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Beetles hear the heat

Thick skin seems to let some insects perceive fire

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1:44am, August 8, 2008
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Fire-sensing beetles have an outer skin tough enough to let them sense the infrared radiation from a blaze up to 10 kilometers away, scientists report online and in the Aug. 15 Journal of Experimental Biology. The study provides yet another clue for researchers trying to design beetle-inspired heat sensors capable of detecting forest fires sooner.

It’s almost as if these fire beetles can “hear” heat, says Helmut Schmitz, an entomologist at the University of Bonn in Germany and coauthor of the new study.

For years, Schmitz and fellow entomologists have speculated that the fire beetle Melanophila acuminata uses modified hair sensors to pick up signals indicating where intense heat is originating.

“But this is the first study to show how the mechanism works,” Schmitz says. “It is another little stone overturned as we try to understand how the beetles’ heat sensors work and how we might one day make artificial fire-beetle infrared sensors.”

Each sensor consists of a tiny sphere that wraps around an inner, fluid-containing microcompartment and a single nerve cell, or receptor, sensitive to mechanical pressure.

Similar hairlike “mechanoreceptors” are found on just about any insect, Schmitz says. But, in the Melanophila beetle, the microcompartment contains fluid that expands in response to heat radiation emanating from a nearby fire. The expanding fluid is thought to press against the tip of the receptor anchored inside the sphere. Of course, Schmitz says, this mechanism can work only if the fluid-containing layer sits inside a sphere rigid enough that it would not swell when heated.

To determine if the beetle’s receptor had such a tough shell, the scientists tested the hardness of each receptor layer. They found that the outer layer was two times harder and 1.5 times stiffer than the fluid-containing middle layer. The difference was great enough to conclude that the sensor’s tough sphere would not expand when heated and would act as a pressure vessel for the insects’ heat-sensing receptor, Schmitz says.

The researchers’ use of dehydrated specimens, however, jeopardizes the results, says Julian Vincent, a biologist and mechanical engineering professor at the University of Bath in England who was not involved in the research. To really show how the layers expand and contract, the layers need to contain fluid levels similar to what was there when the beetles were alive, he says.

“But these sensors are horrendously small, only about 20 micrometers in diameter,” he concedes, “so I admire what Schmitz is trying to do.”

Schmitz and his colleagues at research institutions in Bonn also tested the hardness of the cuticle layers in the neck area of the Pachnoda marginata beetle. Based on theory, the scientists expected to find hard exocuticle there. Instead, they found that the outer region of the neck area was covered by a pad consisting of softer, spongier material encasing a thin strip of even softer, squishier material.

It isn’t clear whether other beetle species share this unusual, spongy layering structure, Schmitz says, but he thinks it may act like cartilage, allowing beetles to rotate their heads.

Vincent says he would like to see more data collected before scientists draw either conclusion.

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