29 April 2012

The amazing acid-proof ant

Most terrestrial insects actively avoid liquids, particularly water, because they are too small to escape from its surface if they fall in, owing to the strong hold that the liquids surface tension holds over them. Even larger insects are no different and dislike water, even though some of the bigger species may be able to paddle around to some degree and eventually escape (if they are not eaten by a bird or a fish first).

The diver ant, Camponotus schmitzi, however is a species of carpenter ant that goes against the insect social norm and actually depends on water to survive. Well, depends on acid that is; not water... The fearless ant lives in an exclusive mutualistic relationship with Nepenthes bicalcarata, a species of  carnivorous pitcher plant that lives in Borneo, where it actually dives into the plants digestive juices in order to scavenge for its prey!

A small group of diver ants waiting underneath the lip of N. bicalarata until a insect drops into the acid pool at its base. The ants will then 'liberate' the plant of its meal, eating it themselves. Although this action robs the plant of its meal initially, the ants mainly scavenge large insects and throw back what they don't eat. This also helps to remove debris from the acid pool, keeping it fresher and therefore more attractive to potential prey animals; meaning the ants also benefit the pitcher plant.

If you've never heard of pitcher plants before then you may be confused as to why they have a pool of acid in their base, but it's actually fairly simple - pitcher plants, like the more famous Venus Fly Trap, are carnivores that live by feeding off insects that they have lured into their trap. Whereas species of Venus Fly Trap clamp insects that have landed on their surface and then actively secrete acid to digest them, pitcher plants use a 'pitfall trap' mechanism. This involves a small hole in their top that their prey must climb into to reach the attractive-smelling puddle of liquid in their base. However, this liquid is actually a mixture of a powerful acid and other digestive juices and once the prey species has crawled inside, it is prevented from escaping by tough downward-pointing spines. Unfortunately, the hapless animal then faces a slow and grisly death where it is digested alive.

Nepenthes sumatrana, a species of pitcher plant that can be found throughout the Phillippines. Its bright colouration and sweet smell help to attract its prey. Some of the larger species of pitcher plant, like those found in the Amazon Rainforest, can even attract and eat mice or frogs as prey!

The observation of diver ants has found that they spend most of their time hidden under the lip of N. bicalarata and run down into the base of the plant when an insect falls into its deadly pool of acid and digestive juices. Amazingly, the ants then dive head-first into the acid and work together to pull out the now dead arthropod and begin to eat it in safety on the shore of the pool. Therefore, the ants still get a meat meal but do not have the same risk of injury that most predators face while killing their prey.

Or do they? This strange hunting method seems to have a major and very deadly risk - obviously, the pitcher plants acid should also kill the ants! Scientists are still baffled by why it doesn't and it is thought that the short exposure time of the ants to the digestive juices allows them to survive, along with some other physiological adaptation that helps provide them with some immunity to the corrosive effects of the acid.

One physiological adaptation that has been identified however, is that the ants have a fluid on their outer cuticle (layer of skin) that makes them less water-repellent and therefore, helps to negate the action of surface tension. This allows the ants to dive underwater and leave the acid pool freely and without this chemical surfactant, they would be trapped on the surface of the acid pool like other insects; eventually dying.

A unit of C. schmitzi workers dragging a dead insect (either a grasshopper or cricket) from the pitcher plant's deadly digestive juices.

What is also interesting, is that the ants appear to sprint through the acid rather than swim; using the same tripod sequence as when they move on land - they move the first and last legs on their left side along with the middle leg on their right side and then the front and last leg of their right side with the middle of their left. This simple method of swimming suggests that the ants have evolved fairly recently, since the ancient and truly aquatic insects have all developed much more complex patterns of swimming than this.

Thus, the ingenuity and unique adaptations of diver ants allow them survive in a very hostile organism that by all rights, should kill the ants; turning them into its next meal...

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