Zoologger: Infrared-sensing beetles born in fire

Species: Melanophila consputa
Habitat: coniferous forests of western North America, hoping for an arsonist to come calling

On the morning of 10 August 1925, a bolt of lightning struck a large oil storage tank in a remote area of California. The oil immediately caught fire and burned fiercely. By the evening it was so hot that it suddenly boiled over, sending flames shooting over 100 metres into the air, and spilling boiling oil onto the ground. The resulting lake of fire burned for another two days.

Before the fire could be extinguished, something strange happened ? untold numbers of charcoal beetles descended, swarming near the blazing oil.

We know what they were doing there: charcoal beetles are drawn to heat, and only breed in the smouldering remains of forest fires. But their nearest habitat was dozens of kilometres away. How did they know where to come?

Smokin' hot

Charcoal beetles belong to a genus called Melanophila, commonly known as fire-chasers or fire beetles. They are part of a larger family called the jewel beetles, most of which have bright iridescent colours. Fire beetles, however, are as black as coal.

They flock to forest fires, where the mated females lay eggs in the charred wood. The larvae burrow into the wood and feast, safe from the defences that a healthy tree might employ to get rid of them.

To help it find a suitable conflagration, each beetle has a special set of sensors mounted just behind the "hips" of its second pair of legs. These can detect infrared radiation, the invisible rays of heat given off by fires. Similar heat sensors have been found in the wings of some butterflies.

Each sensor array contains around 70 individual organs called sensillas, which contain a small pocket of water. This expands when heated, pushing against a receptor at the base of the pocket.

But the question remains: how good are these heat sensors? The closest large population of charcoal beetles was probably in the forests of the Sierra Nevada, around 130 kilometres away. Did they really feel the heat of the fire?

Into the fire

Infrared radiation is probably the most reliable way to spot a fire from a distance, says Helmut Schmitz of the University of Bonn in Germany. The beetles are active during the day, when the flames would be hard to see from a distance. The beetles can also sense smoke, but smoke plumes can be disrupted by crosswinds, so are rather unreliable.

With Herbert Bousack of the Peter Gr?nberg Institute in J?lich, Germany, Schmitz calculated how much heat the oil fire must have emitted, and thus how sensitive the beetles' sensors needed to be.

Based on the size of the oil tank, it must have been putting out around 20 kilowatts of heat per square metre. That means the beetles needed to sense an extraordinarily faint signal of 0.13-0.41 milliwatts per square metre.

Lab tests suggest that each sensilla can only sense signals of 0.6 watts per square metre ? which is a problem as the real signal was a thousand times weaker. But Schmitz says those lab tests were crude, and probably didn't tap the beetles' true abilities.

It may be that the beetles can combine signals from multiple sensillas, which would allow them to spot much weaker signals. The sensors may also make use of a phenomenon called stochastic resonance, in which random noise (in this case, heat from other sources) actually improves the sensitivity of the system.

Maybe we should start making insect-based fire alarms.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0037627

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