Discovery News has published a blog about the "suicide bomber" ant, including my photograph from the AAA. The workers of several species in this group of carpenter ants will rupture their body when confronted by an enemy, spewing a toxic yellow glue that instantly kills themselves and any ants in the vicinity. These ants are currently being studied by Diana (Dinah) Davidson, recently retired from the University of Utah, who kindly showed me their behavior in Brunei.
How is this defense used in practice? The details aren't known, but I doubt if workers enter the territories of other colonies to disrupt their societies, in the manner of a human terrorist. Nor would it make sense for the workers to self-immolate so readily on a battlefield -- the supply of fighters would soon become exhausted. My guess is that this is the species' way of handling lone scouts from other colonies that wander into their territories. (Actually, naughty me, that's how I took this picture: I used a food bait to lure foreign ants up a tree occupied by this colony.)
A foreign scout may be as dangerous as a human spy if she returns home with knowledge about the colony or its resources. Killing her without hesitation is worth the loss of a worker's life.
For on suicide by ants in the service of colony defense, see AAA pages 126-8.