Rhizophagus perforatus

This my first Rhizophagus was found in a vane trap in my garden. It seems there are few records in Norfolk compared to some other species of Rhizophagus so perhaaps a surprising one to start with. The ID was challenging but seemed to resolve clearly by the end. I began using Duff and got to the Rhizophagus couplet 9 straightforwardly. At that point it became trickier - the punctures at the sides of the elytra were fairly weak but not much more so than at the middle, so were they weak enough to not be ferrugineus? I wasn't sure. The pronotum seemed to be broadest well behind the fore angles which favoured ferrugineus. I couldn't see the tibial spines clearly at first but the centre of the prosternal process seemed rather flat (the borders were clearly raised but the area between the borders seemed pretty nearly flat) which pointed away from ferrugineus.

My next step was to check Hackston's key. Hackston described the elytral punctures for ferrugineus as being very distinct at the sides, little finer than at the centre. Though mine were little finer than at the centre, I wouldn't say they very distinct - but the other option required them to be much finer than at the centre (usually) which they weren't. Looking at photos of ferrugineus online they seemed to have much stronger punctures all over the elytra than mine. Hackston also described the raised area on the prosternal process better and included a small photo and this made me feel more strongly that it could not be ferrugineus. Hackston says that male ferrugineus has the first segment under the abdomen broadly and shallowly indented which did seem, very subtly, to be the case - but Hackston doesn't say this is not also the case for the alternatives. At this point I didn't know if it was male or female but Hackston provided another character for males that meant that if mine was a male it definitely could not be ferrugineus - there were no converging hair lines on the metasternum.

Both keys led to perforatus if I was correct in eliminating ferrugineus, but as there don't seem to be many Norfolk records of this species I felt I needed more proof - and to find out what sex it was. By now I had examined quite a few photos of both ferrugineus and perforatus and although the pronotum shape of the perforatus was usually more like that described in Duff than mine was, the difference was pretty minimal and the photos of ferrugineus were considerably more obviously broader in front of the middle and for the most part also lacking such obviously protruding fore angles.

Next I checked the Beetles of Europe website and encouragingly this had photos of the aedeagus for both species (and other species) so if mine was a male I should be able to resolve it confidently. Next job was to dissect it and find out. In the meatime I removed one of each of the tibia (fore, mid, hind) and examined them more carefully. I'm not entirely clear if the counts of external spines descibed in Duff are supposed to exclude any spines at the apex, but if they do then the spine count for each leg fitted perforatus/parallelocollis and not ferrugineus (and if they are supposed to include the apical spines then they don't all fit anything). By now I had reached a stage where I was happy it was perforatus, but it was nonetheless very reassuring when I dissected the cleared abdomen and found that it was a male and that its aedeagus matched perforatus perfectly (and ruled out other species).

male Rhizophagus perforatus showing head from above and below, antenna, pronotum, side of pronotum, prosternal process (2 views), base of elytra (2 views), elytral striae (2 views), underside of abdomen and metasternum, underside of base of abdomen (detached), hind tibia, mid tibia, fore tibia and aedeagus (from side and flat), North Elmham (Norfolk, UK), 10th May 2024