Tiny insect soldiers with butch forearms are actually medics

Many insect colonies have troops of soldiers, which defend their nests with special weapons like massive jaws or chemical guns. Kladothrips intermedius is no exception – this tiny insect, known as a thrips, has soldiers that supposedly crush their enemies to death with butch forearms. But contrary to appearances, these big arms aren’t all that useful for fighting. Instead, they’re living pharmacies. Christine Turnbull from Macquarie University and Holly Caravan from Memorial University of Newfoundland have found that the thrips warriors are actually healers.

There are thousands of thrips species. Most of them are solitary animals, but a few species form colonies like those of ants and bees. They induce plants to form hollow outgrowths called galls, which the thrips then live in.  A single queen gives rise to two castes: workers, which do menial, and soldiers, which are thought to defend the colony against other invading thrips, such as Koptothrips dyskritus.

Early studies showed that soldiers could use their unusually large first pair of legs to crush invaders. But later ones found that these defenders weren’t any better at killing enemies than their peers with smaller arms. To check this result, Caravan staged gladiatorial fights between Kladothrips soldiers and Koptothrips invaders. Sure enough, she found that the size of a soldier’s forelimb or body had no effect on its odds of besting its rival. So why are they so big?

The team think that the soldiers do indeed defend the nest, but against the fungus Cordyceps bassiana. The bane of insects everywhere, Cordyceps fungi sprout throughout the bodies of their hosts. These macabre infections can destroy entire colonies. Rather than fending off similarly sized invaders, the soldier thrips defend against these microscopic ones.

Turnbull demonstrated that by creating extracts from the chemicals on the soldiers’s outer coats. These extracts could prevent Cordyceps spores from growing, and the essence-of-soldier proved to be a better anti-fungal than eau-de-worker.

The concept of soldiers as medics helps to explain some odd bits of their behaviour. For a start, these so-called soldiers seem remarkably reluctant to actually get into fights. If Koptothrips invades, the big-armed defenders will eventually defend, but they’re in no hurry to do so. This might be because they aren’t warriors at all.

This is a nice story, but it still doesn’t explain the size of the soldier’s big forearms. Perhaps these limbs provide sanctuary for antifungal compounds, but that’s something that Turnbull and Caravan still need to check. Caravan says that her group has found bacteria in the thrips galls that can kill other microbes. They’re now using a high-powered microscope to scour the insects’ arms, and other parts of their bodies, to look for places that could harbour the beneficial bacteria.

We know that other social insects use their medicinal chemicals to ward off infections. For example, leafcutter ants also have bacterial partners, which secrete chemicals that kill other microbes. But that’s the workers’ job. It’s not clear how many species have soldiers that double as medics, although there are at least two more examples:  the soldiers of two termite species also carry secretions that hold back the growth of fungi.

Reference: Turnbull, Caravan, Chapman, Nipperess, Dennison, Schwarz & Beattie. 2012. Antifungal activity in thrips soldiers suggests a dual role for this caste. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0184