SEPARATION OF RED TWIN-SPOT AND DARK-BARRED TWIN-SPOT CARPETS (Xanthorhoe spadicearia and Xanthorhoe ferrugata)- ColinPlantThe separation of these two moths is a problem for many people. The Red-twin-spot usually has a red median band on the forewing, but in some forms this band can be very dark or almost black. The Dark-barred Twin-spot, as the common name implies, has the median band of the forewing dark - it is in fact usually black. However, there is a form that has a reddish band. [Actually it is the typical form that has a red band - those with black bands being referable to ab. unidentaria. The red-banded typical form appears rare].One of the key features for separation has been the presence (Dark-barred) or absence (Red) of a notch on the INNER edge of the red/dark median forewing band at the costa (leading edge). This character is mentioned in Bernard Skinner’s Colour identification guide to Moths of the British Isles (Viking, 1984) and is repeated in the 1998 revision. The character is repeated by Paul Waring and Martin Townsend in their recent identification guide (Waring, Townsend and Lewington. 2003. Field Guide to Moths of Great Britain and Ireland. British Wildlife Publishing) and has been widely used by many - including myself. Recently, however, John Chainey suggested to me that that this character may not, in fact, be quite as reliable as is widely believed. Accordingly, I examined the genitalia of the males in my collection of both species. This is, in fact, achievable by merely brushing away the scales at the tip of the abdomen, from below, using a Woodcock’s pin-feather or similar. The two species are immediately and easily separated by the length and shape of the processes that arise from the inner face of the valves (see photos. There are also illustrations in Peder Skou’s book The Geometroid Moths of North Europe Brill/Scandinavian Science Press, 1986; the females are also illustrated in this book). As a result of this examination I am able to report that THE NOTCH CHARACTER DOES NOT WORK. I have examined males as follows:
Another result is the discovery that there do not appear now to be any valid records of the red-banded form of Dark-Barred Twin-spot Carpet (ferrugata) in Hertfordshire - the few that I am responsible for are all Red Twin-spots (spadicearia) and I feel it is unsafe to accept others unless they are checked. The dark-barred form is the Red Twin-spot IS present in Hertfordshire (about 5 percent based on my own material).Identification in the field, of live moths, may be a problem and it is recommended that if you do not collect you should at least bring the live moths back home for a closer look in daylight. The Red Twin-spot usually has a white band on the outside of the red/dark median band and the other characters in Waring and Townsend seem to work OK. However, some of my dissected male Dark-barred Twin-spots also have a whitish outer edge to the band. The reality is probably that many examples may not be identifiable from a live insect. Tip of abdomen of Red Twin-spot Carpet Xanthorhoe spadicearia |