This is a moth that seems to be particularly captivated by light and finds its way frequently in to the moth trap.
The dark arches (Apamea monoglypha) is generally single brooded flying from June through until August and it is quite common throughout the British Isles. In the south, however, and especially in Dorset it can have a second brood in September to October if the weather is right so it is a species that keeps cropping up for most of the autumn.
Like many moths it is a lover of red valerian and buddleia and as our garden is blessed with both of these the dark arches is going to be a regular visitor.
The larvae feed on the roots and stems of grasses, notably the very common cock's-foot and it overwinters as a larvae, pupating in the spring before emerging in June.