History has long attributed the development of the gear to the ancient Greeks, over two thousand years ago. But now it turns out that human beings did not, in fact, invent the gear. Whether natural selection or intelligent agency deserves the credit may be up for dispute, but mechanical gears have recently been discovered in the biological realm. Ladies and gentleman, meet the planthopper insect Issus.
The gears in question are found on a structure near the top of each of the insect’s hindlegs, called the trochanter. Humans have a trochanter, which is near the top of the femur connecting to the hipbone. When people have hip replacement surgery, part of what is normally replaced is the trochanter. Arthropods have an analogous structure in their legs, which is rigidly attached to the femur, and articulates with the insect’s version of the hip, called the coxa. This Wikipedia image below labels the basic anatomy of an arthropod leg:
Now that you know a little about arthropod leg anatomy, next time you eat crab legs, you can impress your friends (or gross them out) by naming the parts, including the trochanter!
In any case, the trochantera of the hindlegs of Issus nymphs have interlocking gears, so that one leg can’t move without the other moving as well. Some beautiful images from National Geographic and elsewhere are seen below:
The technical paper in Science explains how the gears coordinate movement to ensure that the little bugs jump straight:
When one leg moves first at the start of a jump, its gear teeth will engage with and transmit power to the other stationary leg inducing it to move. The left and right power-producing muscles are innervated by independent sets of two motor neurons each, but all four motor neurons carry highly synchronized spike patterns that should help to ensure that the same amount of force is generated in each leg. This neural mechanism assists the synchrony of the leg movements but cannot deliver the level of synchrony measured during jumping. Thus, the primary role of the gears is to ensure that the hind legs move synchronously within microseconds of each other.
Popular Mechanics adds some further details about the gear:
The gears themselves are an oddity. With gear teeth shaped like cresting waves, they look nothing like what you’d find in your car or in a fancy watch. (The style that you’re most likely familiar with is called an involute gear, and it was designed by the Swiss mathematician Leonhard Euler in the 18th century.) There could be two reasons for this. Through a mathematical oddity, there is a limitless number of ways to design intermeshing gears. So, either nature evolved one solution at random, or, as Gregory Sutton, coauthor of the paper and insect researcher at the University of Bristol, suspects, the shape of the Issus‘s gear is particularly apt for the job it does. It’s built for “high precision and speed in one direction,” he says. “It’s a prototype for a new type of gear.”
At National Geographic, Sutton explains that by mimicking these gears in human technology, we may be able to improve machine function and minimize friction between gears:
Modern machines, such as 3-D printers, could easily create gears with these shark-fin teeth. Sutton is really excited by the prospect, and suspects that they may perform better in very small machines. “Modern machinery often doesn’t work at very small scales,” he says. “Friction doesn’t matter so much when you have two big gears next to each other but when you get small, friction starts killing you.”
The planthoppers might help to solve that problem. “We’re still being impressed and shocked by what we find in the back garden,” says Sutton.
So are these gears the only ones in known in nature? It turns out there are other examples, as paper in Science explains:
Elsewhere in the animal kingdom, apparently ornamental cogs occur on the shell of the cog wheel turtle Heosemys spinosa and on the pronotum of the wheel bug Arilus cristatus (Hemiptera, Reduviidae). The hearts of crocodilians have a cogwheel valve that closes during each heartbeat and can increase the resistance in the pulmonary outflow. In some insects, a row of regularly spaced protrusions work like clockwork escapement mechanisms to produce sound. In such stridulation mechanisms, a plectrum is moved across the row of teeth at a rate of 2500 to 5000 teeth per second, whereas the similarly sized gear teeth of Issus spin past each other at almost 50,000 teeth per second. Despite working under very different mechanical conditions, the similar tooth morphologies of the two structures suggest constraints that enforce a particular geometry.
Despite the fact that there are other gears in nature, Smithsonian Magazine has an article titled This Insect Has The Only Mechanical Gears Ever Found in Nature, which observes that “This seems to be the first natural design that mechanically functions like our geared systems.” What’s that about a “natural design”? Worried that readers might start to think that “natural design” implies actual design, the magazine is quick to add:
“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,” Sutton said. “These gears are not designed; they are evolved — representing high speed and precision machinery evolved for synchronisation in the animal world.”
The paper in Science tried to head off the same kind of dangerous ID-friendly thoughts, stating: “The gears in Issus, like the screw in the femora of beetles, demonstrate that mechanisms previously thought only to be used in manmade machines have evolved in nature.”
Wait a minute. How do we know these gears evolved, as opposed to having been designed? Because we know that everything in biology evolved. And how do we know that everything evolved? Because we know that nothing was designed. Right. But how do we know that nothing was designed? Because we know everything evolved.
Ah, got it now. Everyone clear?