13 September 2013
Mechanical gears seen in nature for the first time
by Will Parker
Gear mechanisms, previously thought to only exist in man-made machines, have also evolved in nature, according to scientists who say they have made the first observation of mechanical gearing in a biological structure.
The gears were identified in the hind-leg of juvenile Issus leafhoppers and they bear a remarkable resemblance to the cogs found on every bicycle and inside every car gear-box.
Issus leafhoppers are a plant-hopping insect found in gardens across Europe. The researchers say the insect has hind-leg joints with curved cog-like strips of opposing "teeth" that intermesh, rotating like mechanical gears to synchronize the animal's legs when it launches into a jump.
Each gear strip is around 400 micrometers long and has between 10 to 12 teeth, with both sides of the gear in each leg containing the same number - giving a gearing ratio of 1:1.
Unlike man-made gears, each gear tooth is asymmetrical and curved towards the point where the cogs interlock - as man-made gears need a symmetric shape to work in both rotational directions, whereas the Issus gears are only powering one way to launch the animal forward.
The findings, reported in the journal Science, were made using a combination of anatomical analysis and high-speed video capture of the insects movements.
Lead investigator Malcolm Burrows, from the University of Cambridge, said each gear tooth has a rounded corner at the point it connects to the gear strip; a feature identical to man-made gears such as bike gears - essentially a shock-absorbing mechanism to stop teeth from shearing off.
"The gear teeth on the opposing hind-legs lock together like those in a car gear-box, ensuring almost complete synchronicity in leg movement - the legs always move within 30 'microseconds' of each other, with one microsecond equal to a millionth of a second," Burrows explained. "This is critical for the powerful jumps that are this insect's primary mode of transport, as even miniscule discrepancies in synchronization between the velocities of its legs at the point of propulsion would result in 'yaw rotation' - causing the Issus to spin hopelessly out of control."
Such precise synchronization would be impossible to achieve through a nervous system, as neural impulses would take too long for the extraordinarily tight coordination required. "By developing mechanical gears, the Issus can just send nerve signals to its muscles to produce roughly the same amount of force - then if one leg starts to propel the jump the gears will interlock, creating absolute synchronicity. In Issus, the skeleton is used to solve a complex problem that the brain and nervous system can't," said Burrows. "This emphasizes the importance of considering the properties of the skeleton in how movement is produced."
Interestingly, the mechanistic gears are only found in the insect's juvenile stages, and are lost in the final transition to adulthood. It's not yet known why the Issus loses its hind-leg gears on reaching adulthood. The study points out that a problem with any gear system is that if one tooth on the gear breaks, the effectiveness of the whole mechanism is damaged. While gear-teeth breakage in nymphs could be repaired in the next molt, any damage in adulthood remains permanent.
While there are examples of apparently ornamental cogs in the animal kingdom, the Issus is the first example of a natural cog mechanism with an observable function, says Burrows.