Water-plantain Straw Gynnidomorpha alismana

Formerly given the vernacular name Water-plantain Conch.

This was one of two worn conches that I picked out of my moth trap on the same night. The other was more worn and was clearly going to need gen det to resolve. This one looked more interesting, but I was in a hurry and potted it up to look at properly later. For reasons that I can no longer remember, I didn't do that before putting the gen dets to one side to come back to later in the year. In hindsight I should have realised that it was a Gynnidomorpha and at least a strong candidate for alismana based on what I could see of its external appearance.

Examining its genitalia I was quickly able to confirm that it was a Gynnidomorpha. I could easily eliminate Coast Straw permixtana and Fen Straw minimana as they have much narrower tips to the socii. The socii of Bartsia Straw luridana have broader rounded tips, so that was out too. Separating alismana from Saltern Straw vectisana on male genitalia is not so straightforward. Looking at the images at mothdissection.co.uk and the figures in Chambon I couldn't coninvce myself of any consistent differences. MoGBI wasn't much help either - the diagrams for alismana include a couple of arrows as if pointing out the diagnostic characters, but I can't make sense of what it's getting at (in one case I'm not even sure what bit its pointing to). Chris Lewis's website quotes Pierce & Metcalfe, providing the following descriptions for the socii - "broad, slightly arched, and quickly reduced to a point set with fine hairs" for alismana and "broad, gently arched and tapered to a point set with short, stout hairs" for vectisana. I'm not convinced I can see that in the images on mothdissection.co.uk or elsewhere.

Neither species is expected in North Elmham, with vectisana being a saltmarsh species present along the north coast of Norfolk and alismana mainly restricted (in Norfolk) to the Broads and SE Norfolk though there have been a couple of recent records at Cranberry Rough in the Brecks (the only records in this vice-county). Size is often the most obvious difference with vectisana being 9-11mm and alismana being 11-14mm, so I set about measuring mine. The forewings were 5.1/5.2mm and with the thorax being about 1.2mm across I calculated the wingspan to be around 11.5mm - near the bottom end for alismana but too close to the top end of vectisana for this to be sufficiently ruled out on size alone. Having said that the wings were devoid of any cilia at the apex so if these had been included in the measurement it would have bumped it up by maybe a mm or so and put it more firmly in the alismana camp.

In the end it came back to the external characters - in particular the extensive dark area at the dorsal end of the central fascia and the dark dorsal area behind this. I think this supports my ID of alismana.

male Water-plantain Straw Gynnidomorpha alismana showing genitalia (various views, before and after setting), North Elmham (Norfolk, UK), 21st August 2023

This one was at one of Brian Jones' patches where he kindly allowed Dave N and I to join him for what turned out to be an amazing night's mothing. It was the first record for the site. I put it down as gen detted though I don't recall precisely which characters I was going on. I suspect that like the one above the external appearance and size would have played a part in the ID.

male Water-plantain Straw Gynnidomorpha alismana showing genitalia, Aldeby (Norfolk, UK), 10th July 2018