Image caption Ageing workers assist soldiers in defending their colony

A species of termite has been found to inflict more damage on its enemies as it ages.

When defending their colony, some termites "explode", releasing chemicals that injure intruders.

A previously unknown crystal structure has been discovered that raises the toxicity of their chemical weapons.

As worker termites grow older, they become less able to perform their duties.

Yet this newly discovered structure allows ageing workers to better defend their colony. The research was published today in Science .

When faced with a threat, many termite species employ a type of altruistic suicide known as "autothysis" in order to deter attackers.

For the good of the colony Many species of "eusocial" insects deploy suicide defence strategies

"Autothysis" is a special kind of suicide where an organism ruptures an organ

These ruptures release toxic compounds, killing the animal and injuring its target

Autothysis was first documented in carpenter ants from Borneo Discover more about termites See inside a huge termite mound

In a few species, workers join "soldier" termites in the defence of their colony and perform these acts of suicidal defence.

However, a twist to this system has been discovered in a species from French Guiana.

"My PhD student, Thomas Bourguignon, was studying termite community ecology and collecting species when, casually, he found something really special," Prof Yves Roisin from the Free University of Brussels told BBC News.

By rupturing their bodies, Neocapritermes taracua release a toxic chemical that sticks to intruders, holding them fast and corroding their bodies.

"[Autothysis] is usually a one component system. The defensive secretions are stored in salivary glands, but in these species there is a 'backpack' with two crystals carried outside the body. When the termite bursts, the two mix together, producing the more toxic compounds," Prof Roisin explained.

The "backpacks" are formed from pouches on the outside of the body.

Defensive bombs

Although termite societies contain castes of "soldier" individuals with vastly enlarged mandibles that have evolved for the purpose of attacking intruders, workers can join fights and perform defensive suicides should the need arise.

The research shows that as workers in this species grow older and more incapable of performing other tasks, they store up crystals that produce a chemical reaction when mixed with glandular secretions. This increases the toxicity of their explosive defence mechanism.

Biologists believe it allows the ageing workers to become more "useful" to the colony as sacrificial, defensive bombs.

How the crystals are synthesised is, as yet, unknown. Also unknown is whether other species in the genus have evolved a similar backpack system.

"There are some five or six species in the genus, but it's the only species [that carries a backpack] we've seen so far," Professor Yves Roisin said.

"It's quite strange."