I found a bug in Longview Washington that looks alot like this one. My cat had it on it's back this morning when I went to work. I took it the my daughter school class room this morning. The bug was a big hit.

It looks similar to Asemum striatum in one of my books, but the size is off. At first I thought it might be a false blister beetle, but really I don't have a clue.

I can see a dozen differences between this one and that species, which we do have in the guide. I don't think it's a cera*mbycid. The eyes are entire and bulging and the tarsi don't seem to have furry hook pads. The feet are too fine and delicate.

The Canadian list shows several western spp. [A. asperatus, A. productus, or maybe A. foveicollis] that should also be in Montana. From another of their websites, I'd say A. productus - best guess. They also have an interesting details page. Yours is a female - recorded length is 15-25mm (did you include the protruding ventrite and to the tip of outstretched mandibles? Typically should be from tip of elytra to base of mandibles - that would drop a few mm).

I did measure to the tip of the mandibles but not the tip of the ventrite. That would shave off a few mm.