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They are what fire fighters have long been calling for: low-cost and highly sensitive infrared sensors that automatically monitor large forest areas and trigger an early warning in the event of fire. Zoologist at the University of Bonn have taken an important step towards this goal. They have constructed a forest fire sensor which could be produced more cheaply than commercially available infrared detectors, although it is not yet as sensitive. In identifying a principle of measurement previously unknown in nature and in technology, the scientists have taken the idea from a small insect: the jewel beetle, which lays its eggs in the wood of freshly burned trees, is said to be able to detect forest fires from a distance of 80 kilometres. The biologists now want to perform more tests on their little model to determine the limits of this new method of measurement.
This "fire beetle" loves burnt wood: immediately after woodland blazes the females fly in from far and wide, climb up the trees and lay their eggs in the smouldering bark. Since most other insect species tend to avoid freshly scorched areas, the jewel beetle larvae can develop with hardly any competition. The adult insects owe their sense of burning to clever sensory organs located on their underside. These organs are pits containing a large number of receptors ("sensilla") that are extremely responsive precisely to the infrared (IR) radiation that comes from a forest fire.
"Interestingly, these infrared feelers are modified mechanosensors," explains Bonn zoologist Dr. Helmut Schmitz, "and present us with a completely novel method of monitoring IR radiation." The finger-shaped protrusion of an individual mechanoreceptor is inserted in a tiny sphere made of "cuticula" – the same material that forms an insect's armour. The cuticula sphere surrounds the pressure-sensitive "finger" like a clamp. "Now, our jewel beetle's cuticula is particular good at absorbing thermal radiation with a wavelength of about three micrometers – exactly the radiation that is typically emitted by a fierce forest fire. So when a fire occurs the sphere heats up, expands and, in this way, directly stimulates the mechanoreceptor," says the research scientist. Since the atmosphere is pervious to infrared at these wavelengths, the insects can identify potential breeding grounds from a long distance.
Assisted by his doctoral candidate Martin Müller, Schmitz has reconstructed the sensor using the simplest of components. Instead of the cuticula sphere, the replica uses a polyethylene platelet. Polyethylene absorbs infrared radiation in a similar spectrum as cuticula and also expands in response. This expansion is measured by the scientists. "The whole thing already works quite well, although commercially available IR sensors are better by a factor of 100," says Schmitz, who is nevertheless convinced of that the system can be perfected: "With our simple prototype we are only at the beginning of what is possible." The zoologist is currently looking for industrial partners to work out the specifications of his sensor and mature the technology.
To determine where the limits of this monitoring principle lie he also wants to make precise measurements of his biological model. "There are indications in the literature that the jewel beetle has detected forest fires from distances of up to 80 kilometres; but the data has never been checked and therefore remains unsound." In any case, the beetle is only the starting point: "Our aim is to test all known infrared-sensitive animals to find out how sensitive their sense of thermal radiation is." However, this gift does not appear to be very common in the animal kingdom. Apart from three species of beetle, so far scientists only know of some snakes – the pit viper and giant constrictor – that have genuine IR-sensing organs. In fact, since the beginning of last year a snake experts, Dr. Guido Westhoff, has been collaborating with Helmut Schmitz at Bonn's Institute of Zoology. He is examining the heat sense of reptiles in a project funded by the Deutsche Forschungsgemeinschaft (DFG). Nowhere else in the world are all the known IR-sensitive animals being studied under the roof of one research institute.