How a Half-Inch Beetle Finds Fires 80 Miles Away
02 May 2019
Published by https://blogs.scientificamerican.com
USA – The fire chaser beetle, as its name implies, spends its life trying to find a forest fire.
Why a creature would choose to enter a situation from which all other forest creatures are enthusiastically attempting to exit is a compelling question of natural history. But it turns out the beetle has a very good reason. Freshly burnt trees are fire chaser beetle baby food. Their only baby food.
Fire chaser beetles are thus so hell bent on that objective that they have been known to bite firefighters, mistaking them, perhaps, for unusually squishy and unpleasant-smelling trees.
They have descended on at least one UC Berkeley football game at California Memorial Stadium — rather unfortunately situated in the midst of some recently burnt pine hills — at which an estimated 20,000 cigarettes were being smoked. The beetles’ disappointment on discovering the source of the “fire” was probably only matched by the irritation of the smokers swatting confused beetles attempting to bite their necks and hands.
They have shown up on hot pipes and other equipment at lumber yards and sugar mills, tar plants, cement kilns, smelters, and on one occasion, a colossal 1925 oil tank explosion in Coalinga, California. In that last case, the flames reached hundreds of feet into the air and were visible for over 30 miles … but the nearest plausible beetle-bearing forest was 80 miles away.
That last detail implies something amazing about fire chaser beetles: they can sense fires from distances over which car stereos are hard pressed to pick up FM radio.
In fact, because the infrared emission of a burning oil tank of known volume (in this case, 750,000 barrels) can be calculated with reasonable certainty, scientists that studied the Coalinga oil tank explosion have inferred the beetles can detect infrared radiation intensities so low that they are buried in the thermal noise around them. But … how?
Infrared radiation, a proxy for heat, is a reliable source of information about fire because it propagates outward in a clear gradient, dampened only by humidity. It gives a very accurate indication of distance and direction from the source. A highly sensitive infrared sensor can detect a surface fire from space.
A flying fire chaser beetle appears to be trying to give itself up to the authorities. Its second set of legs reach for the sky at what appears to be an awkward and uncomfortable angle.
But the beetle has a good reason. It’s getting its legs out of the way of its heat eyes, pits filled with infrared sensors tucked just behind its legs.
The heat eyes on the sides of fire chaser beetles are filled with about 70 infrared sensilla. Inside each sensillum is a hair-like sensor (called a dendritic tip in the diagram above) that physically deforms when the sensillum expands in response to heat, triggering a neural response.
Such arthropod hair sensors are incredibly sensitive. Spiders possess versions called trichobothria that detect air movement – such as that caused by movement of web or prey – with such sensitivity that they are tripped by levels not much higher than the random movement of air molecules (Brownian motion), or, as the authors of recent paper on the oil-tank-fire-beetle-sensitivity-question put it, “at the limit of the physically possible”.
In addition to containing similar hypersensitive mechanoreceptors, the sensilla in fire chaser beetle heat eyes are found in arrays of 70-90. A signal picked up by more than one of them can be summed up and amplified by the neurons that wire the array. As a result, the heat eye can detect softer signals than a single sensor could.
Finally, it is also possible the beetles are better able to detect a signal buried in noise due to a spooky (to me) phenomenon called “stochastic resonance”. In this scenario, added thermal noise counterintuitively helps a sensor pick up a signal.
A signal below the threshold for triggering a sensor – but still close to it – will resonate by chance with a portion of thermal noise that is the same frequency. When there is more noise, there is more signal at that resonant frequency. Together, noise plus signal adds up to an impulse sufficient enough to trip the sensor when signal alone or signal with less noise would not. Incredibly, the measurement gets more precise in the presence of noise than without.
Though deeply counterintuitive, stochastic resonance has been demonstrated over and over in biological systems, including several times in humans. For example, humans can detect a touch stimulus that would normally be undetectable when they are exposed to a mechanical vibration at the same time. The portion of the vibration that “resonates” with the touch sums to trigger mechanoreceptors in skin. Crazy but true.
Thus, though the fire chaser beetle’s ability to detect fire may seem supernatural, it may operate on physical principles that are also at our very own fingertips.
References
Schmitz, H., and H. Bleckmann. “The photomechanic infrared receptor for the detection of forest fires in the beetle Melanophila acuminata (Coleoptera: Buprestidae).” Journal of Comparative Physiology A 182, no. 5 (1998): 647-657.
Schmitz, H., and H. Bousack. “Modelling a historic oil-tank fire allows an estimation of the sensitivity of the infrared receptors in pyrophilous Melanophila beetles.” PLoS One 7, no. 5 (2012): e37627.
