When trekking through a forest in French Guiana to study termites, a group of biologists noticed unique spots of blue on the backs of the insects in one nest. Curious, one scientist reached down to pick up one of these termites with a pair of forceps. It exploded. The blue spots, the team discovered, contain explosive crystals, and they're found only on the backs of the oldest termites in the colony. The aged termites carry out suicide missions on behalf of their nest mates.

After their initial observation, the team carried out field studies of Neocapritermes taracua termites and discovered that those with the blue spots also exploded during encounters with other species of termites or larger predators. The researchers report online today in Science that the secretions released during the explosion killed or paralyzed opponents from a competing termite species. However, if the scientists removed the blue crystal from the termites, their secretions were no longer toxic.

Back in their labs, scientists led by biochemist Robert Hanus of the Academy of Sciences of the Czech Republic in Prague went on to show that the blue termites always had shorter, worn-down mandibles than others from the same species, indicating that they were older. Then, the researchers removed the contents of the blue pouches and analyzed them. They contained a novel protein that is unusually rich with copper, suggesting that it's an oxygen binding-protein. Rather than being toxic itself, it likely is an enzyme that converts a nontoxic protein into something toxic.

"What happens is when the termites explode, the contents of the back pouch actually interact with secretions from the salivary gland and the mixture is what is toxic," explains Hanus. It's the first time two interacting chemicals have been shown responsible for a defense mechanism in termites, he says.

Researchers already knew that many social insects change roles in their colony as they age. Moreover, it's well known that a number of species of termites explode, often oozing sticky or smelly fluid onto their opponent. But in previously observed cases, the explosive or noxious material is found in the termites' heads, and the suicide missions are the responsibility of a distinct caste of soldier termites, not aging workers. Since N. taracua have soldiers, it's especially surprising to see workers exploding, says Hanus.

"This is a quirky, funny natural history," says behavioral ecologist Rebeca Rosengaus of Northeastern University in Boston, who was not involved in the study. "What's new and interesting here is that this is found to be an aspect of colony-related age organization," says biologist James Traniello of Boston University. And the placement and chemistry of the blue crystals is unique, he says. The findings illustrate the vast diversity of social structures and defense mechanisms that the more than 3000 species of termites have evolved over time, Traniello says.

One question that remains is exactly how aging triggers the accumulation of the blue crystals. "We're still not 100% sure what the role of the blue protein is," says Hanus. "That's definitely something which we want to perform further research on."