Longitarsus atricillus

This small Longitarsus came to light and proved to be a tricky one to identify. My first attempt reached quite a rare species (Longitarsus kutscherae) but my specimen didn't really stack up against the description or images online. There had been one couplet where I wasn't quite sure which way to go and taking the alternative route I got to a much more likely Longitarsus gracilis. But although that's a variable species the description didn't really add up here either and none of the photos I could find were a good match. So I checked its genitalia. The aedeagus seemed to pretty firmly rule out both candidates but instead seemed to be a really good match for atricillus. Checking back at the key it goes there perfectly so long as I take the pronotum to have a metallic lustre. The key does say this may be difficult to see but boy, it really was. However, it did have coppery reflections on the pronotum when held at certain angles to the light, so I assume this is enough to call it a metallic lustre. This time the description (and online photos) of atricillus seem to match the specimen so I'm as confident as I can be that this is the correct ID.

male Longitarsus atricillus showing close-ups of elytra and pronotum, aedeagus (flat and side view) and hind tibia and tarsus, Hoe Rough (Norfolk, UK), 18th March 2023

These 3 proved a challenge to key as well. My first attempt using Duff went wrong when I looked at the hind tibial spine. I didn't think it looked as much as 0.2 x the first tarsal segment but when I measured it it seemed to be (if only just). That led me to lycopi, an unlikely species and one that Duff descibes as "probably usually fully-winged" - as none of my 3 specimens were fully-winged that added to the doubt. Ignoring the tibial spine the couplets became tricky - I had the same problem as before deciding if the pronotum was metallic. Rather than spend hours going through all the options I decided to go straight to the genitalia. The first one's aedeagus looked familiar, and seemed a good match for atricillus, and it was at this point I turned to look at the above notes on my first. Everything seemed to match that one - it was clearly the same species. The second male was the same and the third was a female. Externally this was very similar and the spermatheca seemed to match up with Duff's diagram for atricillus well. The nodulus was thinner (the outer section being more concave) but otherwise it was spot on (and nothing else in Duff's diagrams had a similar nodulus). The two males were a fraction under 2mm long - I didn't measure the female but it was about the same.

male Longitarsus atricillus showing hind tarsus and aedeagus (side and flat), Hoe Rough (Norfolk, UK), 23rd August 2023

second male Longitarsus atricillus showing aedeagus (side and flat), Hoe Rough (Norfolk, UK), 23rd August 2023

female Longitarsus atricillus showing spermatheca, Hoe Rough (Norfolk, UK), 23rd August 2023

My next one, again from the same site, was easier after the experience of the previous ones. Another female, but this time the nodulus of the spermatheca was as thick as shown in Duff. I think I've got the hang of what the metallic sheen on the pronotum is meant to look like now!

female Longitarsus atricillus showing hind tibia & tarsus, pronotum and spermatheca, Hoe Rough (Norfolk, UK), 26th September 2023