Small Spruce Bark Beetle Polygraphus poligraphus

This beetle was only added to the Norfolk list in 2020 when one was taken at Thursford Wood. I don't know if there have been any other records since or if mine below is the second. It doesn't seem to be very common anywhere - the NBN Atlas only has 23 records as at December 2023.

This was taken at LED lights in the middle of a meadow, but there are some spruce trees fairly nearby. It keyed to Polygraphus fairly easily but telling it apart from Polygraphus grandiclava (which I don't think is known in Norfolk) was more problematic. The shape of the antennal club matched the diagram for poligraphus in Duff but my first attempts at measuring it put it at considerably longer than the scape (eventually, after removing one antenna, clearing it and ensuring it was flat, I managed to get a more accurate measurement which only put it slightly longer than the scape). At first I couldn't see as many as 5 segments in the funiculus (poligraphus has 5, grandiclava has 6) even at 63x magnification, so this favoured poligraphus but as I clearly couldn't make them all out I couldn't be sure. The pronotum had a median keel - the keel itself wasn't obvious but the change of direction in the surface either side of the keel was clear enough - so was this "obsolete" (poligraphus) or distinct (grandiclava)? Obsolete felt closer, but I wasn't sure. I couldn't make out any clear dark marks in the elytral scales at 63x magnification, so this favoured poligraphus too.

Eventually I resorted to removing one antenna, clearing it in potassium hydroxide and examining it in alcohol. This enabled me to get a clearer look, satisfying myself that the shape of the club and its size relative to the scape were ok for poligraphus. It also helped me get closer to counting the funicular segments, although this is still a little confusing. Between the scape and the club it looks like it has one relatively large round basal segment and three tiny segments, however on one side the middle of these "three" tiny segments seems to be divided as if it might actually be two segments - at some angles it looked like two. The only way I can get it to having 5 funicular segments altogether is to assume that this divided segment is in fact two segments, though these appear joined on one side so that from that side it looks like a single segment. Anyway, there's no way at all I can make it look like it has 6 funicalar segments, so I think that's enough to rule out grandiclava.

Small Spruce Bark Beetle Polygraphus poligraphus showing tarsi, foretibia, eye, elytal scales (4 views) and antenna (5 views), Wendling Beck Environment Project (Norfolk, UK), 9th October 2023