Deleaster dichrous

Although this is supposed to be a very distinctive rove beetle, if you don't know it you have to key it out, and that proved to be quite problematic. I always struggle a bit with the subfamily keys because I find it difficult to interpret the keys' attempts at describing the position of the antennae - clearly this isn't easy to describe as different keys contradict each other. In this case I was fairly confident on that character as there was a strong similarity to other Oxytelinae I had just examined, but when other characters failed I wondered if I'd got it wrong. The main problem is that both Lott and Duff require the fourth tarsal segment (if it's got five tarsal segments) to be simple not bilobed in order to get it to Oxytelinae. The fourth tarsal segment on this beetle was not simple, being clearly split to about half way to the base on the dorsal side. Hackston's key is ok in this respect, but there the start of the key to Oxytelinae goes wrong as the sides of the tergites were clearly angled upwards not nearly flat.

Further causing confusion, my specimen was smaller than the bottom of the stated size range in any key. The overall length seemed to be much too small but this is always difficult to assess as rove beetles tend to contract on death (at least if they're kept dry as I usually do), so the measured length is usually less than its length would have been when alive. It's also tricky as the beetles tend to bend at the junctions between head and pronotum and abdomen, and when dry they're hard to straighten without damaging them. Duff provides fore body length measurements as well, which is really helpful as this only goes to the end of the elytra so avoids the problem of the abdomen contracting on death. The forebody length of mine was 3.65mm, sill below Duff's range of 3.7-4.2mm, but close enough to ease my concerns about the overall length seeming much too short.

Most references seem to regard this as a very distinctive species, and other than the size everything else seems to be spot on for this speices. Lott mentions a similarity to Anthophagus in Omaliinae but as the depressions on the head and pronotum exactly matched the descriptions for Deleaster I think this is easily eliminated. Lott also describes "deep, irregular incisions" on tergite 8 and my specimen showed these (I couldn't find a diagram showing how these should appear but there's an individual on GBIF where this is shown and that compared favourably with mine).

I don't have any more up to date information but I understand there was only one Norfolk record of this species up to the end of 2021, but the species seems to be well known for coming to light as mine did.

Deleaster dichrous showing head, pronotum, elytra, tergite 8 (2 views), side of tergites and hind tarsus, North Elmham (Norfolk, UK), 13th June 2025