When I first saw these tiny black and white wasp beetles running around like crazy on a log, I thought they were weird ants. Then I looked closer and saw what they were. The ant in the picture is very small — the beetle is only 4 or 5mm. I had to wait about ten minutes before one of them would stand still enough for a photo!
Coleoptera: Cerambycidae (Longhorn Beetles)
Small black wasp beetle = hime-kuro-tora-kamikiri
Location: fureiai-no-mori Forest Park in Munakata (map here)