Famed biologist E.O. Wilson wrote that while humans send their young men to war, ants send their old ladies. As workers age in insect societies, they play a larger role in nest defense, and new research on a termite species has revealed a link between aging termites and the accumulation of toxic substances on their backs—which they can burst in an act of self-sacrifice when the colony is threatened.
"I think it is a great discovery and a nice combination of behavioral, morphological, and molecular biology," said Olav Rueppell, who studies social insects at the University of North Carolina at Greensboro, and was not involved in the study, by email. "It demonstrates the power of kin selection and social evolution to create novel adaptations."
Inspecting individuals of Neocapritermes taracua termites, which feed on and live in decaying wood, Robert Hanus of the Academy of Sciences of the Czech Republic and colleagues noticed some workers had dark blue spots at the intersection of the thorax and abdomen. When faced with rivals of another termite species, these so-called "blue workers" actively bite their attackers until they are overpowered, at which point they burst their backs to expose the blue dots.
The blue dots, it turns out, are a pair of crystal-like structures encased in pouches that isolate them from the rest of the body. The crystals are formed of a copper-containing protein known to be used by arthropods for carrying oxygen and for immune defense, but this is the first time the protein has been seen accumulating in external "backpacks." Hanus and his colleagues noticed that while individuals without the crystals (termed "white workers") also burst in response to attack, they kill off far fewer rivals than their blue worker brethren.
To determine the toxicity of the crystals, Hanus and colleagues tested the efficacy of bursting if they removed the crystals from blue workers as well as implanting the crystals into white workers. While adding crystals made the white workers more deadly, they didn't reach the killing potential of the naturally blue workers. Beneath the crystal pouches, the researchers discovered a set of salivary glands, which secrete a substance that mixes with the blue crystals when the termites burst their backs, suggesting the interaction between the crystals and secretion is important to produce toxicity.
Self-sacrifice is well known in insect societies. Honey bees, for example, usually die after they sting an attacker because, rather than detach cleanly, the stinger rips out many of their internal organs. "Social insect workers are similar to the cells in our own body and sacrifice themselves in different contexts, just like some of our cells," said Rueppell. "The ‘superorganism’ concept has been used to illustrate the similarities between social insect colonies and multicellular bodies."
But while insect societies have specialized soldiers, it is also common for aging workers to play a larger role in defense as their usefulness to the colony wanes. In the present study, Hanus and the team were able to correlate decreasing mandible sharpness with increasing blue crystal weight. So, as workers became less able to carry out their tasks of collecting food and sculpting the nest, they were also becoming more prepared for an enemy attack.
"These adaptations are only expected in highly social insect societies, where individual workers have a next-to-zero chance to become reproductive themselves," said Rueppell. "It exemplifies adaptations that can only evolve in social species."
J. Šobotník et al., "Explosive Backpacks in Old Termite Workers," Science, 337: 436