Posted by: scrubmuncher | January 14, 2011

Unsporting ants

The trap ant, Allomerus decemarticulatus, is a small, unassuming South American insect, staying out of sight much of the time in nest pouches among the branches of the tree, Hirtella physophora. In this arboreal environment animal prey can be hard to come by. Any substantial insect the ants would dearly love to get their mandibles into can simply relinquish its grip on the tree and fall or fly to safety.

To secure such evasive prey the trap ant has evolved a devilishly cunning strategy. Beneath the branches of its host tree, worker ants construct small galleries riddled with holes. More intriguing still is the way they actually build these galleries. Using hairs stripped from the surface of their host tree to act as a sort of fibrous matting, saliva and a specially cultivated fungus as resin, the ants construct these peculiar structures.

These galleries have been known about for a long time, but it was wrongly assumed they were simply a refuge for the ants whilst they were away from the nest foraging for food. As it turns out the real purpose of these galleries is far more sinister. The worker ants amass in these galleries with their little heads just inside the holes, their powerful serrated jaws agape. Here they wait in ambush and before long a plump cricket ambles in to view exploring the branch tentatively with its long limbs and sensitive antennae.

Worker Allomerus decemarticulatus waiting to spring their trap (Alain Dejean)

The hairs that clothe the tree’s outer surface are a deterrent to walking insects. The smooth surface of the gallery on the other hand will seem like a fine place to rest or have a snack. Sensing nothing out of the ordinary about the ant gallery the cricket walks straight on to it. Two or three of its clawed feet may probe the holes for purchase and it is then the waiting ants strike. They grab the ends of the cricket’s legs and heave, pinning the cricket to the surface of the gallery. Other workers rush out and begin dragging the remaining limbs and antennae into the holes until the prey is well and truly snared. Other workers then swarm all over the prey and sting it. With the prey immobilized and at death’s door the ants can begin their gory work. Using their powerful jaws they begin butchering the carcass of the cricket, chopping out chunks of flesh to carry back to their developing sisters in the nest pouches.

For this cricket, the leaf-munching party is over. It has fallen foul of the trap ant's cunning ploy and will soon be nothing more than ant-grub food (Alain Dejean)

This amazing trapping strategy ensures the ant larvae have a good supply of protein in an arboreal environment that it otherwise lacking in nitrogen containing food fit for ants. Not only have these ants evolved a unique way of catching animals much larger than an individual in the colony, but the whole set up is the product of a symbiotic relationship between the ant, its host tree (Hirtella physophora) and the fungus. The ant has a safe place to nest in the leaf pouches of the tree. A favor repaid by ridding the tree of herbivorous insects that can’t wait to get stuck into the succulent leaves of the Hirtella tree. The fungus used as an adhesive in the construction of the trap galleries is thus cultivated by the ants and is spread to areas where it might not reach under its own steam.

 

Further reading:

Dejean, A., Pascal, J. S., Ayroles, J., Corbara, B., and Orivel, J. Arboreal ants build traps to capture prey. Nature 434, (2005) 973.

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