We humans think that we are pretty smart. We have invented a dizzying array of machines and mechanisms that have transformed the way we interact with the world. Yet despite all our cleverness, the products of evolution can still outdo us – sometimes by millions of years.
In a recent issue of Science, Malcolm Burrows and Gregory Sutton of the University of Cambridge reported that the nymphs of Issus coleoptratus, a common species of planthopper (a type of true bug), have the first known case of an evolved working gear mechanism. Noticing that planthoppers have legs arranged under their bodies that rotate in the same plane, they reasoned that the nymphs must synchronize their back legs somehow to allow them to jump without spinning out of control, but what mechanism could the animals be using?
To get a better handle on how planthoppers hopped, the duo first used a high-speed camera that captured 5000 images per second to film the tiny creatures. They found that I. coleoptratus was able to begin hopping within 2 ms of stimulus, and was able to synchronize its two back legs within 30 μs of each other – a feat that would be nearly impossible by nerve transmission (which takes sluggish milliseconds).
To take a closer look at what I. coleoptratus did when it jumped, Burrows and Sutton mounted individual planthoppers upside down in Plasticine while they filmed the insects' undersides at 30,000 frames s−1 and watched the action of the legs. They found that the trochanters (the second joint on an insect leg moving out from the body) of the two back legs each had a strip of gear teeth located on their inner surface. When the planthoppers prepared to jump, they rotated their rear legs, which rotated the two cogs at the top of the femur so that the teeth of the gears meshed together as the legs prepared for launch. Then, when the planthoppers jumped, the legs snapped forward with the gear teeth re-engaging as the legs rotated in the opposite direction, to synchronize the movement of both limbs for a successful launch.
The researchers used a scanning electron microscope to look at the fine-scale structure of the insect cogs. They found that each tooth on the gear was tiny – only 15–30 μm high – and separated from the next tooth by another 30 μm. Strangely, once I. coleoptratus moults into an adult, it loses the gears. They hypothesise that this might be because adults do not moult and so are unable to repair the teeth if any were to become damaged.
Burrows and Sutton think that without the gears, jumping I. coleoptratus nymphs would not be able to synchronize their legs in their fast jumps. Understanding how such tiny gear mechanisms work could help engineers develop small machines. It seems that insects have still got the jump on us – in this case, literally.
