Glossy Birch Dot Stigmella sakhalinella

Formerly given the vernacular name Small Birch Pigmy.

This one was initially keyed using the Nepticulidae key at www.kleinevlinders.nl after dissecting. As usual when using this key there are couplets where I think one option is better than the other, but I follow both routes just in case. The second-choice options mostly led to species which were easily eliminated, mainly as the genitalia are very different. A few had more similar genitalia but still different enough for me to be comfortable ruling them out quite quickly. I ended up with a pair of species that were not so easy to separate - Stigmella betulicola (rare form) and Stigmella sakhalinella The position of the fascia favoured sakhalinella but this was marginal and I wasn't happy to settle the ID without working out differences in genitalia.

In the end after comparing images of genitalia at the Moth Dissection website and Lepiforum and the excellent diagrams at svenska_fjarilar I think I found a number of differences between the two species. There's always a risk when working out differences by looking at a relatively small number of images that you pick out something that is coincidental not diagnostic, so I'm not completely sure these are all valid, however I think I found the following differences:

• The cornuti at the base of the aedeagus in sakhalinella take the form of two broad flat plates with a row of strong teeth at their apex. In the images of betulicola it's slightly less clear what's going on - I can't quite tell if the equivalent to the teeth are indeed teeth on plates (in which case the plates are smaller and less distinct) or if they're separate cornuti. Mine was a clear match to sakhalinella.
• There may also be a difference in the cornuti in the middle of the aedeagus but this is less clear to me. Mine resembled sakhalinella but I'm not sure I can rule out betulicola on this basis.
• The outer edge of the valvae near the tip appears to be angled near the tip and concave after this angle on sakhalinella but although it may just be the way the genitalia have been flattened, they appear more evenly rounded on all the images of betulicola. Unfortunately this isn't helpful for mine as I've not managed to set either valvae completely flat - they look more rounded, but I think that's just because they're not flat.
• The uncus appears to be flatter with more rounded corners in betulicola and in a shallow V and more pointed at the corners in sakhalinella. On mine it looks a little flatter than most of the images of sakhalinella but not as flat as betulicola - the corners are clearly further out than the centre and they're sharp.
• Not sure if I've got my anatomy right but both species have curved processed pointing upwards from what I think is the gnathos. I'm not sure about this as there is some variation across the images, but I think they tend to be a little shorter in sakhalinella. Mine matched sakhalinella in this respect.
• The transtila appears to be consistently different between the two species (and this is the character that separates betulicola from sakhalinella and some other similar species in the key at Chris Lewis's British Lepidoptera website). On betulicola there are clear processes pointing down and out from either side of the transtila whereas in sakhalinella these are reduced to roundish swellings giving the transtila a bra-shaped appearance. On mine the transtila is broken in the middle - the left side is missing but the right side is in position and apparently undamaged, and matches sakhalinella.

male Glossy Birch Dot Stigmella sakhalinella showing genitalia (aedeagus and 2 focuses of whole genital capsule), North Elmham (Norfolk, UK), 2nd August 2024

I had previously identified this species but although I was confident enough to count it on my personal private list I was not quite certain enough to record it formally. Back then (in 2018) I was not completely clear on the differences in genitalia between this species and betulicola, plus I had been unable at that time to find any images or descriptions of the genitalia of one of the other similar species. However, given what I now understand I think I was right and now intend to submit the record. The genitalia was badly damaged during the preparation but most of the key bits are present and match sakhalinella (and importantly are wrong for betulicola - and the third species I couldn't find out about at the time turns out to be very different).

male Glossy Birch Dot Stigmella sakhalinella showing pieces of genitalia, North Elmham (Norfolk, UK), 23rd July 2018