Orthoperus corticalis

I had a few challenges reaching an ID for this one. It came to a vane trap in my garden the day after I'd found my first Orthoperus, a nigrescens, elsewhere. But although I caught them at around the same time, I examined and identified them months apart. My first challenge with this one was satisfying myself that it was Orthoperus. I'm often a bit unclear when a key choice depends on whether or not the head is hidden under the pronotum because the head moves! In this case the head was always visible when the beetle was alive, and indeed often protruded well forwards of the front of the pronotum. But in death it retracted in and, looking directly from above it was barely visible. As it was so clearly visible in life I figured that would be the correct way to key it but checked both options just in case. I couldn't get anywhere with the "hidden" option, but I also had a bit of a problem with the "visible" option in that the foremargin of the pronotum did not seem to me to be as deeply emarginate as it should be for Orthoperus. The other characters (transverse scutellum and 9-segmented antennae at least) clearly pointed to Orthoperus though, and looking from in front the pronotum wasn't far off the diagram in Duff.

When it came to the species ID I looked carefully for the setiferous papillae on the elytral microsculpture and couldn't see it, but the key says it needs 100x magnification. My dissecting microscope lit from above only goes up to 63x and my compound microscope is lit from below so not much good for this sort of thing. Eventually I managed to rig up a bright light by the microscope and view the top of the elytra, but at high magnification and light from the side, only a tiny bit of the elytra was visibile and in focus at any one point. But I had a pretty good look around the whole elytra and couldn't make out anything that looked like the setiferous papillae illustrated in Duff. I could see the setae, but the microsculpture itself seemed to be entirely reticulate. If it hadn't been then it would have keyed to nigrescens which I think is the commonest Orthoperus, so I needed to be sure. I also had a problem seeing the row of stronger punctures at the base of the pronotum which corticalis has but atomus lacks. The appendage colour (particularly the dark antennal club) favoured corticalis over atomus and I don't think the latter has been recorded in Norfolk, but not being able to see the punctures bothered me.

In the hope of resolving this I checked the genitalia only to find the spermatheca was oval-rounded not at all like any Orthoperus illustrated in Duff or, after a bit more research, any other member of the family so far as I could tell. In the end I concluded that my beetle's spermatheca was either deformed, not fully formed, or just aberrant - or possibly broken by me during the prep but I couldn't see how unless it was right at the base of the abdomen when I removed it. I didn't manage to photograph it but it basically looked pretty much like the rounded bit at the left hand end of each of the Orthoperus spermathecae diagrams in Duff. By this point I was wondering if I would be able to put a name to this beetle, but then I looked up photos of nigrescens and corticalis. I was able to find high quality images of both of these species (here and here) and the difference in microsculpture was pretty obvious.

When I started to key it I thought I remembered my previous Orthoperus nigrescens but must have looked in the wrong place to check my notes for that one as I couldn't find them. In the end I finally found those notes and reminded myself that with that one I had eventually managed to see the elytral microsculpture best when I removed an elytron and examined it with the light below (and from my photo the setiferous papillae were pretty clear). I had tried that already and not been able to make out any microsculpture at all but in the light of my past experience tried again. This time I discovered that I needed to adjust the contrast slide and then it became clear. At last I had a good view of the microsculpture covering more than just a few cells at a time, and could be 100% sure it was not nigrescens. In reviewing my photos I discovered that although I hadn't been able to make them out clearly looking down the microscope, I had nevertheless captured the larger punctures at the base of the pronotum.

female Orthoperus corticalis showing showing pronotum from above and front and elytral microsculpture (multiple views), North Elmham (Norfolk, UK), 2nd May 2024