Issus nymph revealed first functional mechanical gears in nature

While there are examples of apparently ornamental cogs in the animal kingdom – such as on the shell of the cog wheel turtle or the back of the wheel bug – gears with a functional role have been elusive or rendered defunct by evolution. Scientists at the University of Cambridge have been able to reveal these functioning natural gears used to synchronize the animal’s legs when it launches into a jump.

The researchers used a combination of anatomical analysis and high-speed video capture to find that juvenile Issus, a plant-hopping insect found in gardens across Europe, 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.

The gears in the Issus hind-leg bear remarkable engineering resemblance to those found on every bicycle and inside every car gear-box. Each gear tooth has a rounded corner at the point it connects to the gear strip. Just like with gears found in bikes, it serves as a shock-absorbing mechanism to stop teeth from shearing off.

The gear teeth on the opposing hind-legs lock together to ensure almost complete synchronicity in leg movement – the legs always move within 30 ms of each other. This is critical for the powerful jumps that are this insect’s primary mode of transport, and the synchronization ensures elimination of “jaw rotation” and increases the stability during jumps.

“This precise synchronization would be impossible to achieve through a nervous system, as neural impulses would take far too long for the extraordinarily tight coordination required”, said Professor Malcolm Burrows, Cambridge’s Department of Zoology. “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.”

Interestingly, the mechanistic gears are only found in the insect’s juvenile (nymph) stages, and are lost in the final transition to adulthood. Each gear strip in the juvenile Issus was around 400 micrometers long and had 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.

It’s not yet known why the Issus loses its hind-leg gears on reaching adulthood. The scientists point 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 – a process when animals cast off rigid skin at key points in their development in order to grow – any damage in adulthood remains permanent.

It may also be down to the larger size of adults and consequently their ‘trochantera’­ – the insect equivalent of the femur or thigh bones. According to scientists, the bigger adult trochantera might allow them to create enough friction to power the enormous leaps from leaf to leaf without the need for intermeshing gear teeth to drive it. It could also be attributed to adaptation to their more developed skills after practicing jumping through their life.

“We usually think of gears as something that we see in human designed machinery, but we’ve found that that is only because we didn’t look hard enough”, said Gregory Sutton, now at the University of Bristol’s School of Biological Sciences. “These gears are not designed; they are evolved – representing high speed and precision machinery evolved for synchronization in the animal world.”

For more information, read the paper published in the journal Science: “Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect”.