U.S. Military Looks to Beetles for New Sensors

By John Roach
for National Geographic News
March 14, 2003
This is the last in this week's series of articles on how military and other scientists are turning to nature to find the next generation of powerful secret weapons. Please look under the picture caption for links to the earlier articles. The stories all air on National Geographic EXPLORER at 8 p.m. ET/PT this Sunday on MSNBC.

Some like it hot. Some beetles like it smoking hot.

When a forest goes up in flames normally elusive Melanophila acuminata beetles from miles around head for the inferno in droves, joining a mating frenzy so that the females can lay their eggs in the freshly burned trees.

The beetles are attracted to the smoldering wood because the burned trees no longer have active defense mechanisms such as flowing sap. Larvae feed on the layer of plant tissue that lies between the bark and wood, and as the beetles grow they chew into the wood itself.

"They are adapted to an ecological niche to use fire-burned trees as a food source," said Helmut Schmitz, a zoologist at the University of Bonn in Germany and leading researcher on the beetle's ability to detect fires.

Of great interest to Schmitz, his colleagues, and the U.S. military are the sensors on the beetles' chests that allow them to determine when and where a fire is burning so that they can race towards the flames and get first dibs on the smoldering trees.

The researchers want to mimic the beetles' sensors, which are finely tuned to detect infrared radiation in the spectrum put off by forest fires. Robotic devices built with this technology could detect chemical or infrared emissions more cost-efficiently than current technology.

For example, the U.S. Department of Defense has a variety of systems that use infrared sensors, such as heat-seeking missiles, but in order for the sensors to work they must be cooled to freezing temperatures, which is expensive.

"If you get rid of the need for cooling but maintain sensitivity you have a tremendous gain in weight, size, complexity, maintenance, durability, et cetera," said Hugh DeLong, a contract officer with the U.S. Air Force's Air Force office of Scientific Research in Arlington, Virginia.

The Air Force is funding a portion of the research by Schmitz and his colleagues in the hopes that the scientists can mimic the beetle's sensor system to design a sensor for the military.

Natural Sensor

Melanophila acuminata beetles have what scientists call pit organs in their chests right at the point where the legs are attached to the body. Within each pit are 60 to 70 sensors called sensilla in insects. Each sensillum consists of a little sphere which is made of the same hard skin material that covers the beetle's outer shell. Each sphere is connected by nerves to a highly-sensitive mechanoreceptive sensory cell.

"Incoming infrared radiation is absorbed by the outer sphere and absorption causes the sphere to expand a little bit," said Schmitz. "This micromechanical event is measured by the mechanoreceptor and a neuronal signal is generated."

So when a forest goes up in flames, the infrared radiation excites the little sensors, which alert the beetles to an imminent opportunity to mate. The beetles take off urgently for the flames.

Once Schmitz and his colleagues figured out how the sensor on the beetle functions, they started to work on mimicking this natural approach to engineer what they call a photomechanic infrared sensor.

To date the researchers have built a prototype that can detect the heat put off by a human hand or a lit match from a distance of 12 to 16 inches (30 to 40 centimeters).

"Studies indicate that animals have extremely sensitive systems, but progress in reproducing that sensitivity is slower," said DeLong.

The sensors made by Schmitz and colleagues consist of a little metallic disc that acts as the infrared absorber. The metal chosen depends on the wavelength of infrared that is being detected. For example, Teflon is good for detecting heat put off by a human hand. Polyethylene—the common plastic used in grocery bags—is good for detecting the heat of a fire.

The metallic disc is connected to a mechanosensor, which is currently an inexpensive crystal that gives off an electric charge when disturbed by the expanding metal. "We now want to make it much more sensitive," said Schmitz.

The next step will involve the addition of a highly sensitive so-called capacitive sensor that can measure the expansion of just a few nanometers. As well, they hope to make the sensor smaller, making it faster and thus more sensitive.

Schmitz says the applications of this technology are many, including every field of application where other technical infrared sensors are used today.

"That means we could detect infrared radiation emitted from low-temperature infrared sources like animals or humans as well as infrared radiation emitted from very hot surfaces or fires," he said.

Note: Melanophila acuminata beetles are not the only animals equipped with biological receptors capable of detecting infrared sources at even great distances. Scientists are also studying how blood-sucking insects such as mosquitoes detect warm-blooded animals as well as how IR-sensitive snakes find their prey.

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