I've been hoping to find one of these, and I was surprised at how small this was. Until I got it magnified I wasn't sure if it was a beetle or a mite.

i collected extensively in the Moscow area for years, but got perhaps 5 or 6, all told (there is just one sp. up there, though); other, less specialized, histerid inquilines were more abundant in Formica nests (tons of Monotoma, too)

There's a prairie a little over 30 miles from Madison here where I can reliable find these. One rock I overturned a couple of years ago had 16 H. brunneipennis and a single H. blanchardi under it with Formica montana ants. I may have missed some beetles as it was a free-for-all once the rock was lifted, with ants and beetles heading for the ant nest tunnels going underground.

It's all a matter of being out as much as possible so that you're there to be in the right place at the right time. ;-)

Oh, and I'm always very careful to gently replace the rock into its original position. That way I restore the environs and can come back over and over.

Probably H. brunneipennis. A shot of prosternum would do the trick.

Nice find! And great pics as usual.