|The first specimen, a shiny blue chrysomelid beetle about 4mm long.|
One very clear feature here is the pronotum being considerably narrower than the elytra. The length of the pronotum is about 0.4 that of the elytra and the elytra are about 1.6-1.7 times as long as they are wide. The overall form is distinctive and indicates it is a member of the subfamily Criocerinae.
|A close-up of the pronotum with key features indicated.|
O. obscura (often known as O. gallaeciana in continental Europe) is generally considered to be a fairly short species with a pronotum 0.3 times as long as the elytra and with elytra 1.25 times as long as wide. So, using these features, it would appear to be one of the other two, more elongate, species. However, there is a problem here - O. septentrionis is found in Ireland, but not on the British mainland while O. erichsoni has, since the early 20th century been found only in Somerset (a county in the west of England). This specimen was collected in Essex (a county in the east of England) which means one or more of a few things - either it is one of the above two species well away from its usual area (this is unlikely), it is a species new to Britain (very unlikely), the key is not correct (possible - I'm using my own key which is not yet complete) or it is an unusual specimen of O. obscura (also possible).
So, I decided to start with the specimen and looked for some other images which might show some variation in length:width ratio. This turned out to be quite simple - some pages such as this one showed specimens of similar proportions, while Google Images provided many which were more elongate still (though of course these may not all be accurate identifications). So, I am certain that this is indeed O. obscura, though I will add a note to my key about the variation in proportions.
The second specimen is smaller (about 2.3mm without appendages) and less brightly coloured and is in the genus Aphthona - the leg shown below is twisted but the tiny tibial spur is on the outer side of the lower edge (if in the middle of the lower edge, it would be in the genus Phyllotreta).
|The Aphthona specimen - a male with the aedeagus partly protruding.|
In Britain, only three Aphthona species are this colour rather than being black/blackish - lutescens, pallida and nigriceps. Some fine details of the head, including its colour (not black - actually paler than shown here) strongly suggest that this is A. lutescens, however this species generally has a partly darkened elytral suture (i.e. where the wing cases meet) as shown here. This does not appear to be the case here, but a very close examination showed that the suture did have a hair-thin dark line and that the darker areas such as the femora and apical half of the antennae were starting to darken. Therefore, this appears to be a teneral specimen of A. lutescens i.e. one that has only recently emerged as an adult and has yet to fully develop its coloration. Teneral specimens can often prove problematic as normally distinctive features may be missing. However, I wanted to check to be absolutely certain, so I looked at the aedeagus which was helpfully protruding.
|The aedeagus of the probable A. lutescens.|
Warchalowski, A. (2003). Chrysomelidae. The Leaf Beetles of Europe and the Mediterranean Area. Natura Optima Dux, Warsaw.