Tachyporus pusillus

When I first examined this using Hackston I tentatively identified it as pusillus, but was hesitant as I couldn't make out the microsculpture. Now armed with Duff I'm not so concerned about that. I can't see all of the elytral setae, but I can see a pore where the humeral seta would have been on at least one of the elytra. The measurements and other characters all seem to check out ok for pusillus (at the bottom of the size range).

female Tachyporus pusillus showing maxillary palps and elytra, North Elmham (Norfolk, UK), 2nd May 2021

I normally struggle to see all the elytral setae but on this one I could see them easily - but I still struggled to identify it! It had an outer discal seta on at least one elytron (I wasn't convinced it was on both, but clear on the one), but the couplet in Duff where this comes into play also requires (for the same option) the elytra to be less than 0.85mm long - and these were 0.96mm long, so well over. The elytral setae arrangement and other characters all appeared to be spot on for pusillus but the elytra were so long I couldn't believe it at first. If I went with the larger elytra option and ignored the outer discal seta then I could get it to hypnorum but not without difficulty. The penultimate maxillary palp segment was yellow, but distinctly darker than the apical segment, and the lateral setae didn't seem long and obvious enough. There were other additional setae that weren't present in Duff's diagram for hypnorum (although one of these was in Hackson's diagram for hypnorum).

If it was pusillus it was a relatively pale example and I wasn't convinced either way until I looked at the next beetle. That was a hypnorum and laying the pair of elytra side by side with this one's the difference was stark. Not only were the hypnorum significantly larger (and with a larger darker pattern), the setae were very different. Most obviously the lateral setae were not only longer but also clearly thicker.

The humeral setae were clearly present and under high magnification under the compound microscope some microsculpture was visible (similar to the hypnorum examined with this one, but helping to eliminate tersus).

female Tachyporus pusillus showing maxillary palps (2 views), abdominal tergites and elytra, North Elmham (Norfolk, UK), 3rd March 2022

Here are the elytra on the left compared next to the Tachyporus hypnorum on the right.

elytra of female Tachyporus pusillus, North Elmham (Norfolk, UK), 3rd March 2022 (left) and female Tachyporus hypnorum, North Elmham Cathedral Meadows, 12th April 2022

This appeared to be a very straightforward pusillus except that it too had elytra that were 0.96mm long. Both this and the previous one had the total fore body length (head to elytra) comfortably within range for pusillus, so how come the elytra are coming in so long compared to the key? Well, by the time I got to the next two beetles I examined (both Tachyporus tersus) I figured out where I'm going wrong. I've been measuring the elytra after removing them so I've measured the whole elytra (minus the tab that it is attached to the body by). Prior to dissecting the insect the elytra are partly covered by the pronotum (at least they are in the two tersus I examined next). I bet Duff's measurements are the visible length of the elytra while still in situ - that would make sense of course! For both of the tersus the visible lengths of the elytra prior to dissection were 0.15mm less than the length measured the way I've been measuring these. So on that basis where I measured 0.96mm the "correct" measurement should have been 0.81mm, and in that case both would have keyed to pusillus without any issue! Problem solved!

female Tachyporus pusillus showing maxillary palps, abdominal tergites and elytra, North Elmham (Norfolk, UK), 20th February 2023