Dusky Longhorn Beetle Arhopalus rusticus

I would have identified this one using Mike Hackston's key.

Dusky Longhorn Beetle Arhopalus rusticus, North Elmham Cathedral Meadows (Norfolk, UK), 7th August 2020

The individual below was attracted to light in a mainly deciduous wood but there were a few scattered conifers and historically I gather there have been a lot of larch trees here (many now removed but a few still present). I was not aware of any fires having occurred here but I initially identified it as a Burnt Pine Longhorn Arhopalus ferus based on my assessment of the four characters Duff uses to separate this from the much commoner Arhopalus rusticus, supported by comments in the Hackston key which allayed my concern about the eye hairs. However, having now examined another Arhopalus and investigated it further (the photos at the Beetles of Europe website were especially helpful), I think the keys do not describe the differences in the tarsomeres precisely or accurately enough and both specimens, including the one I originally identified as ferus, are in fact the much commoner rusticus.

• Hind tarsomere 3 did not appear to be as deeply lobed as on rusticus which should be split almost to the base. I judged it to be split to about half way. Duff's diagrams showed it split more than half way on both species but it seemed clear that it was closer to ferus than rusticus. However, Hackston uses this character too but doesn't specify that it must be the hind tarsomere - and the fore tarsomere 3 was split nearly to the base. The real problem here seems to be interpreting what is meant by split. To me "split" means completely divided - so if you look at it in silhouette the split ends where you can't see it any more. In reality the tarsomere is narrow beyond the end of the split, so you might say the "split" continues towards the base, just not cutting all the way through the tarsomere. After examining another Arhopalus I realise that the tarsomere is completely split only 60-70% of the way to the base (the variation depending on the angle) and thus similar to Duff's diagram for ferus, but it is excavated to about 90% of the way to the base. As tarsomere 4 emerges underneath the base of the split it is not always easy to see how far back it is fully split, and may at some angles be easier to see how far back it is merely excavated. Anyway, the photos of the tarsi at the Beetles of Europe website are really helpful here. On rustica the tarsomere appears to be completely split to about 70% and excavated to about 90% whereas ferus has a very much shallower split, very much less than halfway (although it is excavated to just beyond halfway). Compared to those photos both of my specimens clearly match rustica much better than ferus.
• Not mentioned in the literature anywhere so far as I can see, I wonder if the shape of the lobes may be more reliable, or at least easier to see, than the depth of the split. In both Duff's diagrams and the photos in the Beetles of Europe website, and also in an Australasian paper¹ all show the lobes of ferus being much narrower whereas on rustica they are much more broadly rounded. If this is reliable then it further points to both of mine being rustica.
• According to Duff the elytra have a small tooth at the apical angle in rustica but not in ferus. I looked hard and could not see any such tooth on my specimen so I deduced that this supported my ID as ferus. However the specimen that I examined more recently also lacks a tooth at the apices of the elytra - I have checked this from every conceivable angle and simply cannot see any sign of a tooth. Hackston does not use this character and I conclude that it is not reliable or at least not practically useful. There is a bulge at the apex, a thickenening, but it does not protrude beyond the edge of the elyton when viewed flat, or even at an angle - you have to look at it from the side to see it.
• Duff says that ferus has a median setal tuft on the labrum, lacking in rusticus. It is not clear precisely where this tuft of setae should be, but my my specimen had a small patch of gold setae in the middle of the front edge of the labrum and I guessed that this was what was being referred to here. The specimen I examined more recently has similar setae, less gold-coloured and more spread out along the front edge of the labrum. Due to the way they spread along the front edge it would be harder to reconcile this with a description of a median setal tuft. I cannot find a photo of ferus that shows the labrum clearly enough to see what the median setal tuft is meant to look like or precicely where it should be positioned - if it should be right in the middle, i.e. away from the front edge, then neither of my specimens had it, so supporting my revised ID of rustica.
• Duff says that the eyes of ferus should be glabrous between the facets, whereas rusticus has fine hairs between the facets. This caused me to question whether mine really was ferus or not as it clearly had a few hairs between some of the facets. But Duff illustrates this and shows rusticus as having far more hairs than my individual. My specimen only had a few hairs, restricted to the rear half of the eye. It wasn't glabrous but Hackston doesn't describe the eyes of ferus as glabrous and allows for "a few isolated bristles between the facets" in ferus - and that unfortunately allayed my concerns that the presence of a few eye hairs ruled out ferus. Now the next specimen I examined is a little hairier than the other, and thus a better match for the diagram in Duff for rustica, but it's not vastly different from the one I'd identified as ferus and as on that one the hairs are restricted (or nearly so) to the rear half of the eye. The Beetles of Europe website has photos of the eyes of both species and no hairs are visible at all on ferus whereas the photo of rustica is much closer to both of my specimens. This seems to remove any lingering doubt that my specimen was not ferus after all.

¹Wang, Q & Leschen, R A B, 2003. Identification and distribution of Arhopalus species (Coleoptera: Cerambycidae: Aseminae) in Australia and New Zealand. New Zealand Entomologist 26: 53-59

Dusky Longhorn Beetle Arhopalus rusticus showing hind tarsomere 3 (2 views), elytral apical angles, labrum and eye (2 views), Sporle Wood (Norfolk, UK), 27th July 2023

This was the one I examined more recently that helped resolve the ID of the one above.

Dusky Longhorn Beetle Arhopalus rusticus showing hind tarsomere 3 (4 views including both the tarsomere from both legs), elytral apices and labrum, North Elmham (Norfolk, UK), 11th July 2022

I don't recall having particular challenges with these ones which I'm pretty sure would have been identified using Hackston.

Dusky Longhorn Beetle Arhopalus rusticus, Holt Country Park (Norfolk, UK), 1st July 2017

Dusky Longhorn Beetle Arhopalus rusticus, Swanton Great Wood (Norfolk, UK), 27th August 2019