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)
