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Does anyone know how reliable the labial palps (dark with pale tips in Svensson's) are thought to be, as a means of differentiating the pair? Last time I checked, they seemed to be regarded as evidence one way or the other, rather than being definitive in themselves.
Along with the notch for dealing with the "twin-spot carpet" pair, the palps on the 2 copper underwings are not now really considered to be a reliable ID feature. As suggested above the underwing pattern is the key pointer, although this can be (very) difficult to see. One character that has also been used is the relative size of the V's just down for the head. These might help to pick out a moth worthy of further consideration, but are probably not reliable enough to stand alone. If in doubt the old stand-by is "Copper Underwing agg.".
I did see the hind wing which was a copper/orange what should I look out for with the hind wings as we seem to find these this time of year in Dormouse Boxes when doing our monthly survey.
Howardian Local Nature Reserve
To quote from Waring and Townsend..."On Copper Underwing, the copper colour is limited to outer third, and ends abruptly at dark cross-band, and basal two thirds is largely straw-coloured, apart from grey-brown speckled streak along leading edge, making a sharply contrasting effect. On Svensson's, copper continues towards base in trailing half, and grey brown speckling is more extensive, so that basal third is either not, or only slightly, paler."
A small pic of each can be seen here....
Someone else might know of a better pair of comparison pics?
My Flickr photos...
Lat/Lng: 51.5, -3.2
OS grid ref: ST2079