A SENSOR found in a beetle’s “armpit” is the inspiration for a novel heat detector that could one day give early warnings of forest fires.
The jewel beetle, Melanophila acuminata, is legendary among entomologists because it heads straight for fires rather than trying to escape. Burnt trees are its preferred home because their sticky resins and poisonous chemical defences have been deactivated (91av, 7 August 1999, p 36). But how the beetle homes in on fires from a distance of up to 80 kilometres away was a mystery.
The beetle’s sensing mechanism relies on the fact that fires produce large amounts of infrared radiation. Most IR is absorbed by carbon dioxide and moisture in the air, but at a wavelength of 3 micrometres it escapes the absorption processes and can travel long distances. Of course there is always some background radiation of this wavelength from the sun, and at night from objects that radiate heat they have absorbed during the day, but a wood fire creates a lot more radiation at the 3-micrometre wavelength.
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In 1999 Helmut Schmitz, a zoologist at the University of Bonn in Germany, worked out how jewel beetles exploited this. They have clusters of deformable spheres containing proteins and carbohydrates located at the join between the thorax and the second pair of legs. The compounds in the spheres are rich in chemical bonds that vibrate only when excited by radiation at a wavelength of 3 micrometres. The vibration causes the spheres to expand, and this triggers associated nerves.
Schmitz has now built a sensor that mimics the way the beetle perceives the vibrations in the chemical bonds. Instead of the beetle’s smart spheres, he uses a sheet of polyethylene – a polymer whose bonds also resonate and expand at 3 micrometres. When the polymer expands it presses against a piezoelectric crystal positioned next to it, inducing a telltale current in a pair of wires attached to the crystal.
His sensor only has a range of 2 metres at present, but with modifications, he believes he can get it to detect fires up to 10 kilometres away. “In principle we can expect the same kind of sensitivity as the beetle from this device,” he predicts.
Schmitz’s idea is a welcome development, says Dan Lang at the California Department of Forestry and Fire Protection in Sacramento, because commercial sensors that detect IR at long distance and at the crucial 3-micrometre wavelength are too expensive for most fire departments. Their supercooled semiconductors require a constant supply of liquid nitrogen, and cost tens of thousands of dollars.
Schmitz’s device could cost just a few dollars. Lang envisions hundreds of cheap, “beetle-based” sensors being deployed throughout forests, perhaps with wireless links to a central computer, to automatically monitor the outbreak of fires.
