What's in the box? No.10 - a tricky customer

My epic beetle identification marathon continues... in envelope number 3 (of 20+ containing chrysomelid 'flea beetle' specimens collected in the Uk during 2011), I found the following, about 3mm long and a dark metallic blue colour.

The latest beetle specimen in dorsal view.

One of the first things I noticed was the groove running along the rear edge of the pronotum.

The groove along the rear edge of the pronotum.

Many chrysomelids have a groove in this position, but it varies between genera - some are less even in depth than this one and can have short furrows running forward from the ends. Along with features such as colour and shape, this indicates that the specimen is in the genus Altica. This is always potentially difficult as reliable identification of Altica species requires dissection - in this case of a male in order to see the aedeagus (identification of females is detailed in Kangas & Rutanen, 1993). However, the curved sides make A. ericeti unlikely while the elytra are rounded at the rear but not bulbous/widened, so it is probably not A. brevicollis or A. lythri. Similarly, the lack of small dents at the tips of the elytra suggest it is not A. oleracea. This leaves three British species - A. helianthemi, palustris and carinthiaca. These do differ to some extent in terms of the fineness and depth of punctures, shininess/dullness of the top of the head, prominence of the elytral shoulders and so on, but these features are somewhat variable and comparative and so, dissection of males is, as noted above, the only way to make a definite species-level identification. Pins and forceps deployed, the aedeagus was removed.

Aedeagus (ventral view)

Aedeagus (dorsal view)

Aedeagus (lateral view)

Immediately it is clear that this is not A. helianthemi which has an aedeagus that is more clearly broadened towards the tip and S-shaped in lateral (side) view. However the aedeagi of A. palustris and A. carinthiaca are very similar and care needs to be taken. Both are more-or-less straight in lateral view, end in a small point, have transverse wrinkles and are the same overall shape. Fortunately for identification purposes, there are some small but diagnostic differences allowing them to be separated. Firstly, on the right-hand side in the lateral view above, there is a distinct dent about one third of the way down - something that, although not always this clear, is seen in A. carinthiaca but not in A. palustris. Also, although the ventral views are very similar, the arched structures near the tip in dorsal view match those of A. carinthiaca and this is indeed the species we have here.

This is an interesting find as it was only recognised as a British species in 2000 (Cox, 2000) and is found on meadow vetchling (Lathyrus pratensis) in a variety of habitats, mainly in the south and east of England, though there has been a recent (2011) record from further north in Cheshire. Since 2000, there have been numerous records of this species in the UK and, although the Cheshire record has not yet been included on the database, the distribution is shown by the NBN as follows:

Distribution map of Altica carinthiaca in Britain © National Biodiversity Network

The south-eastern distribution is clear (this specimen, from Oxfordshire, is within the usual British range) and although there is evidence of some expansion of range e.g. in Finland since the 1980s (Kangas & Rutanen, 1993), it is likely that this species has been present but overlooked in Britain for some time. Indeed, the difficulty of identification (given that even the aedeagi can be similar in some cases) has meant that it has been confused with both A. palustris and A. helianthemi, the latter when known as A. pusilla var. montana, with museum specimens at least as early as 1939 being attributable to A. carinthiaca (Cox, 2007).

So, we have a species which, although not a rarity, was overlooked until fairly recently, so records are valuable in order to provide information about its British distribution. It is also a good example of the importance of dissection in the identification of some chrysomelid species, especially the 'flea beetles' as well as showing that even once dissected out, care may be needed to accurately use sometimes subtle and variable features of structures such as the aedeagus. Now, where's that 4th envelope of beetles..?

References

Cox, M.L. (2000). Progress report on the Bruchidae/Chrysomelidae recording scheme. The Coleopterist 9(2): 65-74.

Cox, M.L. (2007). Atlas of the Seed and Leaf Beetles of Britain and Ireland. Pisces, Newbury.

Kangas, E. & Rutanen, I. (1993). Identification of the females of the Finnish species of Altica Müller (Coleoptera, Chrysomelidae). Entomologica Fennica 31(4): 115-129.