What's in the box? No.11 - get into the groove

A few weeks ago I wrote about the use of pronotal grooves as identifying characters in some leaf beetles (Chrysomelidae). Continuing through the box of specimens I was ent for identification (I'm now about half way), I found this one, about 2mm long without appendages.

Another small brownish leaf beetle!

Even at this scale, it is easy to see that the end of the hind tibia has a wide curved dent on the outer edge. In close-up, the dent is quite obvious and a fringe of hairs on the upper edge can be seen.

Tibial dent with fringing hairs

This dent means that the beetle is in the genus Chaetocnema - it looks like the fringed dent could be used for cleaning another appendage but I must confess I don't know what its function is and I can't find it mentioned in any of the literature I have or via a fairly quick web-search; if anyone knows, please do post a comment here! Anyhow, moving onto species-level identification, the head needs to be looked at closely to see if there is a keel running across between the antennae. In this case there isn't, but there are several punctures above each eye which I've tried to photograph - it was tricky and the clearest image is here with a puncture indicated by the arrow.

Head of Chaetocnema showing one of the punctures above the eye.

These punctures are helpful as they mean it can only be C. concinna or C. picipes. There has been taxonomic confusion between these species in the past (Booth & Owen, 1997; Cox, 2007), but even without dissection there is a subtle difference in the antennae. In C. concinna, the last antennal segment is clearly asymmetrical, while in C. picipes it is more or less symmetrical (and relatively narrow). In this case, the segment is narrow and symmetrical and the specimen is C. picipes, a beetle with a scattered distribution in Britain, found on Polygonaceae and oraches (Atriplex) as an adult, although the larval feeding behaviour is unknown (a project for someone).

Antenna of C. picipes with the diagnostic last segment arrowed.

References

Booth, R.G. & Owen, J.A. (1997). Chaetocnema picipes Stephens (Chrysomelidae: Alticinae) in Britain. The Coleopterist 6: 85-89.
Cox, M.L. (2007). Atlas of Seed and Leaf Beetles of Britain and Ireland. Pisces, Newbury.