This is a type of tiger beetle, although it looks much different to the spotted one, featured yesterday.
I see these all the time, and we always used to call these beetle ants. But their real common name is wingless tiger beetle.
SEE THEM: Often seen on tree trunks or on railings, runs quickly when disturbed, and despite its name, it is not able to fly. ED: Surely, you don't mean that, as it is called that because it cannot fly. Anyway, this couple are having fun on a railing near Sai Kung.