Stilbus testaceus

Although this is possibly the commonest member of the family, I had come across numerous Olibrus (involving 3 different species) before encountering this genus. The single stria adjacent to the suture and the non-nipple-shaped final segment of the antennae eliminated Olibrus and the pale appendages was probably the most obvious pointer to genus Stilbus (as opposed to Phalacrus).

Species-level identification wasn't easy due to the small size of the beetle and the need for top-lit high-powered magnification. I couldn't make out the apical setae of the prosternal process initially but found that by breaking the specimen in half (between the fore and mid legs) and viewing the front half from the rear, the six setae were clearly visible (ruling out oblongus which only has 4-5 smaller setae). My dissecting microscope wasn't powerful enough to see the elytral microsculpture clearly and the compound microscope is only lit from below, but by removing an elytra an examining this alone there was enough light to just make out the wavy-lined miscrosculpture (more reticulate in both oblongus and atomarius). Just to nail it completely I checked the aedeagus which confirmed things nicely (with its broad rounded tip with nipple-like central protrusion).

This species is supposed to live in grass and hay but this one was found on Spindle (next to a grassy field).

male Stilbus testaceus showing its prosternal setae, elytral microsculpture and aedeagus (flat and side), Wendling Beck Environment Project (Norfolk, UK), 24th September 2021

By the time I examined the next three (in fact two predated the last one, but I didn't examine them until later) I had acquired a better microscope and seeing the prosternal setae and microsculpture was easier. Seeing enough detail in the microsculpture was still tricky but I was pretty confident without dissection. Just to make sure I did check genitalia anyway. These three were all taken in my garden moth trap.

male Stilbus testaceus showing its prosternal setae, elytral microsculpture and aedeagus, North Elmham (Norfolk, UK), 7th August 2020

The female couldn't be confirmed genitalically but I was happy everything else stacked up. Here I've included photos showing the single sutural stria and unconstricted apical antennal segment which rule out genus Olibrus, and the size of the scutellum (which is supposed to help rule out genus Phalacrus, though without comparative material it doesn't really).

female Stilbus testaceus showing its prosternal setae, elytral microsculpture and aedeagus, North Elmham (Norfolk, UK), 7th August 2020

This one showed a darker prosternum which contrasted more with the pale legs, which could have suggested Stilbus atomarius. However I was pretty happy that the elytral microsculpture was linear and the shape of the aedeagus tip clinched it (atomarius also has a central nipple but lacks the broad shoulders, tapering more towards the nipple).

male Stilbus testaceus showing its prosternal setae, elytral microsculpture and aedeagus, North Elmham (Norfolk, UK), 11th July 2022