Philonthus quisquiliarius

I initially used Hackston to key these but could only get as far as Philonthus sp. Then I obtained the Lott & Anderson and finished the job. It wasn't an easy ID - I didn't find it easy to determine whether tarsal and antennal segments were transverse or elongate - the difference didn't seem to be vast. Perhaps a bigger problem was at the couplet where I had to choose between black femora (corvinus) or pale brown femora (quisquiliarius and ventralis). They weren't black, though the front femora weren't very far off, but pale brown? I wouldn't come close to describing them as pale. Then deciding how faint the microsculpure was proved tricky without any reference material. But going through the descriptions of all three species and comparing images in the book and online I'm fairly confident they were quisquiliarius - and this is much the commoner of the three species and in the right habitat (well, just up from the right habitat - it came to light in a grassy field just above a wet meadow, along with heaps of other insects that must have come up from the wet meadow like water boatmen, caddisflies and mayflies). Both were female so I wasn't able to compare aedeagi with the diagrams in the book.

female Philonthus quisquiliarius showing front and hind tarsi, pronotal punctures and microsculpture, Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

second female Philonthus quisquiliarius showing front and hind tarsi, pronotal punctures and head microsculpture, Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

This one turned up in my moth trap at home. I keyed it to quisquiliarius before dissecting it to check the genitalia. Fortunately this time it was a male and so I was able to confirm the ID, and in doing so increased my confidence that I was keying the last two correctly. I think I must have damaged this specimen when re-potting it after its photoshoot as the specimen had its head damaged and twisted half-off the pronotum, but fortunately this damage didn't mask any of the features needed when keying it.

male Philonthus quisquiliarius showing head (2 views), palps, antenna, pronotum (5 views, the last two showing the side border from the side and from above), hind tarsus and aedeagus, North Elmham (Norfolk, UK), 24th June 2023

The next one I examined was after I'd acquired Duff's new volume (I'd trapped it before it was published but saved it). My first attempt led to a dead-end at couplet 55 (umbratilis and sanguinolentus), a familiar problem as I'd reached exactly the same place with a different beetle immediately before I examined this one. Compared to that beetle this was similar in many respects but not all - it had a smaller shinier pronotum and paler legs for starters. So I tried with Lott & Anderson and this time it keyed differently from the previous beetle I'd been looking at, going to couplet 24, a choice between quisquilliarius and ventralis. Here it was problematic as antennal segments 9 and 10 were just about transverse (or at least quadrate - not elongate) but the microsculpture on the head and pronotum were very far from "well impressed". I figured quisquiliarius was the better fit here.

At this point I checked back at this page and noted many similarities between my insect and the ones shown above, so I returned to Duff to see where a path to quisquiliarius diverged from the path I ended up on. It turned out to be couplet 48 where I had judged the eyes to be clearly longer than the temples. Well they were longer, but how much longer is clearly longer? I measured them - the right eye is 0.35mm long and the right temple is 0.32mm - I suppose that's close enough that it could be reasonably described as only slightly longer? Pursuing that route the next couplet was difficult - I couldn't see any linear microscuplture on the shiny pronotum at 63x magnificiation, but it was much too small for the unlikely rotundicollis. Eventually after looking at many angles I could just about make out linear microsculpture near the middle of the pronotum (but not near the sides) - and I note that it was hardly obvious in the individuals I'd seen previously (above). Finally the shorter hind tarsomere 1 got me to quisquiliarius, a far more satisfactory result than anything I'd come across along the way.

male Philonthus quisquiliarius showing head (front and side views), palps (2 views), antenna tip, pronotum (5 views, the last from below), fore and mid femora, fore tarsus, hind tarsus, elytra (with close up of tip) and base of tergites, North Elmham (Norfolk, UK), 12th August 2024