Sphaeroderma rubidum

Supposedly the less common of two similar Sphaeroderma species.

I have found Sphaeroderma identification to be quite problematic so it was a relief to catch a male that could be confirmed by referring to its genitalia. I initially identified this as rubidum based on external characters including the shape of the pronotum and the sparse fine puncturing on the pronotum. Both Hackston and Duff include diagrams of the median lobe, with Duff including lateral and side views. The shape clearly matched rubidum better, in both views, but I was slightly bothered by not being able to marry up the detail of the internal structures shown in Duff with what I could see. This concern was alleviated when I found photos in the Käfer Europas (Beetles of Europe) website and the photo of rubidum median lobe was a perfect match for mine.

This individual came to LED lights in a meadow that has lots of Common Knapweed.

male Sphaeroderma rubidum showing puncturing on the pronotum and elyrta and its median lobe (3 views), Wendling Beck Environment Project (Norfolk, UK), 1st September 2003

After finding the above male I reviewed my previous two records of rubidum.

This one was found in ash keys beside a wildflower meadow. It was a female, and I'm not aware of any differences in the spermatheca between the two species of Spaermoderma, so this didn't help confirm the ID, but I initially identified it as rubidum using the Hackston key. It hadn't been a straightforward ID. It's hard to say if it was "yellowish-red" (testaceum) or "bright reddish-brown" (rubidum) but I felt it was probably closer to the former (and certainly closer to the photo of testaceum in the key than the photo of rubidum). But the body seemed very round ("short oval, almost round" for rubidum and "oval" for testaceum) and I couldn't see the side borders of the pronotum from above (favouring rubidum). The pronotum puncturing seemed pretty minimal but without comparative material this was of limited use. On balance I felt that it fitted rubidum best.

Looking at this again today, the first thing I notice from my photos is that the overall colour was clearly different and less brown compared to the male rubidum above. I now have the Duff key which I don't think I had back in 2020 when I first identified it. Duff doesn't mention the visibility of the side borders of the pronotum but describes the shape of the pronotum. I am having some trouble interpreting this - at first I thought it supported my original ID of rubidum as compared to Duff's diagram of testaceum the pronotum is less rectangular, the sides curving in towards the front more. But looking at photos of testaceum online they don't look as rectangular either. The sides are straigher than rubidum at the base but curve in towards the front sooner than shown in Duff's diagram. The straigher sides at the base mean there's a distinct indent between the curve of the sides of the pronotum and the curve of the sides of the elytra, whereas this is more continuous on rubidum. In that respect mine looks more like testaceum, but it's a pretty subtle difference.

Looking at my photos of the puncturing on the pronotum and elytra, the pronotum at least looks like it is more closely and coarsely punctured, if only marginally, compared to my male rubidum above. That would favour testaceum perhaps, but on the other hand the puncturing wasn't obvious enough to be able to make it out on my photos of the whole insect. Admittedly they're not such high quality images as some of the photos of testaceum I'm looking at online, but on those testaceum photos the punctures are pretty easy to see. So I'm not sure, for a number of reasons I think this one might in fact be testaceum, but I'm not convinced either way. Maybe with more experience I will feel confident to call it one way or the other, or perhaps if you're reading this as someone who knows the species better than me you might be able to advise me. But until then I think this is best left undetermined.

female Sphaeroderma sp. showing punctures on pronotum and elytra and spermatheca, North Elmham Cathedral Meadows (Norfolk, UK), 14th August 2020

This was my first Sphaerodroma and after photographing it in the field I attempted to pot it for closer examination. I was unsuccessful as it dropped into the undergrowth, so I only have this (and an almost identical) photo to identify it from. With no previous experience of the genus I initially suspected it was testaceum based on the colour as rubidum is supposed to be a bit browner. I think the pronotum shape fits rubidum better and reidentified it as that following my second Sphaeroderma which I thought at the time was rubidum. But in view of the difficulty I had identifying that one, and the uncertainty I have about it now, I'm not sure it's safe to put a name to this one either. It was on a Burdock leaf which doesn't really help as I don't think either species is normally associated with Burdock (both thistles and knapweeds, the plants associated with the two species, grow in the vicinity).

Sphaeroderma sp., North Elmham chapel (Norfolk, UK), 25th March 2020