What's in the box? No.6 - taxonomic confusion and big shoulders.

OK, after my brief foray into palaeontology, it's time to get back to what I arguably know a bit more about - tiny invertebrates. You may have seen previous posts like this one and this one about the specimens that land on my doormat having been sent to me for identification as the organiser of the UK Chrysomelid Recording Scheme. I usually know that something is on its way as I ask for a little warning from contributors to make sure I don't get sack-loads of beetles when I'm busy, on holiday etc, especially as the organiser role is voluntary... This case was no exception as the collector had been in touch to ask about a specimen that appeared to be Aphthona ?atratula. Now, the ? in the name isn't a typo - it's there to indicate that there is some taxonomic confusion surrounding this species in Britain. Basically, A. atratula was referred to as A. atrovirens Foerster, 1849 by Pope (1977) but this species is not found in Britain (Cox, 2000). This means that British specimens of 'A. atrovirens' are probably A. atratula Allard, 1859. So, that's actually quite straightforward isn't it... well, actually, no. The problem is that various authors in continental Europe (e.g. Konstantinov, 1998) consider A. atratula to be the same as A. euphorbiae (Schrank, 1781), the Large Flax Flea Beetle. However, there are some quite convincing differences between British specimens of A. atratula and A. euphorbiae suggesting that they are actually separate - including the spread of A. euphorbiae with cultivated flax which is not seen in A. atratula (Cox, 2007). I happen to agree that they are separate species but officially this is not certain and so the ? remains. In any case, that's enough of the finer points of Chrysomelid taxonomy for now; let's have a look at the specimen itself.

Dorsal view of the Aphthona specimen, length 2mm excluding appendages. This is how it arrived in the post - very neatly (unlike my own card-mounting), glued onto a small card in a protective tube - thanks to Jon Cole for the good quality specimen!

First of all, it is an Aphthona - although I didn't get a clear photo of it, there is a spur on the outer side of the lower edge of the hind tibia (honest) - if it was in the middle of the lower edge, it would be in the genus Phyllotreta - see why the 'flea beetles' are considered so tricky to identify! I'm not going to go through the whole process of keying out Aphthona specimens here, but let's look at some of the final features required to make a species-level identification.

A close-up of the front half in dorsal view.

Looking at the pronotum, you can see some small 'punctures' - these are actually moderately coarse as finer ones would look more-or-less like a matt surface. So, it can't be A. melancholica which has very fine punctures - it is, as expected, either A. atratula or A. euphorbiae (remembering that Konstantinov, 1998 and several other important authors would stop at this point, considering the species to be the same). Both can be shiny black with a bluish reflection as seen here, however do have a look at the 'shoulders' of the elytra (wing cases). Here, they are well developed and are somewhat bulbous - this is characteristic of A. euphorbiae as is the size with A. atratula reaching no more than 1.7mm and having weakly developed shoulders. This is a fairly good indication that the specimen is actually of the common and widespread A. euphorbiae, but isn't strictly conclusive - to be really certain, dissection of the male genitalia is required (as is so often the case with small chrysomelids), and handily, this is a male.

Ventral view of the aedeagus.

Dorsal view of the aedeagus.

Side view of the aedeagus.

These images can be compared with those here at the excellent 'European Chrysomelidae' website. They clearly belong to A. euphorbiae - the dorsal view for example bears the same small polygonal structure at the base of the indent at the tip and the side view shows the same curvature. A. atrovirens (remember the taxonomic confusion...) however does not show the same small polygonal structure and the curvature differs slightly in that the inside edge of the curve straightens towards the tip.

So, we have a common species, but one that provides an opportunity to work through the taxonomy of Aphthona. Though often associated with cultivated flax/linseed (Linum usitatissimum) which they may kill by feeding on seedlings and cotyledons (Cox, 2007), it is actually found on many plant species in a wide range of habitats - as in this case where the beetle was collected (by beating in late October) from a Field Maple (Acer campestre) by a lake. Now, a box of about 20 little packets of beetles has arrived so I suspect this series will be updated soon...

References

Cox, M.L. (2000). Progress report on the Bruchidae/Chrysomelidae Recording Scheme. The Coleopterist 9: 65-74.
Cox, M.L. (2007). Atlas of the Seed and Leaf Beetles of Britain and Ireland. Pisces, Newbury.
Konstantinov, A.S. (1998). Revision of the Palaearctic Species of Aphthona Chevrolat and Cladistic Classification of the Aphthonini (Coleoptera: Chrysomelidae: Alticinae). Associated Publishers, Gainesville, FL.