We found these ants below in Anduki (Seria, Brunei):
The ants pictured belong to the genus Cataulacus, a group of arboreal-nesting ants widely distributed across the Old World tropics. The exact species is likely Cataulacus latissimus, one of the more sizable of the dozen or so Indo-Australian species, known to occur in West Malaysia, Sumatra, Singapore, and of course Borneo.
Thanks for the photo,
Alexandra Westrich & the AntAsk Team