A large Neocapritermes taracua soldier termite with four worker termites—two… (R. Hanus )
Esteemed biologist E.O. Wilson famously observed that "a principle difference between human beings and ants is that whereas we send our young men to war, they send their old ladies."
And so do termites.
When they become too old to help their colonies perform risky foraging tasks, elderly members of a termite species called Neocapritermes taracua provide one final service by sacrificing themselves to defend against predators, researchers reported last month in the journal Science.
As rival termites close in on their nest, the selfless termites break apart their bodies to create a toxic, sticky goo. That goo seals off tunnels that lead to the termite nest and sticks to would-be predators, disabling or even killing them.
With their abdominal walls broken, the termites die too.
Thomas Bourguignon discovered this while collecting termite samples in French Guiana for his dissertation on community ecology.
He also noticed an unusual mechanism they use to carry out these suicidal attacks: They build up pouches on their abdomens that contain a mysterious blue substance.
Bourguignon and his colleagues dubbed these pouches "explosive backpacks." They seemed harmless on their own, but their contents became deadly when mixed with salivary secretions.
Some of the termites had elongated blue spots on their backpacks, and these workers were particularly combative in their encounters with members of another termite species called Labiotermes labralis. Other Neocapritermes termites lacked the blue spots; these so-called white workers were less aggressive against invaders.
Back in his lab at the Universite Libre de Bruxelles in Brussels, Bourguignon and his colleagues examined the blue substance. They determined that the spots were "a pair of crystal-like structures" made with a previously unknown protein, according to their report.
Whatever was in these crystals was certainly potent. When a group of 40 Labiotermes termites was exposed to a drop of the goo produced by blue workers, 26 of them died and 11 became paralyzed within an hour. By contrast, a drop of goo from a white worker killed only five enemies and paralyzed four in the same amount of time.
To test whether the blue crystals were indeed the deadly ingredient, the researchers removed them from the blue workers' pouches. Sure enough, their goo became less noxious, killing only seven Labiotermes termites and paralyzing eight. On the flip side, adding the crystals to the white workers' goo made it more lethal, killing 18 enemies and paralyzing five.
What's in the blue crystals that packs such a punch? The scientists discovered that the previously unknown protein contains 7 to 11 nanograms of copper per crystal. The copper gives the crystals their blue tint.
The older the termites got, the more worn-down their mandibles were and the larger their blue crystals grew, the research team reported. The increasing size of the crystals — and thus their larger "explosive backpacks" — make elderly termites ideal candidates for suicidal missions, said Bourguignon, now doing postdoctoral work at Hokkaido University in Japan.
"We are trying to understand the evolution of such a complicated a new system," said Yves Roisin, who was Bourguignon's thesis advisor and senior author of the study. "We are wondering what other related species explode in a suicidal fashion."
Olav Rueppell, a biologist at the University of North Carolina at Greensboro, who was not involved in the research, said the finding "is a great discovery."
"It shows that each species is worth studying in its own right and that the biological diversity is full of surprises."