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A THIN curl of smoke, a crackle of pine resin and a sudden flickering glow: at the first sign of fire in the forest most animals are off as fast as their legs or wings can carry them. As the fire spreads and the temperature soars above 700 掳C, the exodus swells. Look closely, though, and you might spot some traffic heading in the opposite direction-straight for the fire. For Melanophila beetles, a burning pine forest is irresistible, drawing them from far and wide. They come in hordes to mate in the glow of the fire. And, as the flames subside, the females lay their eggs beneath the bark of the burnt trees.

The beetles鈥� ability to spot distant fires is legendary. Their desire for burnt pine is so strong that they are said to make journeys of up to 80 kilometres to reach a fire. So how does an insect just a centimetre long detect a fire from so far off?

This is a question that has been taxing biologists for most of the century. But in the past few years, researchers in Germany have begun to unravel the beetle鈥檚 secrets. Helmut Schmitz and his colleagues at the University of Bonn suspect that the beetles get the first hint of a fire from tiny traces of chemicals in stray whiffs of woodsmoke. But they home in on the blaze with the help of a pair of supersensitive infrared detectors designed to pick up the radiation given out by a forest fire. These detectors, which are unlike anything else in nature, have now begun to attract attention from an unexpected quarter-the US Air Force. Its researchers, ever on the lookout for better ways of detecting the heat signatures of vehicles, tank engines and even individual soldiers, think they might learn a thing or two from the beetle.

As far as the beetles are concerned, their ability to spot a roaring log fire is simply part of their survival strategy. A healthy tree is bristling with defences against wood-boring beetle larvae. A Melanophila grub unfortunate enough to hatch beneath the bark of a living pine will almost certainly be poisoned by the tree鈥檚 chemical weapons or drowned in sticky resin. If it survives long enough to start chewing, the tree responds with a burst of cell proliferation that will squeeze the tiny larva to death. 鈥淎ll wood-feeding insects depend on trees that are sick or suffering-when their defences are down. Best of all is a tree that has just been killed by a fire,鈥� says Schmitz.

There are a dozen or so species of Melanophila beetles around the world. All of them flock to fires to mate and lay their eggs. Unlike their more glittering relatives, these particular 鈥渏ewel beetles鈥� are coal-black. 鈥淵ou can hardly see one when it is on the bark of a burnt tree,鈥� says Schmitz.

Earlier this century, biologists realised that you only ever saw the insects when there was a fire blazing somewhere. They were clearly extremely sensitive to some sort of signal from burning trees.

Forest fires generate clouds of smoke, and trees-especially pines-burn noisily, crackling and spitting as their highly inflammable resin explodes into flames. But experiments suggested that the beetles didn鈥檛 fly towards a source of smoke, nor do they seem to respond to the sound or visible light from a fire.

As well as visible light, however, fires give off infrared light. Although it鈥檚 invisible to our eyes, we can feel it as warmth on our skin. And infrared radiation seems an ideal cue for the beetles. Most of the infrared produced by a hot wood fire has a wavelength between 2 and 4 micrometres, quite distinct from the longer wavelengths produced by, say, the warm body of a mammal. The infrared from fires also travels well through the atmosphere, unlike longer or shorter wavelengths, which are absorbed by water vapour in the air.

Early researchers also spotted odd organs under the beetle鈥檚 middle pair of 鈥渁rmpits鈥�. They could hardly miss them: the pits are big enough to see with the naked eye, forming depressions about a tenth of a millimetre deep-a big dent in the side of such a small insect. And the beetles fly with their middle legs lifted up high, so the organs are perfectly placed to receive infrared signals.

Nuts and raisins

In the 1960s, William Evans, a Canadian entomologist, found that if he focused a beam of infrared light onto a pit organ, the beetle twitched an antenna. If he aimed the beam at any other part of the beetle, nothing happened. He also found that infrared radiation with a wavelength of 3 micrometres prompted the strongest reaction and concluded that the 鈥減it organs鈥� must be some sort of infrared detector.

But for thirty years, this remained a plausible but unproven theory. No one checked whether incoming infrared triggered a response by a sensory nerve cell, the first crucial step in detecting a signal and making use of the information. Then Schmitz and his colleague Horst Bleckmann decided to investigate the powers of one particular beetle, Melanophila acuminata. After a forest fire north of Berlin, they collected charred, grub-infested wood and waited for the grubs to develop into beetles. Reared in the lab on an unusual diet of unsalted peanuts and raisins, the beetles were soon ready to take the firespotting test.

What Schmitz and Bleckmann found was extraordinary: M. acuminata鈥檚 pits are packed with dozens of tiny but highly effective infrared sensors, each able to pick out the wavelengths of infrared emitted by a blazing wood fire. They were nothing like the well-known infrared sensors in the facial pits of rattlesnakes and pit vipers, which detect much longer wavelengths over much shorter distances.

