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Bugs Lead the Way to Better Detectors

Photonics Spectra
Jun 2001
Daniel S. Burgess

Photonics regularly draws the inspiration for new technologies from the natural world, such as microbolometers and green fluorescent protein from rattlesnakes and jellyfish, respectively. It should come as no surprise, then, that insects are doing their part as well. Researchers hope that investigating the insects that are attracted to forest fires will lead to the development of better infrared sensors.

Pyrophilic insects such as these specimens of Melanophila consputa travel to forest fires to mate and lay their eggs in newly dead wood. Researchers are studying
Melanophila's novel infrared sense organs in the hope of developing better IR detectors. Courtesy of Nathan M. Schiff.

Some pyrophilic insects locate burning forests by smell, but others detect and follow the infrared radiation emitted by a fire. The beetle Melanophila acuminata, for example, sports IR-sensitive organs on either side of its thorax, and Merimna atrata, an Australian beetle, employs microbolometerlike sensors on its abdomen.
With funding from the US Defense Advanced Research Projects Agency's Controlled Biological Systems program, Helmut Schmitz at the University of Bonn is looking for new detector designs in the insects. He estimates that Melanophila can detect a 10-hectare fire from up to 12 km with its unique IR detectors. He explained that the beetles' sensors comprise 50 to 100 cuticular spheres, each 15 µm in diameter. The spheres are innervated with a mechanoreceptor that measures the expansion of the cuticle as it absorbs incoming radiation.

The biological sensors are ideal models for more efficient detectors because they operate while the insect is in flight and at an average temperature of 30 °C. Melanophila's photomechanical sensors display a peak sensitivity of between 2.5 and 4 µm and a sensitivity threshold of 60 µW/cm2, Schmitz said. Comparable data for Merimna is unavailable, but he noted that the beetle could detect a temperature change of 0.7 K.

'Ugly bug ball'

Nathan M. Schiff, a research entomologist at the US Department of Agriculture's Forest Service in Stoneville, Miss., studies pyrophiles, including wood wasps and species of Melanophila. He cautioned that it may not be possible to prove why the insects follow fires, but he noted that many lay their eggs in dead wood and suggested that they may be competing for fresh resources.

Schiff also theorizes that flying into fires may be analogous to a behavior in some butterflies called hilltopping, which drives them to congregate at the top of the nearest hill. These butterflies reduce competition among their offspring by depositing one egg per available food plant, he explained, so they must gather to find others of their species. Pyrophilic insects like the wood wasps that lay many eggs in one place may have evolved a similar instinct to avoid mating with their neighbors, which are likely to be siblings. "I call this behavior fire-topping, and it promotes outcrossing."

In this view, burned-out forests are geographical cues to assemble for mating, much as points of high contrast do for the formation of gnat leks. "They're the discotheques of the pyrophilic insect world," Schiff said.

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