Heterocerus fenestratus

The Hackston key and the Duff key use different sets of characteristics to narrow down the Heterocerus species but I didn't find either straightforward. Using Duff, the mesotsternal fore margin was hard to see - and the tubercles on it even harder. This seemed to point to fenestratus/fusculus rather than marginatus but I wasn't completely certain. The second character for splitting marginatus was the pattern of markings on the elytra but after comparing the descriptions with photos I wasn't convinced I really understood what it was getting at. A couple of the characters given in Hackston supported eliminating marginatus, though I wasn't clear about all the characters given there.

Using Duff I had already eliminated obsoletus but using Hackston this was harder to be so sure about. I wasn't clear what smooth vs. notched looks like on the stridulatory file. Are the little ridges down the file what it means by notches (or rather are the notches the gaps between the ridges)? If so mine was notched. I also couldn't decide if mine had an elytal epiplueral ridge - it seemed to have a really vaguely-defined raised area that at a stretch could perhaps be described as a ridge, but not as clearly defined as I suspect it means. I was pretty sure Hackston ruled out obsoletus, and more sure that Duff did, so on to fenestratus vs. fusculus.

I couldn't see any yellow on the foremargin of the pronotum, broad or narrow, other than the yellowish corners. But otherwise the characters given in Hackston pointed clearly to fenestratus rather than fusculus, except that the front femora weren't as clearly yellow as described or shown in the photo there. Duff starts with the shape of the foremargin of the pronotum rather than the colour - convex in the middle in fusculus and convex or truncated in the middle for fenestratus. It looked pretty straight in the middle on mine, but given the foreangles pointed forwards the whole of the fore margin could be described as concave. Not the middle though, so favouring fenestratus. Duff agrees with Hackston about the femoral colour - pale yellow in fenestratus. On mine the femora were yellowish, but not wholly (the bases were darker on all legs) and not, I would say, pale yellow. The front femora in particular were quite dark and dirty yellow, especially on the male. My female had well-decarcated broad yellow edges to the sternites as Duff describes for fenestratus (usually) but these were much less distinct on the male.

All things considered I was favouring fenestratus for both but in considerable doubt about one in particular. Fortunately the more doubtful one was the male, and that meant I could confirm it using genitalic characters. The deep apical notch of the tegmen clinched it as fenestratus. The male was 4.5mm long and the female 4.2mm.

male Heterocerus fenestratus showing underparts, stridulatory file, mesotsternal fore margin and aedeagus (front and back), Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

female Heterocerus fenestratus showing underparts and mesotsternal fore margin, Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

These three came to light at a differnt site within the same project area. The identification process was similar but this time the femora were more obviously pale yellow (again, not wholly). The female was larger this time (4.2mm vs. 3.3mm for both males). Again the female was the only one with obvious wide yellow edges to the sternites (indistict and less extensive on the males).

female Heterocerus fenestratus showing close-up of elytral pattern, antenna, side of metasternum and side of sternites, Wendling Beck Environment Project (Norfolk, UK), 6th July 2023

male Heterocerus fenestratus showing close-up of elytral pattern, antenna, side of metasternum, side of sternites and aedeagus, Wendling Beck Environment Project (Norfolk, UK), 6th July 2023

male Heterocerus fenestratus showing close-up of elytral pattern, antenna, side of metasternum, side of sternites and aedeagus, Wendling Beck Environment Project (Norfolk, UK), 6th July 2023

After a bit more practice these have now become much quicker and easier to identify. The genitalia were examined (but not photographed) in the male below (first photo - the others are females).

male and 3 x female Heterocerus fenestratus, Hoe Rough (Norfolk, UK), 23rd July 2023

I think I'm getting the hang of these now. Three out of these four (the first three photos below) I found straightforward but the fourth was a little trickier. The elytral markings adjacent to the suture weren't so clearly paired as on the others (I think I understand what this means now!) and the base of the elytra was dark. But there were pale markings fairly near the base (certainly welll within the basal third), and a pair of very indistinct ones right near the base. Apart from the femora not being all that pale again the other characters all checked out for fenestratus and in the end I was confident that's what it was.

4x Heterocerus fenestratus the fourth showing pronotal bead, mesosternal foremargin, rear underparts and elytral base, Wendling Beck Environment Project (Norfolk, UK), 9th August 2023

This one, the first Heterocerus I've found in my garden moth trap, had an entirely yellow abdomen (at least the underside, but I think also the tergites). This made me wonder if it might prove to be a different species as by now I've had both males and females of fenestratus and none have had such extensive yellow on the underparts. The outer tubercle on the mesosternal fore margin was substantially larger than the inner one and on this character alone the ID seemed to favour marginatus rather than fenestratus. However marginatus does not have the same elytral pattern which I now recognise on fenestratus - on that character this looked just like another fenestratus. I've not seen marginatus yet but checked lots of photos of marginatus online and they all had a consistently different elytal pattern. So on that basis I assume this is just another fenestratus. Had it been a male I would have confirmed it genitalically but the abdomen did not contain an aedeagus (or any sclerotised structures at all) so I assume it was a female.

presumed female Heterocerus fenestratus, North Elmham (Norfolk, UK), 27th August 2024

These weren't the first times I'd encountered the species. I don't recall if the previous time was more straightforward but so far as I can see them in the photos the femora appear to be pale yellow which would certainly have helped. This one would have been identified using Hackston alone. Like the ones above it came to LED lights.

Heterocerus fenestratus, North Elmham Cathedral Meadows (Norfolk, UK), 10th July 2021

This one seemed to be much hairier (long hairs) compared to previous ones, and the underparts were almost yellow, quite unlike any of the previous ones. I was convinced it would turn out to be a different species, but keying it carefully using both Hackston and Duff, it came to fenestratus. I may have mis-remembered how hairy the previous ones were, but the extensively yellow underparts were certainly different from previous examples. I assume it was simply an unusual fenestratus. It was 3.93mm long.

female Heterocerus fenestratus showing mesosternal foremargin, Wendling Beck Environment Project (Norfolk, UK), 15th September 2023

3x Heterocerus fenestratus, Wendling Beck Environment Project (Norfolk, UK), 11th August 2023

2x Heterocerus fenestratus, Wendling Beck Environment Project (Norfolk, UK), 6th September 2023

Heterocerus fenestratus, Wendling Beck Environment Project (Norfolk, UK), 8th September 2023

Heterocerus fenestratus, Hoe Rough (Norfolk, UK), 23rd August 2023

Heterocerus fenestratus, Wendling Beck Environment Project (Norfolk, UK), 14th August 2024