Tiny planthopper insects are among the most impressive jumpers in the insect world. Experiencing a remarkable 700 g force as they take off, the 20 mg athletes can accelerate to 5.5 m s–1 in 1 ms. They're gone in the blink of an eye, which fascinates Malcolm Burrows from the University of Cambridge. ‘Doing very fast things poses particular problems, not only for their nervous systems, but also the skeleton and the muscles,’ says Burrows. He explains that it is impossible for muscular contraction to deliver power fast enough to launch these ballistic leaps. ‘A muscle can do one of two things. It can contract extremely rapidly, but then it doesn't generate much power, or, it can contract slowly and generate lots of power,’ explains Burrows. And, as if that wasn't difficult enough, planthoppers have to coordinate their hind legs to push off within 30 μs of each other, less than one-third of the duration of an action potential, otherwise they spin hopelessly out of control. Curious to find out how the minute insects have solved the dual problems of producing enough power and coordinating their hind legs, Burrows decided to film the insects' leaps with an extremely high-speed camera (p. 469).
Travelling to Aachen in Germany, he collected the tiny insects from an ivy plant in the garden of his colleague, Peter Bräunig, and filmed the insects jumping at 30,000 frames s–1. Analysing the high-speed footage, Burrows could see that the insects initially pull their legs up under their body, where they hold them cocked for several seconds before the legs are released suddenly, sending the insect sailing into the air.
Having filmed natural jumps, Burrows wanted to get a closer look at the insects' jumping action, so he restrained the tiny jumpers on their backs in Plasticine and tickled their legs to get them to try to leap. Focusing on the insect's pleural arches, part of the insect's thorax, he could see that the structures bent as if they were storing energy. And when Burrows shone UV light on this region, he could see the telltale blue fluorescence produced by the elastic protein, resilin, which could store energy as the pleural arches are deformed. He had found the elastic storage mechanism that powers the insect's lift-off.
According to Burrows, once the insect has cocked its leg, huge muscles in the thorax contract slowly, bending and storing the energy in the elastic pleural arches, ready to be released in an instant as the pleural arches recoil and the insect suddenly leaps; just like the release of energy stored in an elastic catapult.
Having found the insect's catapult mechanism, Burrows began looking for the synchronisation system that coordinates the hind legs' simultaneous push-off and realised that small protrusions from the top part of each hind leg touched as the legs were cocked. Could this mechanical linkage provide the synchronisation necessary to prevent the leaping insects from spinning?
Filming a dead insect as he tried to move one of the legs by pulling on its tendon, Burrows was amazed to see that both legs moved in perfect synchrony. ‘I had to bring Peter into the lab and tell him “Have a look at this! Am I seeing something or what?”’, laughs Burrows. With no neural or muscle activity to move the dead insect's legs, it was clear that the mechanical linkage between the two legs was responsible for both legs pushing off simultaneously.
Having found the planthopper's unique mechanical linkage that synchronises the legs, Burrows is keen to find out more about the neural systems that control the insect's super-fast leaps. He says ‘there's a lot of fun ahead’.