Bembidion lunulatum

This was found in a moth trap and I spent ages attempting to key it and completely failed. There were various points in the key where I tried both options, just in case I'd made an error, but everything I tried ended up with a species or group of species that I either knew was wrong from experience, or were clearly wrong from either the description or photos online. I used both Duff and Hackston but couldn't get anywhere sensible! Some characters weren't easy to see as the beetle was covered in moth scales, which slowed me down but ultimately didn't prevent me from seeing anything. In the end, in desperation, I ran a photo past ObsIdentify. That offered me an 83% probability of octomaculatum, which was clearly wrong, 3% Bembidion sp. which was right but not very useful, 2% lunulatum and 1% fumigatum. I checked all three species and the first high res photo I found of lunulatum (on Wikipedia) looked absolutely perfect in almost every respect, unlike any of the species I had looked up so far.

Both keys require lunulatum (and the rest of the subgenus Philochthus) to have curved or rounded sides of the pronotum right up to the hind angles, not concave or sinuate before the hind angles. On my beetle the sides of the pronotum were rounded but there was a slight concavity immediately before the hind angle. Slight enough that had I been using Hackston alone I might have pursued the "rounded" option, but Duff explicitly says that the sinuation may need to be looked for carefully, so I took it that even a slight dip before the hind angle ruled out the curved option. Apparently not, as other photos of lunulatum show exactly the same shape as mine.

The pattern on the elytra was not at all clear, not only because of all the moth scales covering it. The elytra were paler throughout than most lunulatum seem to be in the centre, but I think it may have been teneral. The elytra seemed soft and had rolled up towards the apex on death, which doesn't normally happen with beetles I catch. The pale spot on the sides of the elytra wasn't really apparent until I removed an elytron and examined it with light from behind. The length of the specimen was 3.0mm but with the head and pronotum angled downwards and the tip of the elytra also bent, I think it's safe to assume that the live length would have been closer to Duff's range of 3.5mm-4.0mm (Hackston gives 3-3mm which is ok anyway).

Bembidion lunulatum showing frons, pronotum, rear of pronotum with humeral angles and elytron, North Elmham (Norfolk, UK), 28th June 2025