In these snakes, the infrared radiation from warm-blooded prey warms up a thin membrane richly supplied with nerve endings which detect heat directly. But Melanophila鈥檚 pit organs seemed to work by some mechanical means. All the evidence suggested that the team had found a novel type of detector.

Each pit organ is packed with around 70 small rounded knobs. These individual sensors, or sensilla, line the bottom of the pit and bulge up through a thin coating of soft cuticle. Each sensillum consists of a sphere suspended from the cuticle by a short stalk (see Diagram). Slotted into a small notch at the bottom of the sphere is the fine tip of a sensory nerve cell.FIG-mg21985101.JPG

In most respects, the sensilla and their sensory nerve cells resemble insect touch sensors, which consist of stiff hairs with a nerve cell at their base. The hairs are made of exocuticle, a mixture of chitin and protein made hard by crosslinks between the molecules. The slightest disturbance of a hair triggers a nerve impulse. The pit organs are unusual in that there is no hard exocuticle anywhere. In place of the stiff hairs there are the spheres-and these are made from softer, more flexible endocuticle, the material that makes up the inner layer of an insect鈥檚 skeleton. Although endocuticle is also a combination of proteins and chitin, it lacks the crosslinks that toughen the exocuticle.

The structure of the sensilla suggests that they evolved from touch sensors, so do they really detect infrared light? And if so, how? In the lab, Schmitz and Bleckmann exposed individual sensilla to infrared radiation with the same range of wavelengths as a forest fire, taking care to exclude visible light or warm air that might give false signals. They found that an exposure as brief as 2 milliseconds to infrared radiation with a power of 24 milliwatts per square centimetre-about twice as much infrared as your hand is giving out-was enough to trigger a string of signals from the nerve cell at the base of the sphere. But even exposure to 5 milliwatts of radiation-the lowest intensity possible with their apparatus-triggered a brief response.

The experiments hinted at how sensitive the pit organs might be. But how do they achieve this? During the experiments, the team discovered that the sensilla also responded to touch. A slight movement of a sphere also set off a nerve impulse. What they had here, they proposed, was a 鈥減hotomechanical鈥� sensor, something never before seen in nature. Somehow, incoming radiation was being converted into a mechanical force strong enough to put a tiny amount of pressure on the nerve cell below and set off an impulse.

With help from Manfred M眉rtz, a physicist at the university, they came up with a theory of how it works. The key is the sphere, a relatively large ball around 15 micrometres across that is ideally suited to absorb infrared radiation. While most chemical bonds in organic molecules absorb infrared light, endocuticle is especially rich in bonds that absorb light with a wavelength of 3 micrometres-which corresponds almost exactly to the maximum output from a forest fire.

Infrared of this wavelength makes the bonds vibrate more vigorously, and this energy is quickly converted to heat. A rough calculation shows that exposure to 5 milliwatts of infrared would raise the temperature of a sphere of similar hardness-made from rubber or resin, for instance-by 0.01 掳C. This tiny change would be enough to make the sphere expand and press down on the tip of the nerve cell, deforming the cell鈥檚 outer membrane by about a nanometre.

Touch sensors in other insects and spiders do respond to such tiny changes, but this is near the limit of their powers. 鈥淲e speculate that the sensillum is adapted to give maximum expansion as the infrared reaches it,鈥� says Schmitz. 鈥淪ofter material expands more in the heat.鈥� At the US Air Force Research Laboratory in Ohio, biologist Morley Stone and his colleagues will be looking closely to see if the endocuticle has novel ingredients which might explain the extraordinary efficiency of the pit organs.

鈥淭here may be a group of components in chitin designed to absorb infrared photons of a given wavelength,鈥� says Schmitz. 鈥淚t might be that endocuticle has all the right properties and there is no need to invoke any special molecules.鈥� Perhaps, he says, the beetle鈥檚 only trick is to suppress the hardening process that turns endocuticle into exocuticle. The way the sphere is built may also play a part, suggests Schmitz, by somehow focusing the swelling onto the tip of the sensory cell.

A quick calculation suggests that a beetle should be able to detect a 10-hectare fire from 12 kilometres away. But this almost certainly underestimates the beetle鈥檚 capabilities. If the researchers鈥� apparatus could produce infrared of lower intensity, they think they might set off nerve impulses with as little as 500 microwatts. And because a beetle has a whole array of sensilla, which would increase the signal to noise ratio, they could be picking out even tinier amounts of infrared-enough perhaps to account for the reports of journeys of 80 kilometres or so.

Wax works

The need to improve the signal to noise ratio might also explain the presence of dozens of wax glands between the domes of the sensilla. The tiny glands secrete prodigious amounts of wax-enough to cover the entire pit. Their exact role is unclear. They might be evolutionary leftovers: touch sensors are usually well equipped with wax glands. They might prevent the organs drying out or protect them from contamination by smoke particles. Schmitz thinks they might also help to keep the temperature of pit organs constant, preventing the air that flows over the beetle in flight from cooling them down. Any changes in temperature would interfere with the expansion and contraction of the spheres, and confuse the signals.

