Tachinus rufipes a myrmecophile?

I found this rove beetle under a stone in the middle of a Lasius flavus colony. As a potential myrmecophile it was pretty convincing, as it was in a chamber right in the middle of the colony, though at a distance from the ants. I collected it with a few L. flavus and kept it alive with them for a few days before deciding to work out what it is.

I originally thought it might be Lamprinodes saginatus, a species associated with ants, including Lasius. However, I'm not a coleopterist and the Staphylinidae are particularly difficult, so I am indebted to Don from the BWARS forum for suggesting an identity from these pictures and describing a bit about the genus.

Apparently it is most likely a male Tachinus rufipes, a not uncommon species in the UK. Tachinus tend to live in grass tussocks, decaying vegetation, dung, carrion, etc, so seem to be generalists. None are listed in Donisthorpe's The Guests of British Ants (1927) as being myrmecophilous. Whether or not this species should be regarded as a myrmecophile probably depends upon how many other times it's found in ant nests, but it is certainly not a species that is dependent upon ants. Most likely the ants were providing suitable conditions and a possible food source. This relationship may not come without a price to the beetle, as both antennae in this specimen are missing segments at the end.

The site it was collected from is one of my favourites and is usually the first site I visit each year. It is a steep south-facing grassland in a sheltered valley and has a lot of flat stones on the ground under which ants nest. Early in the year whole colonies are easy to find just under these stones, along with their myrmecophiles. It was originally visited by Cedric Collingwood in 1960; I checked it out 40 years later and rediscovered Formica cunicularia. These F. cunicularia all seem to be doing something they shouldn't, as they are polygynous, so this year I collected a colony with five queens to observe. Other species recorded include:

• Myrmica sabuleti
• Myrmica scabrinodis
• Myrmecina graminicola
• Lasius alienus
• Lasius brunneus
• Lasius flavus
• Formica fusca
• Claviger testaceus
• Platyarthrus hoffmannseggi

plus a few as yet unidentified possible myrmecophiles, most of which are more nondescript rove beetles. There aren't that many species of ant in the UK, so any site with eight or more species and frequent myrmecophiles is somewhat exciting.