Once you get into mothing you will see lots and lots of moths with names that refer to the colour of their hindwings. They refer to it as an “underwing” because the forewings which are usually less colourful but might be wonderfully patterned nevertheless, cover the hind wings where that flash of colour is seen. It’s presumably an adaptation to give predators with colour vision, such as birds, a bit of a shock when the moth flicks its wings open.
As regulars will know, the first one that got me into the scientific mothing lark was the Copper Underwing, but there are Large Yellow, Lesser Yellow, Least Yellow, Broad-bordered, Lesser Broad-bordered, Lunar, Straw, there are at least a dozen more, the list goes Today first time for me a very large Red Underwing (about 40mm from nose to the end of its forewing). The wingspan is between 65 and 75 mm.
It’s quite a common moth over much of England and Wales. A night-flyer in August and September, it will come to lights and sugaring. Its larvae feed on Poplar and Willow.