Ilybius subaeneus

The ventral suckers on the foretarsi make this a male I think. Duff also splits males from females according to the presence (females) or absence (males) of a median notch on the apical sternite. Important to remmeber here (I didn't at first!) that Duff defines a notch as a "deep and narrow incision" so the shallow indent that mine showed doesn't count as a notch.

This showed no sign of the median keel on the apical sternite that was present on the quadriguttatus I had previously examined but it did show much stronger longitudinal furrows at the apex which Hackston's key provides as a character for subaeneus and not quadriguttatus. Hackston says these should reach over half the length of the segment. The central ones didn't quite, but some towards the edges did reach back more than half way. There was also a short transverse furrow in the centre of the basal half of the apical sternite - I'm not sure if quadriguttatus shows that too or not.

The fore tarsal posterior claw lacked a ventral tooth which helps eliminate quadriguttatus. The ventral edge was expanded slightly beyond the middle but I think it would be a stretch to describe this as a tooth (but I was glad to have previously seen and photographed what the tooth looks like on quadriguttatus - shallow but clearly pointed unlike on this one).

Hackston also describes differences in the elytral microsculpture with "more or less polygonal mesh" for subaeneus and "very elongate along the axis of the beetle, at least in the front third" for quadriguttatus. I hadn't found that very helpful with my previous quadriguttatus as it was only clearly elongated near the suture, so I'n not placing too much importance here either - it was clearly elongate in some places and more polygonal in others.

This beetle had a ventroexternal ridge on hind tarsomeres 1-4 which Duff mentions as a supporting character for subaeneus but does not clarify whether or not it may be shared by quadriguttatus. At just over 10.1mm the size also supports the ID as subaeneus with quadriguttatus being slightly larger and aenescens/guttiger being slightly smaller.

male Ilybius subaeneus showing metasternum, apical sternite, front tarsus, front tarsal claw, hind tarsus and elytral microsculpture (3 views - base around suture, a little further down around suture and mid way down to side), Wendling Beck Environment Project (Norfolk, UK), 11th June 2023