Cassida prasina

I found this really challenging to identify. There were a number of couplets in Duff which I struggled to interpret, though in the end I think I worked out what they all meant. Hackston was easier to follow, but failed to take account of prasina sometimes lacking the red marks by the scutellum as this one did, and consequently hit a dead end.

I was reasonably confident which way to go at Duff's first Cassida couplet, though the tiniest of swellings at the inner base of the claws made me wonder if that counted as appendiculate (given that Duff says that may be hard to see). The other characters that went with appendiculate claws didn't seem to match, and just in case I followed that lead and easily ruled out both options there. Couplet 2 was easy - the broad explanate elytral side margins are very obvious.

I struggled to understand couplet 3 at first, despite the photos illustrating the differences. Part of the problem is that with mentions of rows of punctures that didn't seem to refer to the striae bounding each elytral interval, I wasn't clear which spaces represented which elytral interval. The description of a number of rows of punctures between intervals 3 and 5 (where I would expect two of the rows to be described as striae) confused me further. Eventually I worked it out, though I'm still not entirely clear why it's described how it is. Anyway, there were a few smaller punctures in a row between striae which I think is what it was referring to. Just in case I tried the other path but was unable to reach a satisfactory conclusion there.

Couplet 4 was easy - the femora were green not black. Though this eliminated the commonest species, rubiginosa, photographs of which seemed to be a pretty good match for my beetle. I spent a while wondering if the femora colour was a reliable way of ruling this out, but then realised the size was wrong too (rubiginosa 6.0 to 8.0 mm - mine was 4.9 mm.

Couplet 6 was tricky. Firstly the outer part of the pronotum was nestled beneath the elytra and it took some effort to get it to stay out long enough to study it in detail. There was a small lobe on the base of the pronotum - was this a "weak angle" or a "distinct tooth"? I felt it wasn't toothy enough to call it a tooth so presumed the former, so probably not denticollis on that basis. But this couplet also gave me a choice between the basal elytral margin being "not or only very finely black and +- smooth or at most obscurely crenulate" or (for denticollis) "distinctly black... and clearly denticulate with a series of small black, blunt teeth." Well it was clearly (but also narrowly) black, and it had a series of small black projections. I would have described these as crenulations rather than teeth, but they were clear, not (I would say) obscurely crenulate. Moreover a high quality image I found online showing (or at least purporting to show) denticollis had "teeth" that I would say weren't much more toothy vs. crenulate than mine. So this wasn't altogether clear, but the description of denticollis didn't match my specimen, plus it's a rare species that I don't think occurs in or anywhere near Norfolk.

Finally the last couplet was relatively straightforward, although I had to view the beetle at the right angle to see quite how costate the intervals were and this took a little while to twig.

So it seems that it is prasina, despite the lack of reddish spots at the elytral base (which Duff only describes as "usually" present). Note that it did look like it had a pair of reddish spots just in from the elytral base roughly half way between the scutellum and the sides, and these can be seen in the photo of the live insect below. However under the microscope the colour here appeared to be coming through the elytra from the point where the wings attach to the thorax - this is the corner of the thorax that comes into closest contact with the underneath of the elytra so shows through more clearly here. This was confirmed by lifting the elytra.

Duff illustrates some of the Cassida median lobes, but not prasina. Hackston does inclue diagrams of prasina's median lobe. I think this specimen matched Hackston's diagrams of prasina better than any of the species illustrated in Duff.

male Cassida prasina showing underside, various views of elytra and pronotum base, mesepisterna/mesepimera, hind claws and aedeagus median lobe (side and tip flat), Hoe Rough (Norfolk, UK), 23rd July 2023