Somewhere in the Amazonian understorey, beneath a great forest canopy, a cricket leaps on to the stem of a shrub. It’s the last leap it will ever make.
It never gets to flex its legs again. It’s stuck, like fluff to Velcro. Dozens of tiny jaws appear on the plant’s surface, each grabbing part of the unfortunate insect’s body, holding it fast.
Over the next hour, the spread-eagled cricket is dispatched, dismantled and butchered, piece by piece. Its body vanishes, seemingly into the plant’s tissues.
Yet the plant itself is not some kind of monstrous, carnivorous chimera. Instead, the insect’s brutal treatment is meted out by the plant’s own private army of ants.
While fascinating, there is nothing unusual about a relationship between ants and a plant. Their lives have become intertwined in many ways, and there are so many examples that such relationships even have a name: myrmecophily.
The level of intimacy expressed in this relationship varies widely. Some plants offer a bribe of sugar or protein ‘treats’ to recruit ants to work as a security force or seed-dispersal agency; others provide accommodation.
But one of the most complex myrmecophilous relationships is the one with which we started our story, between the shrub Hirtella physophora and the ant Allomerus decemarticulatus.
How does the trap ant kill its prey?
This ant is barely 1.5mm long and, as always with these insects, its superpower is not size but numbers. The colony has up to about 1,200 individuals, spread over their host in barracks, or domatia.
These are formed from of a pair of fleshy lobes at the base of the leaves, which curl inwards on themselves, making room for around 40 workers. The plant also supplies food in the form of carbohydrates. Extra-floral nectaries ensure that sweet stuff is on tap.
Yet the ants’ needs go beyond sugar alone, and the really distinct element of the relationship is that the ants use materials also provided by the plant to build a trap. First, they cut stiff hairs known as trichomes from the plant’s stems and arrange them into a complex, criss-cross scaffold.
Next, they chew a fungus – not for nourishment but to create a living, adhesive paste that grows, over time, across the scaffold. The ensuing matrix of hair and fungus forms a platform along the length of the plant’s stems, with a sheltered cavity beneath in which the ants can move in relative safety.
The surface of the platform is pock-marked with hundreds of ant-sized pores, which turn the entire structure into an intricate snare capable of capturing prey many times the ants’ size. It is the only collectively constructed snare so far found in the ant world.
The workers sit in the pores, exposing only their heads and mandibles, which are cocked and ready to snap shut on any creature that happens to walk or land on the plant. A large cricket makes a meal more than 140 times the weight of individual ants.
In a horrific but easy-to-visualise scene, the ants grab its limbs and pull backwards, while the insect struggles in the opposite direction, where more hidden jaws are waiting to grab it. The ants hold the insect down and more workers are recruited, stinging and biting until the prey is eventually overcome.
While smaller insects rarely get away, larger ones, such as crickets and grasshoppers, do sometimes escape, losing a leg or two in the process.
This may appear unsuccessful, yet it still works: the cricket is put off the idea of eating the plant and the ants gain a considerable meal.
It’s an extraordinary tripartite relationship. The ants get a supply of prey; the plant gets an ever-alert, highly organised security force; and the fungus gets both a place to grow and nourishment from the ants’ waste.
For this ant-fungus-plant symbiosis, it’s a case of win, win, win.
