Leiopus nebulosus

Leiopus nebulosus sensu lato used to be known as Black-clouded Longhorn Beetle, but since it has now been split into two species (Leiopus nebulosus sensu stricto and Leiopus linnei) I'm not sure what happens with the vernacular names.

I encountered a few difficulties working out if this one was nebulosus or linnei using Duff. The only difference in pattern described is in the pronotum where it's supposed to be uniformly covered in grey pubescence, without ochreous spots, in nebulosus, and "covered with a mixture of brownish and ochreous pubescence forming a pattern of spots or a weakly defined median band" in linnei. Mine doesn't look uniform, and I would say the pale pubescence was buff not grey, but there aren't clear spots like there are on many photos of linnei, and on close inspection under the microscope the colour is uniform - the apparent variation being down to variation in hair density and thickness (so with more or less of the black base showing through) not hair colour. Duff describes differences in the width of the frons measured between the lowest points of the eyes. The eyes aren't perfectly round so near the bottom of the eyes there is a bit of a corner. I'm guessing Duff is measuring the frons between these corners? Otherwise if you measure from the very bottom of the eyes your practically going round the sides of the head and I'm not sure that's even the frons still. I'm not sure why he doesn't measure where the eyes are closest as that seems less ambiguous to me, but maybe the difference between species isn't as strong there? Anyway, taking the bottom corners of the eyes the distance is 1.04 mm, so comfortably within the range for nebulosus and marginally outside the range for linnei - but if you measure the distance between the very bottoms of the eyes it would be well outside the range for nebulosus and into clear linnei territory if not beyond. Without comparative material I couldn't do anything useful with the relative curvature of the frons which Duff refers to.

Tergite 8 is helpfully referred to by Duff as a genital sclerite - which gave me confidence that I hadn't miscounted tergites, given its similarity to the apical external tergite which I think is tergite 6. It seems to match nebulosus best. It was 0.84 mm long - supposed to be c. 0.8 mm for nebulosus and about 1.0 mm for linnei. The length of the hairs seemed to fit nebulosus better - they're longer on linnei. My only hesitation here was that it didn't seem to be appreciably more glabrous in the centre as in the diagram in Duff for nebulosus.

Looking at the genitalia the problems continued. Firstly there are supposed to be differences in the shape of the parameres. My specimen has asymmetrically rounded tips to the parameres and is fairly well swollen at the base, which should point to linnei. But the shape doesn't quite match the diagram in Duff. Thanks to Mike Hackston's key I found the paper where linnei is first described*. Looking at the photos of parameres in there I'm not sure I can make sense of the described differences, so whilst I feel this looks better for linnei to the extent that I understand them, it's not at all clear enough to me, not sufficient to over-ride other characters pointing to nebulosus. The median lobe will clinch it...

The shape of the apex of the median lobe is different between the two species - rounded in nebulosus and pointed in linnei - this should be straightforward... But the median lobe comprises two parts, one in front of the other, and guess what - the front one in my specimen was pointed, exactly the shape shown in Duff for linnei and the rear one was rounded, exactly the shape shown in Duff for nebulosus. The back one was slightly longer - but only marginally so - so I figured that was probably the one to be looking at, but I wasn't confident. Fortunately the paper describing linnei has photos that show both parts, and indeed it is just the back part I needed to be looking at. So after a rather tortuous journey, and with some remaining unanswered questions, I am reasonably confident this one is nebulosus. Subsequent experience with Leiopus linnei has increased my confidence, the difference in pronotum (pattern and punctures) and frons width being evident.

* Wallin et al. (2009). Two sibling species of Leiopus Audinet-Serville, 1835 (Coleoptera: Cerambycidae) from Europe: L. nebulosus (Linnaeus, 1758) and L. linnei sp. nov. Zootaxa 2010: 31– 45. Accessed here.

male Leiopus nebulosus showing close-ups of pronotum, frons (from front and side), tergite 8, parameres and various views of median lobe (lastly with parameres), North Elmham (Norfolk, UK), 2nd August 2022