Sunday, January 21, 2007

Nature Carried Away The Bell - III

Keeping in touch
Great ingenuity and millions of man hours have been invested in developing the different parts of the information system in a modern aircraft. There are the Pitot-static tubes to provide data on air speed, the aerial for the Instrument Landing System, the Angle of Attack probe that shows the angle of wing to airflow, aerials for radar and identifying friend or foe, the ultra-high frequency communications radio, and a good deal more besides. All these require still more computers to feed everything into a data system for the pilot. It is remarkable that so much knowledge, memory and information can be crammed into so small a space.

Few insects, though, would be much impressed. They already receive similar information, together with many other messages essential to their well-being, via a pair of hair-like feelers, or antennae, on the head. The antennae carry tens of thousands of sensitive cells for direction finding, scenting, testing temperature and humidity and many other tasks, according to the needs of particular species. Some of these receptors are geared to distinguish between a whole library of smells, others to reject all others and home-in on only one or two. Female silk moths, for instance, emit a scent that a male can pick up from miles away if the wind is favourable. Bees combine scent and colour sensitivity to identify a profitable food source. They will fly past almost any other blossoms upon sensing the odour and brilliance of a field of oilseed rape.

Honeybees' antennae are also alert to the atmosphere of the hive. If the temperature rises above a desirable level, the workers simultaneously fan their wings, and some go out in search of water to cool it. Species of ant that protect sap-sucking aphids for the sake of the syrup they secrete use their antennae to induce them to part with it; they tap signals on the aphids' backs.

Another parallel between flying insects and aircraft instrumentation is instanced by the Johnston's organ, a mechanism at the base of the antennae that reacts to air currents. The stronger the current, the more impulses the organ conveys to the insect's muscular system, causing its wings to beat harder. Thus flight speeds are kept constant. The organ also helps the insect to maintain course with one wing injured. One antenna directs more muscle power to the damaged member, while the other throttles back the whole wing to synchronise movement.

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