Clifden Nonpareil Catocala fraxini
Wingspan 75-95 mm.
This is the Victorian collector's classic all-time favourite, the 'Blue Underwing'. In the British Isles it is now only an immigrant, though was formerly resident in certain parts of Kent and Norfolk during the middle part of the 20th century.
Nowadays, only a handful per year are recorded, mainly in the south and south-east of England, September being the most likely month.
The large greyish caterpillar feeds mainly on aspen (Populus tremulosa)