So all a beetle has to do is potter about waiting for the infrared 鈥渟tarting gun鈥� and then join the race to find a fireside mate? Not quite. A beetle living deep among the leaves of a tree is unlikely to feel the warmth of any infrared on its pit organs. The signal will be blocked, perhaps by a hilly landscape, certainly by twigs, branches and even by the beetle鈥檚 own legs as it walks about. The only chance it has of detecting infrared radiation from a blaze is to fly above the tree, raise its middle legs and start searching. This is why the beetles also need smoke detectors, suggests Stefan Sch眉tz, a chemical ecologist at the University of Giessen in Germany. 鈥淭hey need a trigger to start them off. We think that trigger might be traces of odour of burnt pine.鈥�

Sch眉tz has found that M. acuminata has sensitive smoke detectors somewhere on its antennae, although he has yet to find exactly where. The smoke detectors are sensitive to many of the volatile chemicals given off by a wood fire, but are most sensitive to guaiacol (2-methoxyphenol) and two closely related molecules. All three are given off when lignin-one of the main components of wood-burns.

Beetles can sniff out these chemicals in concentrations as low as a few parts per billion, making them more sensitive than the most efficient detector in Sch眉tz鈥檚 lab. 鈥淚t鈥檚 probably these that are responsible for detecting burning trees over long distances,鈥� he says. The detectors also allow the beetle to pick out pine fires from their particular chemical signature. 鈥淓xperiments show that it鈥檚 more responsive to chemicals from pines than, say, oak trees,鈥� says Sch眉tz.

With such good smoke detectors, a pair of infrared sensors might seem unnecessary. But smoke blows around, forming breakaway plumes, or settles out on the surfaces of plants. If a beetle simply followed the smell of a fire it could find itself travelling a very roundabout route. 鈥淭heoretically, it is possible to fly to the source by smell alone but it would cost a lot of energy because the beetles would have to fly in a rather zigzag manner, and sometimes even backwards,鈥� says Schmitz. Infrared emissions from a fire are a more reliable guide, providing a consistent signal which grows stronger as you approach a fire, regardless of the weather.

Once above the tree tops, a beetle probably makes a few search flights until it locates the direction of the infrared. The next challenge is to keep on the right flight path. Schmitz thinks the beetle stays on course by continually comparing information from right and left pit organs in the same sort of way that a grasshopper locates sounds with a pair of hearing organs on its forelegs. He likens it to the way a pilot approaches a runway: any deviation from the optimal flight path triggers a bleep in the pilot鈥檚 headphones, warning that a change of course is needed.

The smoke signals probably become important again as the beetle nears the conflagration. 鈥淚t can decide if the tree is suitable for laying its eggs in-checking that it really is a pine tree,鈥� says Sch眉tz.

Fire alarms

Both of the beetle鈥檚 detectors are more sensitive than Schmitz and Sch眉tz have been able to measure so far, so no one knows exactly how good they are. The pit organs are not as sensitive as the best artificial devices, but these have to be cooled to the temperature of liquid nitrogen, cost huge amounts and frequently go wrong. However, the pit organs are at least as good as the current range of uncooled detectors, of the sort used by police in night vision goggles or by the fire service to discover hotspots in burning buildings.

Most of these sensors rely on exotic inorganic materials such as indium-antimony or gallium-arsenide to absorb infrared radiation. 鈥淲e鈥檙e interested in finding out how a beetle accomplishes the same sort of sensitivity with an organic material,鈥� says Stone. 鈥淚f we could understand that we could open up a new era of infrared detection.鈥�

Despite the sceptics who think that attempts to mimic biological materials are naive, the US Air Force is funding a large programme of research into biological infrared detectors, concentrating on those of rattlesnakes and now Melanophila beetles. 鈥淲e aren鈥檛 saying that we want to put a snake or a beetle in a sensor or even make it out of the same material. We want to find out how they organise the structure of the material. Then perhaps we could build an inorganic structure in a biologically inspired way,鈥� says Stone.

And Melanophila could also help in the development of fire detectors that can sniff out traces of complicated molecules in smoke, rather than just simple gases such as carbon monoxide or hydrogen. 鈥淎t the moment, commercial fire detectors can鈥檛 do that,鈥� says Schmitz. 鈥淚n future we might be able to adapt sensors to the special needs of the sort of fire you want to detect-in a forest or in a warehouse-and make it cheap,鈥� he says. 鈥淓volution has given this little beetle a fire detection system that works at the limit of what is possible for physics. It鈥檚 naive to say you can build this system-but you can learn from it.鈥