For most creatures, fire is a complete disaster. It is hard to see how anything benefits when an inferno sweeps through a forest: except for the fire beetles, Melanophila acuminata. They are the first to occupy a scorched site, converging in their millions from distances of up to 10 km. Free of predators, they gorge on roast remains and mate, depositing their eggs beneath the bark of burned trees. But how do these tiny insects sense a blaze over such great distances?
Helmut Schmitz from the University of Bonn in Germany explains that Melanophila beetles are equipped with exquisitely sensitive infrared receptors that may detect blazes. But unlike most infrared receptors, which sense temperature with thermosensitive neurons, the fire beetle's infrared receptors are modified hair sensors that originally detected subtle movements. So how do the insects convert heat into mechanical stimuli? Schmitz explains that the receptors contain fluid that he suspects expands as it heats up and presses against a motion-sensitive nerve cell deep in the receptor; `the beetles could be described as hearing heat,' says Schmitz. But for the mechanism to work, the fluid must be contained in a pressure vessel that does not expand when heated. Schmitz and his PhD student Martin Müller decided to investigate the material properties of Melanophila infrared receptors to see if they are hard enough to stand the pressure(p. 2576).
But first the team had to find some fire beetles. Knowing that forest fires had swept through Spain in the summer of 2006, Schmitz and Müller set off 9 months later to collect charred tree trunks infested with beetle larvae from a burned forest near Cardona. Returning to their Bonn laboratory, Schmitz waited for the larvae to metamorphose into beetles before he could begin investigating the sensor's mechanical properties.
Dying thin dried sections of the cuticle with Mallory trichrome stain,Müller could clearly see that the cuticle around the dome-like receptor structure was composed of three layers; the external exocuticle, reinforced with onion-like chitin layers; the mesocuticle, encasing the pressure-transducing fluid; and the endocuticle, beneath the receptor. But how hard were each of these materials? The duo needed a sophisticated technique to measure the cuticle's mechanical properties on a nanoscale.
Striking up a collaboration with materials scientists Maciej Olek and Michael Giersig at the nearby Forschungszentrum caesar, Müller used nanoindentation to measure the cuticle hardness and stiffness inside the infrared receptor. Schmitz explains that this groundbreaking technique has only recently been used on biological samples, and the receptor's internal structures could only be analysed by cutting ultrathin sections from dehydrated receptors embedded in resin, identifying different regions in the receptor before selectively probing them with a nanoindenter. After months of painstaking analysis, Müller found that the external exocuticle was twice as hard, and 1.5 times as stiff, as the spongy mesocuticle. The exocuticle is tough enough to act as a pressure vessel, allowing the beetle to convert the fluid expansion caused by heat into a mechanical sensation.
Schmitz suspects that the differences in the hardness of the cuticle materials will be even greater in natural hydrated samples, and is keen to measure the native receptor's material properties with freezing techniques. Ultimately he hopes to accurately model the expansions and pressures generated in the insects' extraordinary infrared receptors and build fire beetle-inspired infrared detectors. But until then, fire beetles will remain one of the few creatures that `hear' heat.