When Jan Šobotník from the Academy of Sciences of the Czech Republic and Thomas Bourguignon from the Free University of Brussels in Belgium were examining termites (Neocapriterme taracua) in French Guiana, they were surprised to see that some individuals had pale blue spots on their abdomens. But the real shock came when the pair picked up the spotted insects – they exploded in their hands.

To investigate whether the suicides might be part of a defence strategy, Šobotník, Bourguignon and their colleagues pitted spotted and non-spotted termites against two rival species of termite – known to compete with the others for land and food. When the spotted termites were physically unable to defend themselves with their jaws, they would commit the ultimate sacrifice and burst a pouch on their backs, releasing a toxic liquid that quickly paralysed and killed any other termites it touched.

While non-spotted termites would also burst when they were threatened, they appeared more reluctant to do so. Their poison was less potent too: liquid extracted from spotted workers was five times more effective at killing attackers than that of non-spotted insects.

On closer inspection, the team found that the blue spotted termites' jaws weren't as sharp as those of their non-spotted counterparts, indicating that they were older. Team member Robert Hanus, also from the Academy of Sciences of the Czech Republic, thinks the insects develop their spotted toxicity once they're too old to forage for food.

"To ensure that they remain of use to the colony, the older workers take on a defensive role," Hanus says. "The younger workers will do anything to avoid exploding because they know they are not very toxic so their suicide is a waste to the colony."

Under the microscope, the spots appear to be crystals encased in pouches that sit on top of the insects' salivary glands, says Hanus. When the pouch bursts, it's the mix of crystals, saliva and haemolymph – a liquid similar to blood – that makes the resulting liquid so toxic, he reckons. "Some unknown chemistry must take place," he says.

Self-detonation as a defensive tool is known to be used by social insects like termites, but it has never been seen accompanied by this type of chemistry. Most insects simply compress their abdominal muscles to release the contents of their intestine, which normally disturbs their attackers rather than killing them.