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Sunday
, June 9, 2002
Nature

Beetle that emits stink to repulse foe
Nutan Shukla

IN the past 30 years, chemists and biologists have successfully collaborated in research projects that have provided a wealth of fascinating information about chemical defence in animals. Emitting a stink is perhaps one of the most effective ways of secondary defence for a cornered animal.

A spectacular example occurs in the aptly named bombardier beetle, found in several parts of the world. It is a star defender in this miniature theatre of war. If grabbed by a frog or toad, the beetle discharges its chemical weapon, leaving the unfortunate amphibian with a sore and throbbing tongue. If an ant holds on to its leg, the beetle swivels its abdomen around and squirts a boiling blast of noxious chemicals directly at it. Under constant attack from a swarm of ants, the beetle is able to discharge the scalding fluid 20 to 30 times, and make good its escape.

These insects have dark blue-black wing cases, and an orange head, thorax and legs, and have colours that warn off the foe. A bombardier beetle has an internal chamber which acts as a reservoir for chemicals which it produces: hydroquinones and hydrogen peroxide. When attacked by a predator, the liquid is forced into what is called an explosion chamber containing special enzymes. The enzymes convert the hydroquinones into unpleasant-smelling quinones and the hydrogen peroxide to water and oxygen. The pressure of oxygen causes the beetle to eject an explosive spray from its anus which can be pointed at the predator.

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The spray is not only nasty-smelling but is also hot because of the chemical reactions which have just taken place. The liquid is sprayed in pulses, like a machine-gun, at a rate of 500 to 1,000 pulses per second. By pulsing the jet, the beetle achieves greater spray impact, but more importantly, it does not blow off its own behind. The pulsed nature of the reaction prevents overheating in the explosion chamber.

The whole system is effective enough to intimidate even the most determined of predators, but orb-weaving spiders have developed a strategy that defeats the bombardier beetle. If a beetle is dropped into such a spiderís web, the spider gently wraps it up in silk without stimulating the release of spray. The spider then bites the beetle, who immediately fires off its spray, but with little effect, for the silk wrapping prevents the liquid from travelling.

Another less-advanced relative of the bombardier beetle, one of the paussine beetles, delivers its spray in a different way. The gland openings are spaced out on each side of the body, near the tip of the abdomen, but instead of directing it at the predator, it employs a principle of physics, hitherto only known to have been used by man. The principle is the coanda effect, and it is seen when milk has the annoying propensity to curl round the lip of the jug and on to a table cloth.

In the case of paussine beetle, there is a flange on either side of the abdomen, next to the gland openings. When attacked from behind or the side, the abdomen can be directed at the attacker, but the beetle can not point its abdomen to the front. So, if the insect is under frontal attack, the jet of boiling liquid can bend a full 50 degrees, following the curve of the body, and be directed forward.

Yet another relative, a metrine beetle, does not squirt a jet at all, but simply allows the chemicals to boil out from under the flanges of its wing covers. During an attack, the beetle froths in a bubbling mass of boiling secretion, the bursting bubbles sending tiny droplets of chemicals in all directions. Surrounded by this defensive vapour, the beetle is able to walk nonchalantly through a whole swarm of would-be predators without coming to any harm.

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