Trixagus carinifrons agg.

Slightly frustratingly, I am not able to fully resolve this my first example of a member of the family Throscidae. It was found in my moth trap, was just under 3mm long and I got it to family using the Unwin key fairly easily. Then the fun began.

On paper, separating Trixagus from Aulonothroscus should be straightforward - the latter has more or less entire eyes while Trixagus has its eyes deeply incised. I could see something running through the eyes, but at first I wasn't sure if this was merely pattern or a true division. It seemed to be an excavated region through the eye, shaped as shown in Duff's diagram of Trixagus obtusus eye, but it didn't seem deep - I thought I could even see facets at the bottom. I didn't think the eye was completely incised, but it did have some kind of shallowly-excavated hairy region - was this the incision I was looking for for Trixagus? Other beetles I've looked at with incised eyes have an incision of material that clearly isn't part of the eye - a bit of the head running through (or maybe over?) the middle of the eye. Except very shortly at the base, that wasn't what I was seeing here.

I turned to Hackston to see if that threw any light on the matter. Hackston's picture of the eye looked much like mine (except it was a species with a shorter incision), so seemingly confirming that what I was looking at really was an incision, not some other eye feature. But it also provides an additional character, namely the presence or absence of furrow each side of the metasternum to accommodate the tarsi. My beetle seemed to have a slightly excavated area on the sides of the metasternum which looked like they could accommodate the tarsi (the tarsi were hovering just above it). If I took that to be a furrow - which didn't seem like too much of a stretch to me - it would point to Aulonothoscus.

In the end I ended up clearing the head in potassium hydroxide solution to get a better look at it, and after comparing with high quality online images of Trixagus (unfortunately I couldn't find any high quality images of Aulonothoscus that showed the eye clearly) I was satisfied that the excavation I was seeing in the eye was indeed the incision referred to in the keys. My beetle was Trixagus.

Beyond the incision the eye was only three facets deep, so that ruled out the commonest Trixagus, dermestoides. The frons had a pair of evident keels which ruled out obtusus, and the length and straightness of the keels pointed to either carinifrons or meybohmi. Unfortunately these two can only be separated by male genitalia, and upon dissection this one proved to be a female.

female Trixagus carinifrons agg. showing eye, metasternum, frons (from front and above), eye after clearing (2 views), antenna, genitalia and elytron, North Elmham (Norfolk, UK), 10th September 2023

This one was unfortunately also a female, so like the last could be either carinifrons or meybohmi (I think the more-or-less parallel frontal ridges eliminate leseigneuri). Again I found it rather difficult to see the critical details of the eye incision, not least because the back of the eye was hidden beneath the pronotum. It was a bit clearer after a spell in potassium hydroxide after which I could pull it clear from the pronotum, but even then although I was satisfied the incision extended more than half way across the eye I wasn't convinced unincised bridge was no more than three facets wide. Eventually after a longer spell in the potassium hydroxide, a dip in alcohol and then allowed to dry the eye became semiopaque making it much easier to see.

I also had a closer look at the metasternum which Mike Hackston's key (which incidentally doesn't cover meybohmi and leseigneuri) says should lack a lateral furrow to accomodate the tarsi in Trixagus. There are clear furrows along the anterior edge of the metasternum which accommodate the mid femora, and these furrows are angled sharply back towards the edge of the beetle but more shallowly and not for long. The tarsi do sit in these furrows but the furrows are not deep or long enough to fully accommodate them. I presume they are more obviously able to acccommodate the tarsi in Aulonothroscus.

female Trixagus carinifrons agg. showing metasternum, head from 3 angles before clearing, again after a short spell in KOH, each eye after clearing and alcohol and genitalia, North Elmham (Norfolk, UK), 13th August 2025