Reader Stephen Barnard, occupier of Paradise in Idaho, called my attention to a new article in Science which, sadly, I don’t have time to read (I’m off to Cracow in an hour or so). But it reports that the nymphs of an “issus” (a genus of planthopper on the order Hemiptera, or “true bugs”) have gears on their legs: the first example of a cog-like mechanism in animals. I’ll give the abstract and then show a YouTube video that will tell you how the gears look and work.
Here’s the paper’s abstract:
Gears are found rarely in animals and have never been reported to intermesh and rotate functionally like mechanical gears. We now demonstrate functional gears in the ballistic jumping movements of the flightless planthopper insect Issus. The nymphs, but not adults, have a row of cuticular gear (cog) teeth around the curved medial surfaces of their two hindleg trochantera. The gear teeth on one trochanter engaged with and sequentially moved past those on the other trochanter during the preparatory cocking and the propulsive phases of jumping. Close registration between the gears ensured that both hindlegs moved at the same angular velocities to propel the body without yaw rotation. At the final molt to adulthood, this synchronization mechanism is jettisoned.
You can find the Science paper here (reference below, but article is behind a paywall). The video shows the entirely fortuitous way this phenomenon was discovered:
Here’s a figure from the paper, with its caption:
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”.
. . . 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; 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 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.
“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.
. . . “These gears are not designed; they are evolved – representing high speed and precision machinery evolved for synchronization in the animal world.”
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.
The question then becomes why the nymphs have gears but adults do not. The authors give a speculative answer:
It’s not yet known why the issus loses their 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.
Burrows, M. and G. Sutton. 2013. Interactive gears synchronize propulsive leg movements in a jumping insect. Science 341: 1254-1256 DOI: 10.1126/science.1240284