Ceratapion gibbirostre

I really struggled to identify this. I examined it at the same time as a Ceratapion onopordi, so that was easy to eliminate, but choosing between gibbirostre and carduorum was problematic. Both Duff and Hackston major on the shape of the rostral swelling, rounded on carduorum and angled on gibbirostre, with Duff stressing that this must be viewed from directly above. Now there's a question as to what "directly above" means here, as the swelling is on the curve of the rostrum. Should the beetle as a whole be level, or should it be angled back slightly so that the tangent to the rostrum at the point of the swelling is flat? Anyway, either way, it seemed to me to be clearly rounded, which should make it carduorum (apparently the scarcer of the two species, though by no means rare).

However, there were two problems with calling it carduorum. Firstly it was a touch under 2.6mm long, whereas the bottom end of the size range for carduorum given in both keys is 2.7mm. Close enough maybe? Secondly Duff describes the elytra as "bright metallic blue", whereas gibbirostre can be blue or green. This was clearly green - bluish green maybe, but more green than blue. This seemed less of a problem when I looked at photos of carduorum online - they all seemed pretty green to me, although not as green as mine. So, though I retained a niggling doubt, the shape of the rostral swelling seemed pretty unambiguously round, it surely had to be carduorum. To remove the niggling doubts I decided to check the genitalia. Duff doesn't show the genitalia and Hackston only uses genitalia to compare onopordi to the other species, not do differentiate between the other species whose genitalia aren't illustrated. But there are photos of gibbirostre genitalia at coleonet.de and the Weevils of Greece website. The latter also has genitalia photos for the non-UK Ceratapion damryi but I cannot find any photos of carduorum genitalia. Still, if the genitalia were clearly different from gibbirostre, that would probably be sufficient to convince me it was carduorum. Well, looking at the Weevils of Greece website first, it was different - the V incision of the tegmen diverged more widely near the point of the incision, and the inner sclerite seemed narrower with fewer teeth along its side. The latter difference was less convincing when I looked at the coleonet.de photos but the V still looked wider at the incision. However I then set the genitalia on a microscope slide and as I compressed the tegmen slightly while putting the cover slip on, it splayed out so that it now looked much more like the photo at the Weevils of Greece site.

So now I'm in the position where the genitalia do not look substantially different from Ceratapion gibbirostre but I don't know what they should look like on carduorum. Size and colour suggest gibbirostre, but still apparently the most important character fits carduorum. At this point I brought out a lens attachment for the microscope to look at the rostrum more closely. With hairs on the swelling it's not super easy to discern the outline so I thought this might help, and it did make it look a little more angled than I'd previously thought, but not convincingly so. However I noticed that if I tilted the beetle further back so that I'm looking at the swelling from partly in front (so no longer directly from above no matter how you understand that), it suddenly looks strongly angled, much like the diagram in Duff for gibbirostre.

I know this isn't looking directly from above as you're supposed to, but together with the size, the elytral colour and the likeness of the genitalia to gibbirostre, I'm now of the opinion that it is much more likely to be gibbirostre. I am retaining the beetle so that I can look at it again when I find more of them.

male Ceratapion gibbirostre showing base of elytra, pronotum, frons, frons and rostral tooth, rostral tooth (3 views, the last with the beetle tilted so looking partly from in front), median lobe (flat and side views), internal sclerite in median lobe and tegmen (2 views tilted differently and third view flattened under a cover slip), North Elmham (Norfolk, UK), 7th August 2025

To give more of an idea of how green the elytra were, here is the same beetle together with a much bluer Ceratapion onopordi.

female Ceratapion onopordi, North Elmham (Norfolk, UK), 17th August 2022 (above) with male Ceratapion gibbirostre, North Elmham (Norfolk, UK), 7th August 2025 (below)