Philonthus concinnus

I initially keyed this using Duff and got to couplet 55, a choice between umbratilis and sanguinolentus. At first I tentatively plumped for umbratilis given that it didn't have any red on the elytra and it was small for sanguinolentus, but I wasn't convinced and checked in Lott & Anderson, quickly realising there were other problems with this choice. Most objectively the dorsal puncture row had five punctures (excluding one at the hind margin and one off-line at the front margin). The elytra are just about transverse which is perhaps a bit more subjective (though measurements seem to confirm this). I was less convinced about the relative lengths of the hind tarsomeres. According to Duff there is a form of sanguinolentus (var. unicolor) with entirely black elytra so theoretically that is a possibility but it seemed unlikely.

Using Lott & Anderson it keyed to their couplet 34, a choice between sanguinolentus and rufipes. Lott & Anderson don't allow for the possibility that sanguinolentus can lack red markings so it keys to rufipes. Apart from the fact that rufipes is rather rare in Norfolk, I don't think it can be rufipes - Duff shows rufipes as having much smaller eyes relative to the temples.

So is it sanguinolentus var. unicolor? I suspected not - not only is that presumably an unusual form of a species I haven't found in its usual form but it's also too small. Unfortunately it was a female so its genitalia didn't help to resolve it so in the absence of a satisfactory solution I decided leave it unidentified for the time being.

I'd been back through both keys a couple of times but couldn't see where I might have gone wrong (assuming it was my error and not the keys' errors) so decided to keep hold of it until I've got a bit more Philonthus experience under the belt. That wasn't long - the very next beetle I looked at was a Philonthus and initially gave me similar problems, seeming to key to a dead end at umbratilis/sanguinolentus using Duff. That was a very similar beetle in many respects but had a smaller and perhaps shinier pronotum and paler legs - and it keyed to quisquilius using Lott & Anderson. It turned out that it was quisquilius - the eyes were longer than the temples but apparently I'd misjudged the difference between "clearly longer" and "slightly longer". In view of that I revisited the relative eye/temple length of this one to see if that might be where I went wrong here as well. This time the eye was longer than that of the quisquilius (0.43mm vs. 0.35mm) and the temple shorter (0.25mm vs. 0.32mm), so I think I was right this time to judge it as being "clearly longer". So that didn't really help, but it did make me more confident that I hadn't made a mistake in this part of the key.

The only other place where I hadn't been very confident was my judgement of the head shape at couplet 3 (Duff). I'd judged it to be transverse, or at least close enough to be one of the "doubtful cases" that the key said was treated both ways. What if I'd been wrong about this? In that case it would key to concinnus, and yes, the description matched almost perfectly. My estimate of size was slightly under the limit given in Duff for concinnus, but when the beetle is dried and folded up it's always tricky to judge size accurately so I'm not overly worried about that. Looking at photos of concinnus online some do sseem to have slightly more elongate heads but some look very close to mine. But to be certain I needed to understand why it hadn't keyed to concinnus using Lott & Anderson either - and there it transpires it was down to the elytra colour. According to Lott & Anderson the elytra should have a faint dark green metallic sheen which I had not been able to detect (and still can't). If I took it as having this then it would have keyed straight to concinnus. But does the absence of green mean it can't be concinnus? Well according to Duff no - he describes concinnus elytra as "black to brownish, sometimes with a very faint metallic dark green lustre". So I think my inability to detect green on the elytra does not exclude concinnus, and all things considered this is left as the only feasible identification.

female Philonthus concinnus showing head (2 views), antenna tip, palps (from above, below and side), pronotum from above (4 view), below and side, front tarsus, hind tarsus, elytra (2 views) and basal abdominal tergites (2 views), North Elmham (Norfolk, UK), 2nd August 2024