Lesser Swallow Prominent.
(Pheosia gnoma)

Nicely outlined on a stone wall and showing the difference between a Swallow Prominent and the very similar Lesser Swallow Prominent.

Information:
The key to which is which lies in the intensity and length of the white mark in the trailing corner of the fore-wing. Here, the wedge shaped bright white dash intrudes into the black streak less than half way, therefore it is a Lesser Swallow Prominent. There is no feathering on the antennae so this one is a female. The markings shown are typical but may vary slightly in colouring.
They can be seen flying either in April-June or in August. They are common, resident in most of Britain in or near to woodland, gardens, parks, moorland and heaths generally.