We humans like to think we’re pretty clever. Fire, the wheel, the interlocking toothed cog – but as of today we can scratch the toothed cog off “humans only” list.
Through a combination of anatomical analysis and high-speed video capture of normal Issus movements, scientists from the University of Cambridge have been able to reveal these functioning natural gears for the first time. The findings are reported in the latest issue of the journal Science.
The gears in the Issus hind-leg look amazingly like those found on every bicycle and inside every car gear-box (and your LEGO Mindstorms kit). Each gear tooth has a rounded corner at the point it connects to the gear strip. This is just like human-made gears such as bike gears. Rounded gear teeth let them mesh with one another smoothly without bits chipping 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 (that’s thirty millionths of a second – very very precise). And at that physical scale the Issus needs that precision, because without it there’d be little chance of a jump in a straight, predictable direction.
“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 lead author Professor Malcolm Burrows, from 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. 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.”
“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,” added co-author Gregory Sutton, now at the University of Bristol.
“These gears are not designed; they are evolved — representing high speed and precision machinery evolved for synchronisation in the animal world.”
The mechanistic gears are only found in the insect’s juvenile — or ‘nymph’ — stages, and are lost in the final transition to adulthood. These transitions, called ‘molts’, are when animals cast off rigid skin at key points in their development in order to grow.
They don’t know for sure why the Issus loses its hind-leg gears on reaching adulthood, but they have a guess: 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. The larger size of the adults’ ’trochantera’ — the insect equivalent of the femur or thigh bones – may allow the dispensation of the toothed cogs. The bigger leg body segments might allow them to can create enough friction to power the enormous leaps from leaf to leaf without the need for intermeshing gear teeth to drive it, say the scientists.
Each gear strip in the juvenile Issus was around 400 micrometres 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 human-made gears, each gear tooth is asymmetrical and curved towards the point where the cogs interlock — as human-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.
There are examples of cogs in the animal kingdom other than Issus, but where they do exist, they appear to be purely ornamental, such as as on the shell of the cog wheel turtle or the back of the wheel bug. Working gears up to now is something we’ve never found.
The Cog Is Not Dead. It lives, in Issus.
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