Yellow Meadow Ant Lasius flavus

I have undoubtedly seen this species many times before, but this is the first one I've keyed out. Earlier in the spring I had moved my moth trap one morning and discovered a nest of yellow ants beneath it. I didn't have much time that morning so decided to retain one another day to secure the ID, but I never saw any more activity there again. Finally in the autumn one appeared in the bottom of my moth trap, so I took the opportunity to finally identify one properly.

I initially used the Naturalists' Handbook key by Skinner & Allen to workers and came unstuck at a couple of points. Firstly the hairs on the scape do not lie flat. I wouldn't even describe them as decumbent. They clearly arise away from the surface at an angle of perhaps 35-40 degrees, slightly down-curved at the tips but still well proud from the surface of the scape. So not erect perhaps, but slanting, I'd say so. At least "slightly slanting" which is all they need to key to sabularum/meridionalis/umbratus in Skinner & Allen. I then had a problem - and this problem arises whichever way I went on scape hairs - with the relative length of the erect hairs on the gaster vs. the maximum width of the hind metatarsus. Both routes required me to choose between the erect hairs on the top of the gaster being either 0.3 x or 0.6 x the maximum width of the hind metatarsus. The gaster is covered in hairs, short fine hairs that lie, pretty much, flat on the surface and a few longer hairs that are clearly erect. I presume it means these, unless it calls these bristles, but then there aren't any erect hairs. The problem is these are over 70µm long whereas the maximum width of the hind metatarsus is 35µm. Surely double the length wouldn't be described as either 0.3x or 0.6x? (Actually, looking at my photo I think the apex of the metatarsus may be slightly wider than where I measured it, but still no more than 40µm so it makes no real difference). So what does it mean, or have I misidentified some anatomy? I have no idea.

Struggling to make sense of that key I turned to Lebas et al. Here sabularum etc. (and also mixtus) are ruled out by the absence of erect hairs on the underside of the head and a proportionately shorter petiole scale. So, as suspected all along, it seems that this was flavus.

Yellow Meadow Ant Lasius flavus showing propodeal spiracle, antennal scape, head from front (focuses showing head shape and detail of face), length of hairs on gaster (two) and width of hind metatarsus, propodeal scale and underside of head, North Elmham (Norfolk, UK), 10th September 2023

I bought the updated Naturalists' Handbook (Skinner & Jarman, 2025) before I examined this one. My first impressions are that this is a huge improvement, with additional information provided at many couplets, both for improved clarity and additional confirmatory characters to check. However, even with this and it's extra figure clarifying how slanting hairs are defined, I struggled to call this one the correct way to get to flavus. As before, taking the other option led to sabularum but with no erect hairs on the underside of the head (and sabularum being a very much rarer species) I am confident that this was flavus.

Yellow Meadow Ant Lasius flavus showing antennal scape, hind tibia, fore tibia and area between eye and mandibles, North Elmham railway below the Cathedral Meadows (Norfolk, UK), 25th February 2025