Friday 1 May 2015

Identification Focus from Emoth - Les Hill

Identification Focus: Distinguishing Red Twin-spot Carpet (Xanthorhoe spadicearia) and Dark-barred Twin-spot Carpet (X. ferrugata)

To kick off what may become a short series of identification focus articles, I have chosen Red Twin-spot Carpet and Dark-barred Twin-spot Carpet, two species easily confused; however, once you to start to take a critical look at some of their features, they should be relatively easy to distinguish when fresh or at least give the recorder good pointers to identification.

Firstly, the ‘notch’ which may be found on the inner crossline near the leading edge adjacent to the central band is no longer considered reliable and shouldn’t be used. It can be seen from the image below that both specimens may be considered to have this notch depending on the observer’s point-of-view.

Image © Tom Tams. Northumberland Moth Group.

Well-marked spadicearia are quite distinctive, but once worn can be difficult to distinguish as features become obscured. Worn, darker-banded examples of spadicearia could be confused with the blackish-banded ferrugata form unidentaria. Conversely, the uncommon, reddish-banded typical form of ferrugata may also confuse some recorders believing it to be spadicearia. Many requests for determinations from recorders claiming ferrugata turn out to be spadicearia.

The following image by Tom Tams, Northumberland County Moth Recorder, demonstrates the feature differences in forewing upperside features. The use of a photographer’s [18%] grey card help represents the true colours and gives good moth/background contrast. However, these ground colours do vary between individuals.

Here are a few pointers to help you towards distinguishing the two species, but do bear in mind some or all of these can be subjective and may be inconclusive:

 The overall ground colour of spadicearia when fresh is generally richer and warmer reddish-brown than in ferrugata.

 The overall contrast between the central crossband and the rest of the forewing patterning is generally high in ferrugata and low in spadicearia.

 The trailing area is lighter and creamier in ferrugata and darker and greyer in spadicearia.

 The width of the central crossband in Dark-barred Twin-spot Carpet generally steps out more dramatically at one-half than in spadicearia.

 The ‘twin-spot’ is less contrasting with the ground colour in spadicearia, while in ferrugata it is isolated from any colour and high contrasting.

 The white or whitish crosslines either side of the central crossband are generally wider in spadicearia and bordered with a blackish line of similar thickness. The blackish lines are very thin to obscure in ferrugata.

Sometimes these species will rest with their wings held high, completely covering up the forewing upperside features. All is not lost, spadicearia usually shows some of the reddish-brown ground colour on the undersides; however, ferrugata usually show no ground colour, instead it is an almost uniform grey.

Hopefully by using some of these features, recorders will find them useful to help towards distinguishing these two species a little easier. If you do have to aggregate these two species, please don’t record them as “Twin-spot Carpet” This is another species altogether and not related.

Further reading:

Townsend, M., Clifton, J. and Goodey, B. (2010) British and Irish moths: an illustrated guide to selected difficult species

Townsend, M. 2010. Observations on the use of wing-markings and genitalia to distinguish Dark-barred Twin-spot Carpet Xanthorhoe ferrugata (Cl.) and Red Twin-spot Carpet X. spadicearia ([D. & S.]). (Lep.: Geometridae) and on the implications of recording these species. Entomologist’s Rec. J. Var. 122.

By Les Hill, Butterfly Conservation

1 comment:

  1. You are actually lucky in that you have both species in relative abundance in your area, but they are full of tricks as Les alludes to, with the Red sometimes being Dark and the Dark sometimes being Red. If you are happy to keep the odd specimen, they are very easy to separate via dissection. So once positively id'd that way, it means there's a valid record for a site. As per usual, send any specimens to Ina and they will get over to me in the course of time.


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