Small Scabious Plume Stenoptilia annadactyla

Apart from the individual below, only known from the Norfolk and Suffolk Brecks where it was first discovered in 2005.

This came to light in a recently established meadow (formerly arable). I thought the overall colour was quite brown and that it would therefore most likely turn out to be Brown Plume Stenoptilia pterodactlya, the only really common species in the genus. But it seemed a bit odd, and there were dark markings in the first lobe that aren't normally present in pterodactyla. Could it be Twin-spot Plume Stenoptilia bipunctidactyla, I mused, and then rejected that idea as it surely wasn't grey enough. I couldn't think of any other alternatives so potted it for subsequent examination at home.

When I came to dissect it I had forgotten it was potentially interesting and thought was just being ultra-cautious checking pterodactyla, and was not quite as careful as I should have been. As a result the genitalia aren't quite so well prepared as I would have liked, but upon seeing the antrum and comparing it with the diagrams in Hart I immediately realised it was not pterodactyla. But it also didn't seem right for bipunctidactyla, and although Dusky Plume Stenoptilia zophodactylus was never really a contender, that was also ruled out leaving only species that I would not have expected to encounter round here, and of those only the apparently rare (or overlooked?) Breckland speciality annadactyla matched.

The antrum shape is key to the identification - the sclerotised portion is noticeably shorter than on the other species (some references seem to refer the sclerotised section as the entire antrum but the membranous section above it seems to me to be part of the same structure so I'm not sure). It is also more parallel-sided, not tapering as it does in the other species.

There also seems to be a difference in the form of the sclerotised line running through the ductus bursae. This isn't referred to in Hart or other references I've seen so maybe it is not diagnostic, but in this individual as well as most of the images of female annadactyla genitalia I've seen, this sclerite is clearly broadened into a more-or-less club shape at its end (the end nearest the corpus bursae). On all the images I've found of Stenoptilia bipunctidactyla it either doesn't broaden at the end at all, or if it does it's not nearly so obvious or abrupt.

Intriguingly, although annadactyla and bipunctidactyla are meant to be more-or-less identical in external appearance, the online images I've seen of annadactyla all show browner (less grey) moths than all the online images I've seen of bipunctidactyla. Of course that's not a large sample, and the photos in Hart show a greyer annadactyla and a (darker) brown bipunctidactyla, so I don't suppose there's much in this if anything, but it will be interesting to see if there's an average difference given a larger sample. Photos of both species show a white-fringed termen with a single dark spot at its inner end - not helpful for separating annadactyla from bipunctidactyla but could help to rule out pterodactyla.

More than 18 miles away from the nearest known Breckland site, this was quite unexpected at this location.

female Small Scabious Plume Stenoptilia annadactyla, Wendling Beck Environment Project (Norfolk, UK), 23rd August 2024