is bordered on each side with a row of black spots, each of which is touched with a bluish white or a metallic blue. This insect is not by any means common, but has been seen in considerable numbers in certain localities. It is not found in either Scotland or Ireland, and its chief haunts in England seem to be in Huntingdonshire, Cambridgeshire and Northamptonshire. It frequents wooded country, and flies during June and July. The eggs are laid late in the summer on the twigs of the blackthorn (_Prunus spinosa_), and are not hatched till the following spring. The caterpillar, which is pale green, with rows of yellow spots, may be found in May. _The Purple Hairstreak_ (_Thecla Quercus_) This pretty butterfly, by far the commonest of the Hairstreaks, though comparatively very small, reminds one forcibly of the noble Purple Emperor. Its haunts are the same oak woods, and its upper surface, though only a dull dark brown in certain lights, exhibits the same imperial purple reflections when viewed at certain angles. The purple of the male extends over the whole of the wings, but that of the female is confined to a V-shaped patch at the base of the fore wings. In the latter case, however, the purple is much richer than in the male sex.