AboutZoos, Since 2008


Func­tion­ing ‘mechan­i­cal gears’ seen in nature for the first time

pub­lished 15 Sep­tem­ber 2013 | mod­i­fied 26 July 2014

Pre­vi­ously believed to be only man-​made, a nat­ural exam­ple of a func­tion­ing gear mech­a­nism has been dis­cov­ered in a com­mon insect — show­ing that evo­lu­tion devel­oped inter­lock­ing cogs long before we did.

Issus coleoptratus nymphThe juve­nile Issus (coleop­tra­tus) - 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 syn­chro­nise the animal’s legs when it launches into a jump.

The find­ing demon­strates that gear mech­a­nisms pre­vi­ously thought to be solely man-​made have an evo­lu­tion­ary prece­dent. Sci­en­tists say this is the “first obser­va­tion of mechan­i­cal gear­ing in a bio­log­i­cal structure”.

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 Sep­tem­ber 13 issue of the jour­nal Sci­ence.

Issus GearsThe 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; 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. This is crit­i­cal for the pow­er­ful jumps that are this insect’s pri­mary mode of trans­port, as even minis­cule dis­crep­an­cies in syn­chro­ni­sa­tion 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.

In Issus, the skele­ton is used to solve a com­plex prob­lem that the brain and ner­vous sys­tem can’t
Mal­colm Bur­rows, Depart­ment of Zool­ogy, Cam­bridge University »

“This pre­cise syn­chro­ni­sa­tion 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. 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 empha­sises 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.”

Issus gear-jumping

Inter­est­ingly, 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.

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, 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. 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, 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 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.

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 either remain elu­sive or have been ren­dered defunct by evolution.

The Issus is the first exam­ple of a nat­ural cog mech­a­nism with an observ­able func­tion, say the scientists.

Lis­ten to Pro­fes­sor Mal­colm Bur­rows when he talks about find­ing the bugs that led to the sci­ence, and work­ing with artists Eliz­a­beth Hobbs and Emily Tracy and mem­bers of the com­mu­nity in the Lon­don bor­ough of Hack­ney to pro­duce the film ‘Water­folk’. Be sure to watch the video until the end where some footage is pre­sented of ‘Waterfolk’:

The above news item is reprinted from mate­ri­als avail­able at Uni­ver­sity of Cam­bridge. Orig­i­nal text may be edited for con­tent and length.
(Source: Uni­ver­sity of Cam­bridge research news, 12.09.2013)

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