Look, no hands: ants kill termites with airborne chemical weapons

Many insects are armed with venom, which they can inject into their enemies via a sting. The African ant Crematogaster striatula is no exception, but its arsenal has a disturbing twist – its venom goes airborne. The ant can raise its sting and release its toxins as an aerosol spray. Its targets are termites, whose nests it raids. Even without making any contact, the ants can induce seizures in the termites, eventually paralysing them.

All Crematogaster ants have a mobile sting. The sting sits on the ant’s rear-end, which connects to its torso by a flexible stalk, so the ant can aim it in virtually any direction. Aline Rifflet from the Jean-Francois Champollion University Center saw this ability in action when she watched C.striatula take on termites.

C.striatula is a specialised termite-hunter. When it finds a termite, it raises its sting into the air, releasing chemicals that summon nearby nestmates. If the termite is a soldier, armed with powerful jaws, up to 15 ants can gather round. All of them stay a centimetre away from the termite, aiming their stings at it like fencers with swords outstretched. They close in, but they still never actually touch.

Termites don’t retreat – they defend their nests no matter the danger. That is a fatal mistake. After ten minutes of stand-off, the termite starts to shake. It rolls onto its back, with its legs batting the air in helpless convulsions. Within moments, it’s paralysed, and the ants finally move in and grab it.

C.striatula behaves in the same way when it finds other ants in its territory. These intruders have the good sense to flee, even if they’re much larger than C.striatula and even if they have the weight of numbers on their side.

It’s clear that the chemicals released from C.striatula’s sting do three things: they rally other workers; they repel other ants; and they paralyse termites. To learn the secrets of these chemical weapons, Rifflet dissected the glands of workers.

Ants have two glands that feed chemicals into their stings – the poison gland (no prizes for guessing what that does) and the Dufour gland, which releases pheromones for marking trails and raising alarms. But in Crematogaster ants, it’s actually the Dufour gland that acts as the source of venom, which is then weaponised by enzymes stored in the poison gland.

Rifflet made extracts from the Dufour glands of C.striatula workers, and confirmed that they alone could paralyse and kill termites. The glands contain at least 50 chemicals including a very large one – an alkaloid – that can split into smaller components. Rifflet thinks that these smaller molecules are responsible for killing the termites, and she now wants to identify and isolate them. They could even form the basis of a new breed of natural insecticides.

Many animals have projectile weapons: some ants can spray formic acid; the bombardier beetles squirt enemies with noxious burning chemicals; the spitting cobras spit venom; and both velvet worms and spitting spiders can spew immobilising glue.

In all these cases, there is an obvious and noticeable stream of liquid. By contrast, C.striatula’s long-range chemical weapon seems all the more sinister for its invisible nature. Only one other animal has something similar – another ant called Platythyrea conradti . It also raids termite nests. When it encounters a defending soldier, it drops into a crouch and opens its jaws. It never bites, and it doesn’t need to. Glands in its mouth release an airborne poison that paralyses the termites, in the same way that C.striatula does with its sting.

Reference: Rifflet, Tene, Orivel, Treilhou, Dejean & Vetillard. 2011. Paralyzing Action from a Distance in an Arboreal African Ant Species. PLoS ONE. http://dx.doi.org/10.1371/journal.pone.0028571